Large aperture lenses useless on 1.6 crop cameras?

[Wildewinds]

I was listening to a seminar given by a Canon rep and he explained that the smaller sensors on 1.6 crop cameras (20d, 10d, 300, 350) not only changed the focal length, but they also reduced the depth of field. Thus, a very big aperture (<2.0) has an extremely narrow depth of field (I think the example was using a 100mm lens, only .2 meters would be in focus at 2 meters away, and that was at f/2.8 .

Obviously, having a sub 2.0 aperture allows us to take shots in low light situations. However, does it really look good to have only a very narrow piece of the photo in focus? I can only imagine a very limited amount of situations where someone would want a sliver of focused area. In fact, besides doing some lens tests, I've never used 1.8 on my nifty fifty because the narrow DoF detracted from the photo.

Am I missing something? Does anyone regularly shoot at <f/2.0 with one of the smaller sensored cameras?

[Willie]

The smaller sensor will actually give a greater depth of field in relation to a full size sensor, all other things being equal (mm, subject to lens distance, aperature). The smaller the sensor becomes, the greater the depth of field will be, that is why I went to Rebel 350D from G5. It is impossible to achieve the same depth of field with a P&S that you get with DSLR.

Regards,
Willie

[Wildewinds]

That is exactly opposite of what an official Canon marketing rep spent 20 minutes explaining.

[tim]

I think the Canon guy was wrong - the focal length isn't changed at all. Also, it shouldn't make any difference what size your sensor is. All you're doing is cutting off the edges of the sensor, there are no other changes that should make a difference to the DOF.

I'd personally ignore everything that Canon rep said based on those two things you've told us.

I shoot at F1.8 with the 50mm lens in low light, it works well, though you have to be very careful with your focusing.

[rent]

i think the canon rep was correct in this instance. the smaller sensor size equates to a smaller circle of confusion which yields a more shallow dof.

-alex

[tim]

I don't believe the COC changes when you cut the edges off your sensor.

[JaertX]

I don't believe the COC changes when you cut the edges off your sensor.

I agree.

I used to be confused by this too. It all starts with the crop factor being called a "focal length multiplier." It is cropping the image. It's not changing magnification or optics or the focal length at all.

DOF should not change, regardless of the crop factor. That would be like saying putting 110 film in your 35mm would increase the focal length of your lens.

Sheesh.

[raylks]

I think the DOF would not change. Like JaertX said, the APS-C CMOS only crops the image instead of changing the magnification and the focal length of the lens.

[J Rabin]

Wildewinds. Google or Dogpile Bob Atkins article on DoF. What you were told is not true. All things held equal, I think the apparent DoF will change when the reproduction ratio changes going to print. E.g., an image from a 50mm lens on a 35mm film camera gets "enlarged less" making an 8x10 print than the same image on a 22mm dSLR. That's where CoC comes from.
Go read Atkins article. Maybe the Canon guy was getting confused comparing dSLRs with P&S digicams with tiny sensors. On digicams, the sensor is so small, the reproduction so large, it is difficult to obtain selective OOF at low f/stops.

[rent]

oh boy.... this is gonna be one of those long threads. :p

i think the CoC is directly related to the film/sensor size.

CoC is basically how large a point can be before it starts to look like a small disk, rather than a point. so there are lots of variables, but for arguments sake, let's say for a 8"x12" print, viewed from 1 ft away with 20/20 vision, a person can distinguish a point of 0.250mm, anything larger he perceives it as a circle, not as a point. so we say the print CoC is 0.250mm.

a 35mm film (24mm x 36mm) needs to be enlarged 8.47x to get to 8"x12". so we can see the 35mm film has a CoC of 0.250/8.47 ~= 0.030mm

similarly, a 10D/20D sensor (15mm x 22.5mm) needs to be enlarged 13.55x to get to the 8x12. thus has a CoC of 0.250/13.55 ~=0.018mm

what does all this CoC business have to do with DOF? well, DOF is a function of a lens' focal length, its aperture f number, the actual focus distance and guess what, the CoC. a smaller CoC, as in a 1.6x digital sensor, gives more shallow DOF.

whew...

-alex

[ScottE]

The acceptable Circle of Confusion does vary with the amount of enlargement. If your objective is to produce an 8"x10" print you would need a smaller COC with a 1.6 crop camera to get the same acceptable sharpness in the print. This is simply because you are enlarging the original 1.6 time more with the APS sized original to get to the same sized print.

On the other hand, if you wanted to frame the picture the same you would use a wider lens with the APS sized sensor. For example if you too a picture with a 100 mm lens on a 35 mm camera and wanted to take the same picture from the same location with an APS sized camera you would have to use a 62.5 mm lens. Since wider angle lenses give greater apparent depth of field, even with the smaller circle of confusion for the APS sized sensor, the depth of field would be almost the same as the larger circle of confusion on the 35 mm sized sensor.

If you use the appropriate lense to compare, there is not much difference in the depth of field. I believe that some mathemeticians have even demonstrated that the smaller sensor will give slightly superior depth of field.

The real problem comes with wide angle lenses and a small sensor if you want to use a small aperture to maximize depth of field. Since the size of the aperture gets very small with extremely short lenses, diffusion becomes a factor if you stop down very much. You are not likely to get acceptable results shooting a 10 mm lens at f/32. The fortunate alternative is that with a lens as short as 10 mm lens you do not need a very small aperture to get great depth of field. (I am expecting to recieve my 10-22 lens this week so I can check this out for myself.)

Scott

[roanjohn]

Smaller sensor = greater depth of field.........thats why your point n shoot camera gets everything in focus at 2.8!!!!

Ro1

[Wildewinds]

I'm sure someone here has a 1DS and a 1.6 crop camera. A real world test may be able to resolve the issue.

[CyberDyneSystems]

DOF does not change at all,. untill you move further back to compensate for the reduced feild of view.

(oh no.. here comes "IT"....... )http://members.bellatlantic.net/~vze2hdzu/pics/poke.gif

Canon guy was definitely wrong.. on a few counts...

...becuase the difference when you compare a 1.6 to a full frame is not a big difference at all,... not like the difference fram a full frame to a P&S ... so all those large aperure lenses can still work wonders on an APS,. have have thousands of pics that prove it.. and there are millions more out there. It jsut is not such a big difference.

(that is the difference in DOF that does not actually change at all from the smaller sensor,. but rather from your need or desire to move back further to compensate from the smaller feild of view,.. got that?? ;) )

[CyberDyneSystems]

I'm sure someone here has a 1DS and a 1.6 crop camera. A real world test may be able to resolve the issue.


They've been done,. and RoanJohn's example sums it up perfectly,.
Becuase of the smaller sensor,. yoiu are at dramatically different distances.. much much further away.. and thus as you move further back,. we allready know this creates more DOF at the focus point.

[Wildewinds]

Okay, I just googled a Depth of Field calculator. Enter the data yourself and you can see that a smaller sensor (1.6 crop factor) gives a smaller depth of field than a larger sensor.

http://www.outsight.com/hyperfocal.html#nearfar

I used a 50mm lens at 120 inches. A 35mm sensor gave a 15.6" depth of field. At a 1.3 crop factor, the DoF is 12." At 1.6, it's 9.8."

This backs up what the Canon guy said.

[CyberDyneSystems]

Am I missing something? Does anyone regularly shoot at <f/2.0 with one of the smaller sensored cameras?


All the time,. I have f/1.4 in 50mm, f1.8 in 85mm, and 200mm and f/2 in 135mm and I use them very sucesfully for indoor events in low light all the time on both 10D and now 20D. I shoot wide open,. and I get MORE DOF than I do with my 1D MkII but it is barely perceptible.

[CyberDyneSystems]

Okay, I just googled a Depth of Field calculator. Enter the data yourself and you can see that a smaller sensor (1.6 crop factor) gives a smaller depth of field than a larger sensor.

http://www.outsight.com/hyperfocal.html#nearfar

I used a 50mm lens at 120 inches. A 35mm sensor gave a 15.6" depth of field. At a 1.3 crop factor, the DoF is 12." At 1.6, it's 9.8."

This backs up what the Canon guy said.

But,. your feild of view is changed,. so to get the same framing based on the 50mm on the full frame sensor or film,. you would need to back up.. thus your distance is further away than the 120 inches... thus the DOF will end up being significantly deeper.

[rent]

hey cyber! would you agree then if the pictures were taken at the same distance, with the same lens at same focal length at same aperture, DOF will be shallower on a 10/20D than on a 1DMK2 (granted the FOV will be different)?

on the other hand, i concede if the FOV are to be kept the same, the DOF could end up greater on a 10/20D. :lol:

-alex

But,. your feild of view is changed,. so to get the same framing based on the 50mm on the full frame sensor or film,. you would need to back up.. thus your distance is further away than the 120 inches... thus the DOF will end up being significantly deeper.

[CyberDyneSystems]

Alex,. this is what I am reading .. but I have a hard time believing it... simply because ,. as we have said 10,000,000 times before,. with the APS sized sensors,. all we are doing is cropping off the image a little. My own mind can not comprehend how by merely cropping an image,. the resulting crop left over would suddenly have it's DOF effected? It looks like time travel from where I'm standing.
http://members.bellatlantic.net/~vze2hdzu/pics/atom2.gif

If I print an 8X10 photo,. and take scissors to it the top,. bottom , left and right hand sides and leave about 3/5ths of the image left,.. would the DOF in the image have changed? I don't think so.

I just can't grasp it.. and yet all that is written seems to agree with Canon Boy.. I'm thinking that they are assuming at least ONE other change in the cameras mechanics to get that smaller DOF... when you go from 35mm to Large format,. does the distance from the lens to film change? Is that what causes the differnce in DOF?

[tim]

Like CDS said, the COC might change if you step backwards a bit to get the same image in your 1.6 crop sensor, otherwise I still don't think it does. It's not the simplest of subjects, nor one I claim to be an expert in.

[Tom W]

ONe point to remember is that in situations where you would use an 85 mm lens with a normal 35 mm camera, you would have to use a 50 mm lens on the 1.6 sensor camera if you wanted to stand in the same place and frame the same image.

If lighting conditions were such that you needed f/2 to get adequate shutter speed, you will find that the DOF is much shallower on the full-frame than on the 1.6.

For example, say your subject is 10 feet away. You have 2 cameras, a 1Ds Mk II and a 20D. You first meter and shoot your subject at 1/60 second at f/2 with the 1Ds and an 85 mm lens. You know from experience (and the dof calculator) that your depth-of-field for acceptable focus on an 8X10 image is about 6 inches (actually, 5.91 inches). This is entirely adequate to get the subject of interest in focus while still blurring the background adequately.

Next, you grab your 20D. The first thing you notice is that you need approximately a 50 mm lens to be able to frame the same image from the same spot. So you slap the trusty 50 mm lens on your 20D and prepare to shoot. Since the light hasn't changed and you still want 1/60 second shutter speed, you again choose f/2 as your aperture. But the depth-of-field is now a good deal deeper. How much? Almost twice as deep at 11 inches. Why? Because you are now using a shorter lens to achieve the same image from the same spot.

To sum it up, the need to use a shorter lens to obtain the same framing is what drives the deeper DOF that is obtained with smaller sensors. If you had a third camera, a little point and shoot, you might have a 10 mm lens that, at f/2, would have nearly infinate DOF at a 10 foot shooting distance.

So there you have it - in practical use, smaller sensors give deeper DOF not because of their size, but because of the shorter lens needed to obtain the same FOV.

[ScottE]

If I print an 8X10 photo,. and take scissors to it the top,. bottom , left and right hand sides and leave about 3/5ths of the image left,.. would the DOF in the image have changed? I don't think so.

Your analogy is incorrect. If you had a 12x18 photo and cropped an 8x10 photo out of it, then you could compare it to an 8x10 photo printed directly. If both photos were shot with the same camera and lens you would see a loss of resolution in the photo cropped from the 12x18. That's what happens when you make big enlargements. This is why you need finer detail on the small original to get the same detail on the same size enlargement. Finer detail is the same as a smaller circle of confusion. This is why you can't take depth of field tables derived for a 35 mm camera and apply them directly to an APS sized sensor.

Scott

[Wildewinds]

ONe point to remember is that in situations where you would use an 85 mm lens with a normal 35 mm camera, you would have to use a 50 mm lens on the 1.6 sensor camera if you wanted to stand in the same place and frame the same image.

If lighting conditions were such that you needed f/2 to get adequate shutter speed, you will find that the DOF is much shallower on the full-frame than on the 1.6.

For example, say your subject is 10 feet away. You have 2 cameras, a 1Ds Mk II and a 20D. You first meter and shoot your subject at 1/60 second at f/2 with the 1Ds and an 85 mm lens. You know from experience (and the dof calculator) that your depth-of-field for acceptable focus on an 8X10 image is about 6 inches (actually, 5.91 inches). This is entirely adequate to get the subject of interest in focus while still blurring the background adequately.

Next, you grab your 20D. The first thing you notice is that you need approximately a 50 mm lens to be able to frame the same image from the same spot. So you slap the trusty 50 mm lens on your 20D and prepare to shoot. Since the light hasn't changed and you still want 1/60 second shutter speed, you again choose f/2 as your aperture. But the depth-of-field is now a good deal deeper. How much? Almost twice as deep at 11 inches. Why? Because you are now using a shorter lens to achieve the same image from the same spot.

To sum it up, the need to use a shorter lens to obtain the same framing is what drives the deeper DOF that is obtained with smaller sensors. If you had a third camera, a little point and shoot, you might have a 10 mm lens that, at f/2, would have nearly infinate DOF at a 10 foot shooting distance.

So there you have it - in practical use, smaller sensors give deeper DOF not because of their size, but because of the shorter lens needed to obtain the same FOV.

There you have it. You're totally correct. Thanks for the insight.

So, in total contradiction to my initial thoughts, a large aperture is even MORE important on a 1.6 crop camera.

[CyberDyneSystems]

Scotte,. are you now talking pixel desnsity..

My anology was flippant,. ignore that and my point is still the same,.

BUT if the pixel density is the same with both sensors.. or if you really were using 35mm film,. and then a 1.6X peice of 35mm film for the same res.. then nothing else is changing but the size of the film... are you still saying the DOF would change?

I don't think you are?

.. we keep saying "if all else is equal" but if we do change pixel density,. then all else is not equall...

then are you saying that the cahnge in pixel density is where the "second" factor comes from?

[slin100]

CyberDyne, you and the Canon guy are both right because there are two different scenarios.

In your scenario you are preserving the FOV which requires you to either use a shorter focal length or to back up. In the Canon guy's scenario the distance and lens are fixed which means that the FOV is not preserved; the DOF will be narrower when using a smaller sensor. This is the Canon guy's and Rentboi's point and they are right, too. Please don't confuse these two very different cases. There's no contradiction, and no laws of physics are broken. :)

[CyberDyneSystems]

My own mind can not comprehend how by merely cropping an image,.


.... I'm thinking that they are assuming at least ONE other change in the cameras mechanics to get that smaller DOF... when you go from 35mm to Large format,. does the distance from the lens to film change? Is that what causes the differnce in DOF?

Is the pixel density the ONE OTHER change in the cameras mechanics?

This I can easily live with. This DOES result in a second fundamental change in the camera.

[CyberDyneSystems]

Steven,. I still need something other than a change in sensor dimension to change the DOF...
Is it the pixel density?

[tim]

To sum it up, the need to use a shorter lens to obtain the same framing is what drives the deeper DOF that is obtained with smaller sensors. If you had a third camera, a little point and shoot, you might have a 10 mm lens that, at f/2, would have nearly infinate DOF at a 10 foot shooting distance.

So there you have it - in practical use, smaller sensors give deeper DOF not because of their size, but because of the shorter lens needed to obtain the same FOV.

I think we're all pretty much agreed on that point, but using different ways to describe it.

[CyberDyneSystems]

... oh man.. it's 1:25am and I can't beleive I;m invloved in one of these physics lessons! http://members.bellatlantic.net/~vze2hdzu/pics/roflmao.gif

'night all,. I'll be anxious to see how you all made out in the morning :)

[slin100]

If I print an 8X10 photo,. and take scissors to it the top,. bottom , left and right hand sides and leave about 3/5ths of the image left,.. would the DOF in the image have changed? I don't think so.

In this case no, but if you take that 3/5th size image and blew it back up to 8x10, then the DOF is going to be narrower when compared to the original 8x10. Yes, the images will have different FOVs, but you gotta keep the final print size constant, otherwise you're introducing too many variables.

[ScottE]

I used a 50mm lens at 120 inches. A 35mm sensor gave a 15.6" depth of field. At a 1.3 crop factor, the DoF is 12." At 1.6, it's 9.8."

You are comparing apples and oranges. A 50 mm lens on a 1.6x crop camera does not give the same picture as it does on a 35 mm camera. If you stayed the same distance from the subject and used a 35 mm camera you would have to use an 80 mm lens to get the same picture that you got with the 1.6x crop camera.

For example, on a 35 mm camera with an 80 mm lens at f/4 at 10 ft distance you will have a depth of field of 1.12 feet.

On a 20D with a 50 mm lens at f/4 at 10 ft distance you will have depth of field of 1.84 feet.

The pictures will look identical, except the 20D picture will have slightly more depth of field.

If you put the 80 mm lens on the D20 and backed off to 16 feet distance and shot at f/4 the depth of field would be 0.71 feet. The size of the subject would be about the same, but the perspective would be different so they are not really comparable photos. The shallower depth of field is why a telephoto lens can be used to smoothly blur the background.

These calculations were done using the depth of field calculator at:

http://www.dofmaster.com/dofjs.html

Scott

[Tom W]

I think we're all pretty much agreed on that point, but using different ways to describe it.

I'm not sure that everybody was, but I thought it might be good to clarify it for the benefit of someone or other. Anyway, if not for the different lens length (and more importantly, the smaller absolute physical diameter of the aperture that is needed for a given f/stop), the smaller sensor would have a tighter DOF by virtue of it needing a greater degree of magnification to achieve the proverbial 8X10 inch print upon which the standard (sort-of) circle-of-confusion is based. The little lens more than offsets that factor.

[Tom W]

... oh man.. it's 1:25am and I can't beleive I;m invloved in one of these physics lessons! http://members.bellatlantic.net/~vze2hdzu/pics/roflmao.gif

'night all,. I'll be anxious to see how you all made out in the morning :)


You'll never be able to sleep knowing that we're all talking about it without you! :)

[Rob612]

Alex,. this is what I am reading .. but I have a hard time believing it... simply because ,. as we have said 10,000,000 times before,. with the APS sized sensors,. all we are doing is cropping off the image a little. My own mind can not comprehend how by merely cropping an image,. the resulting crop left over would suddenly have it's DOF effected? It looks like time travel from where I'm standing.

Agree on that. The COC size is always the same, I believe - it may probably vary a little with EF-S lenses because those are designed for the smaller sensor and have a shorter backplane.

Since the COC is always the same, no matter what sensor you have, I do not really understand where is the point.

Don't know... maybe its just me, but there is something that is not really clear here :(

[slin100]

Steven,. I still need something other than a change in sensor dimension to change the DOF...
Is it the pixel density?
No, it's the Circle of Confusion. So long as the pixel density is much higher than the Circle of Confusion, you can ignore pixel density for all practical purposes. Besides, even if you use film, it won't change the result.

Think of it this way. A smaller sensor requires more magnification to maintain the same final print size. That means you are viewing the captured photons more critically. Any out-of-focus blur will be more evident. This translates into shallower depth of field. Incidentally, we quantify this higher standard of judgement by using a smaller Circle of Confusion value when using the DOF equations.

Now I just said a smaller sensor results in a smaller DOF, which seems to fly in the face of common knowledge that smaller sensors have greater DOF. What I said is true if you keep the focal length and distance fixed. Now that doesn't make for an interesting comparison because the FOV changes. The point the Canon guy was making, however, was for this case.

We usually want to keep the FOV fixed. As you assert, this requires using a shorter focal length or moving back. Moving back changes perspective which renders the comparison meaningless because you really have two different pictures, so the only choice is shorten the focal length. Changing the focal length has a greater effect on the DOF than changing the sensor size. Focal length varies linearly with the change in sensor size but it varies parabolically (i.e. the square) with focal length. Focal length wins out and the net effect is that a smaller sensor has a greater DOF when the FOV is kept constant.

I hope this explains more clearly why you and the Canon guy can both be right. There are two scenarios at play here.

[tim]

I think we've gone off on a parallel tangent to the original question. I use a F1.8 lens quite succesfully on a 20D. Maybe it's different on a 35mm camera, but to be honest I don't really care, because I don't have one of those! We could discuss (argue?) this until the cows come home, or we could just forget about it and go take some pictures :)

[slin100]

Agree on that. The COC size is always the same, I believe - it may probably vary a little with EF-S lenses because those are designed for the smaller sensor and have a shorter backplane.

Since the COC is always the same, no matter what sensor you have, I do not really understand where is the point.

Don't know... maybe its just me, but there is something that is not really clear here :(
What's clear is that there's confusion about the Circle of Confusion. The CoC cannot be the same irrespective of the sensor and it is not affected by shorter lens registrations, such as with EF-S lenses.

Any point of an object that is not precisely in the plane of focus is always imaged on the focal plane as a circle. This is the Circle of Confusion. If this circle is sufficiently small it becomes indistinguishable from a point when magnified to a standard print size and viewed from a standard distance.

How do we determine what is sufficiently small? Well, that's subjective and varies from person to person. Nonetheless, 30 microns is a common CoC for 35mm film, assuming magnfication to 8x10 with a viewing distance of approx 10 inches.

Now take a smaller sensor. If you want the CoC to end up the same size when magnfied to the same standard print size as before, you have to start with a smaller CoC. In fact, compared to film, a 1.6x factor sensor should use a CoC that is 1.6x smaller. If, subjectively, you found that 30 microns worked for you on film, then 30/1.6 or about 19 microns should work for you on a 1.6x factor camera. 19 microns is what http://www.dofmaster.com uses.

[slin100]

I think we've gone off on a parallel tangent to the original question. I use a F1.8 lens quite succesfully on a 20D. Maybe it's different on a 35mm camera, but to be honest I don't really care, because I don't have one of those! We could discuss (argue?) this until the cows come home, or we could just forget about it and go take some pictures :)
There's nothing wrong with figuring things out by experimentation but there are people out there who've crossed over from film and want/need to know what adjustments to make, and there are also people who want to know the reason why something is so.

Going back to the OP, yes, people do use lenses with apertures < f/2.0. BTW, the DOF of a 100mm lens on a 1.6x camera at 2 meters at f/2 is 0.02m, not 0.2m! :eek:

[Huckaback Photo]

Wilderness

Just thought I would go back to your original 2 questions and let the other debate
move on in which ever direction.

So your topic heading to start with.
"Large aperture lenses useless on 1.6 crop cameras" One of the main points of going and getting a slr body is to have the abillity to interchange lenses.

lenses come in all different mm's and F stops. and of course vary dif PRICE RANGE.
Now if the only thing you ever need to shoot is say a landscape at focal length 35mm
and F stop = f8 we just need a 35mm f8 fixed lens, job done.
if life were so simple.
Pictorially we like to be creative, so we need more choice, we need to control how much of an image is sharp. and therefore (sometimes) strengthening the content which we feel most important.
this is often the reason for spending lots of money on fast lenses and not just for low light photography, or indeed action shots .

Your first question.
Does it really look good to have only narrow piece of photo in Focus ?

Answer = Yes , use the lens as a creative tool, get to know what it is capable of producing at all distances and indeed F stops.

second question.
am I missing something ?

Answer = Yes.
see image "Peace", a good use of wide apperture in my view.
I used 50 mm f1.8 used at f2.5 at 1/6400 sec. iso 320. 1D mk2.

we can now present an image to the viewer which is vary different from that of normal eyesight viewing (ie. we cant selectively put a background out of focus)
although there are times i wish i could.

Regards
Martin (Huckaback Photo)

[Huckaback Photo]

Now then read the story I was trying to put across in my image above, all the info is there and yet it is not all sharp.
in using wide fstop the audience is directed to the word PEACE and the actual Headstone.
the setting is one with that feel of peace about it.
the sort of calm created by the blur in the background.
the peaceful muted colours of the yellow daffodils & greens of the grass.
even the distant landscape blur gives some info as to the enviroment etc.

Do you see enough in this to convey some feeling of peace within my image ?
Does this work ? I think so but all people are different (fortunately)
this image has not been created in Photoshop but toally in camera with the use of wide apperture.
I hope you get the picture.
just get out there and try this technique you may like it..

regards
Martin

[griff2]

Ok folks, before this thread turns into another x-factor, the maths say that depth of fiels IS a function of circle of confusion. Here's an Excel spreadsheet with all the relevant numbers and a graph: as circle of confusion is reduced - smaller sensor - depth of field is also reduced.

What comes out of this is that large aperture, long lenses will have about 50% the depth of field using a smaller sensor at a close distance, e.g. a 200mm lens at f/2.8 5m from the subject will have a DOF ~120mm; on a reduced sensor this will be reduced to ~ 60mm.

[ron chappel]

Oh no i can see this thread has allready turned into a relligeous/maths war.

Here are some simple truths (easily proven by snapping some pics)

a) As frame size is reduced,depth of field increases.
b)Small formats tend to be enlarged more which decreases DOF in comparison to larger formats (the increased circle of confusion mentioned by someone earlier)

....but these two factors aren't equal -the format size has FAR greater influence!!

That's why large format cameras have very little DOF (and use amazingly small apertures such as f64+ to try to increase DOF)
And it's also why tiny format digicams have huge DOF (in spite of having very large apertures such as f2 or even less)

There really shouldn't be any argumant on this subject

So the canon guy was wrong of course (or was missunderstood perhaps?)

[griff2]

ron chappel wrote: That's why large format cameras have very little DOF (and use amazingly small apertures such as f64+ to try to increase DOF)
And it's also why tiny format digicams have huge DOF (in spite of having very large apertures such as f2 or even less)

Maybe, but were dealing with one variable only here - sensor size. So, unless somebody's changed the laws of physics for this universe, I'll stick with the mathematical proof.:D

[Willie]

Wow, I leave for a couple of hours to play hockey and I come back to 3 pages of replies.

Man, I love this forum!!! I learn more from this forum than from all the other web pages combined.

griff2, as the circle of confusion gets smaller, the depth of field increases, it does not get reduced. Your spreadsheet is incorrect, since you are changing the f-stop. The COC does not change as the aperature becomes smaller. Look at all the DOF calculators and you will see that.

Guys, I wonder if anyone had this discussion before digital? "Hey, does my DOF go up or down when I switch from 35mm to medium format?" If not, then maybe they should have, or maybe we should try to understand it in those terms. Take a 100mm lens on 35mm, and then a 100mm on medium format at same aperature, and which do you think will have greater DOF? Why don't we compare these 2 formats like we do now with 20d and 1DsMII? A 35mm film camera has a "crop factor" compared to medium format. Let's assume for arguments sake it's 2x. My old Rebel has a 2x crop factor. Doesn't really make sense, does it? Well, it might if medium format were the standard for most, and Canaon somehow made lenses that fit both formats. Hmm, what would that make my Rebel XT what, 3.2X? You see, we are introducing too much confusion in here by mentioning the crop factor. We only do it because most are used to the 35 mm format, and Canon makes lenses that will fit full frame, 1.3X, and 1.6X. I never used crop factor on my G5. Why not, I guess I should have. I just used to remember that at wide end I had 7.2 mm. Fisheye? No, just the equivalent of 35mm in film format. Oh, now that made sense.

However, DOF is nothing more than some mathematical equations. The only real unknown is COC. Without COC, there is way to calculate DOF. We must make some assumption on it, and I think that the accepted values are probably pretty accurate. COC always goes down as the sensor size gets smaller. I think this has some way of taking the "crop factor" into account. Since you have to blow up a picture more with a smaller sensor, then the COC must be smaller. COC for P&S, 20D, 1DMII, 1DsMkII, medium format, view camera all have different COC, in this case all going p respectively.

When calculating DOF, 50mm lens is 50mm is 50mm is 50 mm. It doesn't matter if it's on Rebel XT or 1DsMII. I bet in some photography class, there might be a question to find the DOF of some particular instance (mm, subject distance, aperature). You wouldn't convert the lens by multiplying it by any crop factor. 100 mm on 35 mm film would be calculated the same way as 100 mm on medium format. Only the COC would change in the equations. Therefore, my 50 mm 1.8 lens on a 1DsMkII would be 50 mm, and it would be 50 mm on my Rebel XT. I know that the pictures would look different, but it's still 50 mm from the sensor. I plug in the numbers into any of the various DOF calculators using some accepted COC, and voila, greater DOF with my Rebel XT.

Let's take this one step further. If REbel XT is 1.6X compared to full frame, and if we use the assumption that the DOF will be REDUCED, then my G5 has like what, 5X crop factor? Wow, imagine how shallow the DOF must be with that camera! Good thing I don't use it much anymore, it just blurred the background too much. :)

I researched all this stuff to death, since I couldn't get good blurred backgrounds with my G5. I finally realized that my only choice was to go DSLR, because with the increased sensor size came decreased DOF.

Regards,
Willie

[O/confusion]

What's clear is that there's confusion about the Circle of Confusion.

I'm confused.:D

regards
0/Confusion

[slin100]

griff2, as the circle of confusion gets smaller, the depth of field increases, it does not get reduced. ... Look at all the DOF calculators and you will see that.

Willie, the DOF decreases when the CoC gets smaller, when everything else (i.e. focal length, distance and aperture) is kept fixed. For example, the DOF of a 50mm at f/2.8 at 10 ft on 35mm film is 2.06 ft using a CoC of 30 microns. The DOF of a 50mm at f/2.8 at 10ft on a 1.6x factor camera is 1.29 ft using a CoC of 19 microns.

The reason why DOF ends up being bigger despite the CoC being smaller is because we usually also change to a shorter focal length in order to keep the FOV constant. The DOF equation clearly shows that a change in focal length has a bigger effect than a change in sensor size. But if you keep the focal length fixed, as when you compare a 50mm on 35mm film versus a 50mm on a 1.6x factor camera, then the only variable is CoC. In this case, to reiterate, DOF decreases as CoC gets smaller.

[Willie]

Ron, I agree with you 100%.

I bet this was never a topic of discussion before digital. :)

Just grab one of the online DOF calculators, plug in the numbers and look at the output.

Of course, make sure to adjust the COC for the various cameras.

Here is a link with COC for digital cameras:

http://www.dofmaster.com/digital_coc.html

Also, here is another link describing COC and how it changes with different sensor size:

http://tangentsoft.net/fcalc/help/CoC.htm


Now, if someone could just point me to a formula for good composition, I'd be set. :)

Regards,
WIllie

[griff2]

Willie, I'm not changing the f stop, that's in the spreadsheet from a previous model. Here's a simplified version.

[PhotosGuy]

I only read half of this stuff, but as I see it, it's like taking a shot on 35mm film & cropping it 1/1.6X from the middle of the pic. Nothing else changes.

[Willie]

slin100 and griff2,
I stand corrected, all else being the same the DOF will go down on my RebelXT vs. full frame. As you mentioned, I did do the comparisons with the different focal lengths. When I had my G5, I thought that the smaller COC would be offset by the shorter focal length to keep the same FOV, but that was not the case. The DOF increased dramatically. So, yes, I guess the Canon guy was right.

I promise to never get in a technical discussion again, at least not right after coming home from a late night of hockey (I can barely stay awake at work right now). :)

Regards,
Willie

[Citizensmith]

Something I'd like clarifying. This whole smaller sensor gives a shallower DOF from the same lens.

It seems from reading the previous post that it needs elaboration. For a given lens the DOF on the sensor will be the same. However, if a small sensor image is enlarged to the same size as a 35mm image the perceptual DOF will be decreased. If you were to look at unenlarged 'negatives' (wonder what the digital version is) the DOF would be identical.

Basically, I fail to see how sensor size can have any effect on the light coming through my lens. The perception of DOF must therefore be a factor of enlargement and the effect on DOF of sensor size is nothing to do with the glass in front of the lens, despite that being how we all end up measuring this.

Does that statement make sense?

[slin100]

I only read half of this stuff, but as I see it, it's like taking a shot on 35mm film & cropping it 1/1.6X from the middle of the pic. Nothing else changes.
Then you should go back and read the other half. Cutting out the middle of the print is not fair for the smaller format. The standard of judgement should be two prints of the same size.

slin100 and griff2,
I stand corrected, all else being the same the DOF will go down on my RebelXT vs. full frame.
To clarify, this is true only when the focal length, aperture and distance aren't changed. I'm happy you saw the error.

[Tom W]

Willie, the DOF decreases when the CoC gets smaller, when everything else (i.e. focal length, distance and aperture) is kept fixed. For example, the DOF of a 50mm at f/2.8 at 10 ft on 35mm film is 2.06 ft using a CoC of 30 microns. The DOF of a 50mm at f/2.8 at 10ft on a 1.6x factor camera is 1.29 ft using a CoC of 19 microns.

That is correct. Remember how the COC is derived - enlargement of the sensor's image to a "standard" print viewed at a "standard" distance is what determines COC. Little sensors require more enlargement to obtain the same print size.

The reason why DOF ends up being bigger despite the CoC being smaller is because we usually also change to a shorter focal length in order to keep the FOV constant. The DOF equation clearly shows that a change in focal length has a bigger effect than a change in sensor size. But if you keep the focal length fixed, as when you compare a 50mm on 35mm film versus a 50mm on a 1.6x factor camera, then the only variable is CoC. In this case, to reiterate, DOF decreases as CoC gets smaller.

EXACTLY!
The difference in focal length (and hence measured aperture in mm) is the larger influence on depth-of-field. And in the real world, we can't take the same shot with the same lens from the same place with different sized sensors or different sized film.

[slin100]

Something I'd like clarifying. This whole smaller sensor gives a shallower DOF from the same lens.

It seems from reading the previous post that it needs elaboration. For a given lens the DOF on the sensor will be the same. However, if a small sensor image is enlarged to the same size as a 35mm image the perceptual DOF will be decreased. If you were to look at unenlarged 'negatives' (wonder what the digital version is) the DOF would be identical.

Basically, I fail to see how sensor size can have any effect on the light coming through my lens. The perception of DOF must therefore be a factor of enlargement and the effect on DOF of sensor size is nothing to do with the glass in front of the lens, despite that being how we all end up measuring this.

Does that statement make sense?
It doesn't really make sense to speak of DOF on a sensor. DOF is, as you said, a perceptual phenomenon. The captured photons need to be imaged in order to evaluate DOF. Unenlarged 'negatives' are simply 1x enlargement.

It's true that a smaller sensor doesn't affect the light coming through the lens. It's how much the captured image is enlarged that affects the perception of DOF.

[Redbird_xo]

The original question by the thread starter was: Large aperture lenses useless on 1.6 crop cameras?

I have a 1.6x sensor camera and a few larger aperture lenses...35 f/2, 50 f/1.8, 85 f/1.8...

I've been enjoying shooting with these lenses so much because of their ability to shoot in ambient lighting condition when needed and getting nice background blur.

[Tom W]

Something I'd like clarifying. This whole smaller sensor gives a shallower DOF from the same lens.

It seems from reading the previous post that it needs elaboration. For a given lens the DOF on the sensor will be the same. However, if a small sensor image is enlarged to the same size as a 35mm image the perceptual DOF will be decreased. If you were to look at unenlarged 'negatives' (wonder what the digital version is) the DOF would be identical.

Yes, you are right.
That is the basis for the different circle-of-confusion figure that is used among the various format sizes.

Basically, I fail to see how sensor size can have any effect on the light coming through my lens. The perception of DOF must therefore be a factor of enlargement and the effect on DOF of sensor size is nothing to do with the glass in front of the lens, despite that being how we all end up measuring this.

Does that statement make sense?

You are correct that the sensor size in and of itself cannot affect depth of field - it is the enlargement process that does that.

But don't forget the effect of the lens aperture. That is one of the other variables that is changed, in practice, when changing formats. F/2 is f/2 in terms of brightness when light hits the sensor, but f/2 on a 50 mm lens (approximately 25 mm diameter) is of a different diameter than f/2 on an 85 mm lens (approximately 42.5 mm). And that diameter is a strong determinant on depth-of-field.

[Tom W]

Somewhere, there's a long, magical formula that lays out this whole thing. :)

[rdenney]

I don't believe the COC changes when you cut the edges off your sensor.

It does if you enlarge to the same print size.

Remember that depth of field is based on an acceptable circle of confusion, and therefore it is a standard, not an effect. You set a standard for the maximum acceptable circle of confusion based on the quality you want and the degree of enlargement. The increased enlargement increases the effective size of the circle of confusion.

If you take the same picture from the same location, you'll use a lens with a shorter focal length if you have a smaller sensor. That lens will have more depth of field, because the aperture is smaller at a given setting. It's really as simple as that. The extra enlargement offsets this somewhat, but not completely.

The f-number is merely the focal length divided by the aperture, as actually measured. Thus, a 50mm/1.8 lens has an aperture 50/1.8=28mm. Any lens in any format with a 28mm aperture will have the same depth of field at the same focus distance, until you consider the effect of enlargement. What lenses have that same aperture size? An 85/2.8, a 135/4.5, and a 200/7, to name three. That means that the 180mm/2.8 lens shot wide open on a medium-format camera will have much, much less depth of field than a 50/1.8 used from the same distance.

My beef with the smaller sensors is that the shorter lenses one uses with them increases depth of field too much for certain kinds of photography. That's why faster lenses are more important for when you WANT a paper-thin apparent focus. To get the same depth of field and framing at the image plane as the 180/2.8 on medium format, I need a 60mm f/1.0 lens on an APS sensor. I'll get a little of the way there because of the extra enlargement, but not all the way by any means.

So, the Canon rep is nuts. Smaller sensors require shorter lenses to achieve the same image compositions from the same camera locations, and those shorter lenses have smaller apertures resulting in greater depth of field. Thus, when one wants narrower depth of field, one requires a faster lens than used with larger formats.

Rick "who thinks hair-thin depth of field is a classic look from the days when portraits were shot on 8x10 with apertures more like 40mm (325mm/f8)" Denney

[slin100]

I just remembered there's a practical consequence to the point made by the Canon guy in the original post. If you have a lens with a distance scale and depth of field markings, you can figure out what your depth of field will be on a 1.6x factor camera by using the depth of field mark for the aperture that is approximately 1 1/3 stops wider than the actual aperture. For example, if the aperture is set to f/4, use the DOF mark for f/2.5.

[rdenney]

Here are some simple truths (easily proven by snapping some pics)

a) As frame size is reduced,depth of field increases.
b)Small formats tend to be enlarged more which decreases DOF in comparison to larger formats (the increased circle of confusion mentioned by someone earlier)

....but these two factors aren't equal -the format size has FAR greater influence!!


Ron gets the gold star (or, he gives me the gold star, heh, heh).

I would add to his first truth to fill in the links of the chain, thusly: As frame size is reduced, lenses become shorter to provide the same view from the same camera position, and shorter lenses have smaller apertures at the same f/stop setting, and smaller apertures have the result that depth of field increases.

And I would augment his second truth thusly: ...the increase acceptable circle of confusion...

His third statement is perfect as is.

Rick "who still shoots important portraits in medium format to get the desired hair-thin depth of field" Denney

[rent]

okay boys and girls... here're the magic formulas:

f - lens focal length in mm
a - lens aperture in f number
c - CoC in mm
d - focusing distance

h - hyperfocal distance in mm
dn - near distance of acceptable sharpeness
df - far distance of acceptable sharpness

h ~= f^2 / (a*c)
dn ~= d*h/(h+d)
df ~= d*h/(h-d)

note these are simplified equations for when the lens focal length is much less than the focal distance (which is true for non-macro photography).

so clearly the DOF (df-dn) is a function of f, a, c and d. f, a and d being equal, the smaller c (CoC), the smaller DOF.

-alex

Somewhere, there's a long, magical formula that lays out this whole thing. :)

[cyclone]

Yes, yes, yes. But instead of talking theoretical equations, can someone with a full frame and a 1.6X put both cameras on a tripod, take a picture with each at the same location, crop the full frame picture to that of the 1.6X picture, make both pictures the same size, and post so we can see this effect in the real world? Or can someone with better search skills than me, find such an example already done. I will if have to, but my full frame is a film camera, and it will take time to develop and scan.

Cyclone

[Tom W]

okay boys and girls... here're the magic formulas:

f - lens focal length in mm
a - lens aperture in f number
c - CoC in mm
d - focusing distance

h - hyperfocal distance in mm
dn - near distance of acceptable sharpeness
df - far distance of acceptable sharpness

h ~= f^2 / (a*c)
dn ~= d*h/(h+d)
df ~= d*h/(h-d)

note these are simplified equations for when the lens focal length is much less than the focal distance (which is true for non-macro photography).

so clearly the DOF (df-dn) is a function of f, a, c and d. f, a and d being equal, the smaller c (CoC), the smaller DOF.

-alex

Well, that solves it. :)
Now we need the formula for determining COC. ;)

[Tom W]

Yes, yes, yes. But instead of talking theoretical equations, can someone with a full frame and a 1.6X put both cameras on a tripod, take a picture with each at the same location, crop the full frame picture to that of the 1.6X picture, make both pictures the same size, and post so we can see this effect in the real world? Or can someone with better search skills than me, find such an example already done. I will if have to, but my full frame is a film camera, and it will take time to develop and scan.

Cyclone

If you take a crop of a full-frame image that is equal in size to that of the 1.6X sensor, and then enlarge them the same amount, there will be no difference in depth-of-field. (that is assuming identical lens, location, and aperture).

The whole concept of the Circle-of-confusion is that it is based on the amount of enlargement that is required of an original "negative" or sensor output to achieve a given standard sized print. Cropping a full-frame output to match the size of a 1.6X sensor gives you the same starting size, which means identical enlargement which means identical circle-of-confusion.

[Wildewinds]

First of all, I totally understand where the logic of my first post is flawed now. That occurred by the second page. The rest of this discussion is starting to go over my head.

Secondly, as to the motives of the Canon rep, he was REALLY pushing the advantage of the big sensored cameras over the smaller 20D's and Rebel XT's (and D70's). This argument he used regarding the shallower DoF for the 1.6 crops was technically correct, but he failed to mention the fact that once you changed the focal length in order to achieve the same frame, you would, in fact, have a broader DoF (as someone explained here).

He did convince me I needed a $6,000 camera even though I just started with SLR's, so I guess he's good at his job.

[Tom W]

First of all, I totally understand where the logic of my first post is flawed now. That occurred by the second page. The rest of this discussion is starting to go over my head.

Secondly, as to the motives of the Canon rep, he was REALLY pushing the advantage of the big sensored cameras over the smaller 20D's and Rebel XT's (and D70's). This argument he used regarding the shallower DoF for the 1.6 crops was technically correct, but he failed to mention the fact that once you changed the focal length in order to achieve the same frame, you would, in fact, have a broader DoF (as someone explained here).

He did convince me I needed a $6,000 camera even though I just started with SLR's, so I guess he's good at his job.

Any time the crop factor and DOF (a combination heretoforth known as IT) are mentioned together, the thread will automatically grow well beyond the scope of the original post. That's because the real definition of "Circle of Confusion" is a group of 2 or more photographers discussing the combination of depth-of-field, sensor size, and aperture.

It is a somewhat complex subject, with many variables. If all those variables aren't defined, the discussion can get very long.

Good luck with your choice whatever it is. And do try to get some hands-on use of the various sizes if you can. Good bokeh is very possible on a 1.6X sensor.

[PhotosGuy]

That's because the real definition of "Circle of Confusion" is a group of 2 or more photographers discussing the combination of depth-of-field, sensor size, and aperture. :D:D:D And, with that, I'm out'a here! ;)

[accord]

Here is the simple explanation I can summarize from what I observe here.

Let us use the eye of a photographer who produces two equal size pictures from two camera systems. We use a reference of 50mm lens on a 1.0 crop factor at a focus distance of 3m. There can be two ways he can produce the same size image from a 1.6x camera system.
1. He uses the same 50mm lens. He step back 1.6x the distance, i.e. 4.8m. He takes the picture.
2. He stays at the same position. He swaps to a 31.25mm lens. He takes the picture.

For method 1, we have the following conclusion for the 1.6x system relative to the 1.0x system.
a. The effective distance from the object is multiplied by the crop factor (1.6x).
b. The focus distance is equivalence to a lens of focal length multiplied by the crop factor 1.6.
c. The DoF is multiplied by the crop factor as he is using a 50mm lens at 1.6x the distance from the object.
d. He needs more than one stop (1.6/1.4=1.14) wider aperture to produce the same DoF.

For method 2, we have the following conclusion for the 1.6x system relative to the 1.0x system.
a. The DoF is multiplied by the crop factor as he is using a 31.25mm lens at the same distance from the object.
b. He needs more than one stop (1.6/1.4=1.14) wider aperture to produce the same DoF.

[ron chappel]

....as to the motives of the Canon rep, he was REALLY pushing the advantage of the big sensored cameras over the smaller 20D's and Rebel XT's (and D70's). This argument he used regarding the shallower DoF for the 1.6 crops was technically correct, but he failed to mention the fact that once you changed the focal length in order to achieve the same frame, you would, in fact, have a broader DoF (as someone explained here).

He did convince me I needed a $6,000 camera even though I just started with SLR's, so I guess he's good at his job.


Aaaahhh,so explains it.
Mr canon wasn't telling the whole story.Basically he was really dumb or just plain lying

[raylks]

Large aperture lens is still very useful and has always an advantage over the slow lens. Consider you shoot sports event where fast shutter speed is necessary, and in low light conditions where you want to freeze the motion of a subject, a fast lens is always preferred to slow lens.

DOF won't change using 1.6x crop factor camera. It just crops out the edges of the image instead of changing the magnification, perspective, zone of sharpness of an image. It is kidding to say that fast lenses do not have value in digital photography. Given a shooting distance, if you want to blur the background to highlight a subject, the only way to do it to use fast lens.

[slin100]

DOF won't change using 1.6x crop factor camera.

Sigh. We just went through this in this very thread. You are incorrect. Please read the thread.

[CyberDyneSystems]

In this case no, but if you take that 3/5th size image and blew it back up to 8x10, then the DOF is going to be narrower when compared to the original 8x10. Yes, the images will have different FOVs, but you gotta keep the final print size constant, otherwise you're introducing too many variables.


Here is where we differ in our thinking,.

The Sensor did not alter the DOF,. your enlarging the print to make it comparable to the out put of the 35mm film print is what altered it. It has nothing to do with sensor size and everything to do with printing.

If we are going to try and keep every thing else the same and only alter the one variable,. that being sensor size,.. (remember,.. All Else Being Equal") ...then we also need to keep pixel density and print dpi identical.

So.. once again,. changing sensor SIZE does NOT change DOF.


Your own argument illustrates that print dpi does....

This is what I was looking for when I asked repeatedly what the second variable was,. what was the second thing that cahnged,. to alter the DOF?

We were both operating on two different sets of assumptions regarding the final image,. I was looking at the file itself,. you were looking at the print on a standard output.

In fact this very subject has been discussed (out put print res/DPI Vs. DOF) in the 10,000,000 posts on the X-factor section.

My methodology would demand that to keep all else remain equal,..

...that the output prints would have to reflect the same full frame Vs. 1.6X input on the output stage... (thus my analogy ragrding taking scissors to an image wasn't so far off),.

But the actual effect would be illustrated far better by printing using identical output DPI...

The resultig two prints side by side would be different in output dimensions ... but the 1.6X print would reflect EXACTLY a 1.6 crop from the full frame print (assuming identical pixel density, which we do,. becuase we are demanding "all else be equal") No change would be present in DOF.

Sensor size does NOT change depth of feild.

[slin100]

Cyber,

I humbly disagree. DPI, for all practical (see P.S. for explanation) purposes, has nothing to do with DOF. Let's forget about digital for a moment and just go back to plain old-fashioned film and chemical prints. There's no DPI whatsoever involved now. All my assertions still hold, namely that two different film formats when enlarged to the same size will exhibit different DOF when using the same lens. How can you dispute this?

As to your analogy about taking scissors to an image, the standard for judgement in photography has always been from a fixed print size. Who's going to compare a 4x6 print from a 10D with a 6.4x9.6 print (1.6x larger) from a full frame camera? That's a grossly unfair test.

P.S. I said DPI has no bearing on DOF for all practical purposes because obviously if DPI is too low, then overall sharpness will become unacceptable. And what is DOF but simply a perception of sharpness? As long as DPI is high enough, it can be safely ignored.

P.P.S. In case you haven't read the entire thread, please do. I don't stand alone in my position.

[slin100]

Cyber,

I went back and thought about the statement, "sensor size does not change DOF". I see where you're coming from but the statement doesn't mean anything to me. DOF is perceptual and can only be evaluated from a print/screen not from the sensor. Sure, the captured photons are the same, but it's what you do with them afterwards that counts.

[pcasciola]

Sensor size does NOT change depth of feild.I agree with your logic, but that would mean are all the online DOF calculator's are wrong. I always thought DOF was completely unaffected by sensor size as well.

I was starting to buy into the theory that higher pixel density could be interpreted as shallower DOF because you are recording more detail, but checking some of the online calculators (life DOFMaster's), I see the DOF does not change with the pixel density of sensor, only with the sensor size. So, I remain completely confused.

Can somebody just do this test already? A couple of f/2.8 pics with a 50mm lens from about 10 feet of a ruler at a shallow angle to the lens should prove this easily, since the difference in DOF at that distance between a 1D Mk II and 10D/20D or 300D shows as about 3-4".

Do I have to go buy a 1d Mk II just to do this test myself? :lol:

[slin100]

Can somebody just do this test already? A couple of f/2.8 pics with a 50mm lens from about 10 feet of a ruler at a shallow angle to the lens should prove this easily, since the difference in DOF at that distance between a 1D Mk II and 10D/20D or 300D shows as about 3-4".

Do I have to go buy a 1d Mk II just to do this test myself? :lol:
You can do this test with one camera. Take a shot, crop out a portion in the center and blow it up to the original size.

[Citizensmith]

Do I have to go buy a 1d Mk II just to do this test myself? :lol:

I'm afraid you do. Also, one instance of the test would hardly be scientifically valid so if you could send me one to I'd be happy to confirm the results.

In the interest of science obviously. No other motives at all.

[CyberDyneSystems]

You can do this test with one camera. Take a shot, crop out a portion in the center and blow it up to the original size.

What's "the original size?"

Again,.. you have to blow it up to change the DOF.. it is not the sensor size making the change to DOF,. only how you print.

By this same logic,. printing at different out sizes changes DOF..
I'm not concerned with "perceptual DOF" as seen in the the output print,. I am concerned with what will be in focus. This is not altered by changing the sensor alone.
The phenomenon you are describing has every thing to do with print resolution and nothing to do with sensor size. You get the identical change in "perceived DOF" by printing at 13X19" as opposed to 8X10"

The sensor did not change then.. nor did it have to.

Near as I can tell, all my assertions are in keeping with Ron Chapels comments, and Rick Denny's... so at this point I take my leave of this discussion feeling my point has been made.

[CyberDyneSystems]

...but that would mean are all the online DOF calculator's are wrong. I always thought DOF was completely unaffected by sensor size as well.

....but checking some of the online calculators (life DOFMaster's), I see the DOF does not change with the pixel density of sensor, only with the sensor size. So, I remain completely confused.

I would theorize that all the DOF calculators are assuming the out put prints are scaled too fit s specific constant format just as Slin argues as an axiom for his hypothesis.

[Tom W]

This is IT!!

:D :D :D

[rdenney]

I would theorize that all the DOF calculators are assuming the out put prints are scaled too fit s specific constant format just as Slin argues as an axiom for his hypothesis.

The only way to understand this is to be explicit about the arithmetic.

Circle of confusion is derived from degree of enlargement only. If we set a standard of, say, 5 line pairs/mm in the finished print and desire a 10X enlargement, we need 50 lines/mm image resolution at the image plane and each line pair includes two features. Thus, if we want the circle of confusion to be the same size as the smallest feature we can see, it will be 1/(5*10*2) = .01mm. To get a print the same size from an APS image as from a 35mm image, we need more enlargement. Thus, 1/(5*16.6*2) = .006mm circle of confusion, for prints of the same size.

Someone posted the formula hyperfocal distance is approximately equal to the focal length squared divided by the product of f-stop and circle of confusion. If we replace f-stop with physical aperture diameter, the formula changes such that the hyperfocal distance is approximately equal to the focal length times the aperture diameter divided by the circle of confusion.

That is, h ~= fo/c, where h is hyperfocal distance, f is focal length, o is opening diameter of the diaphragm, and c is the circle of confusion.

Points of near and far depth of field are related solely to the hyperfocal distance and the subject distance. A situation with a shorter hyperfocal distance has greater depth of field.

So, let's say we want to produce a 10x15 print with a 5 lines/mm resolution. We want to make the picture include the same subject, with the same composition, from the same camera position.

To use 24x36, we would require a longer lens than if we use a 15x23 format, with those stipulations. The longer lens would provide greater magnification to compensate us for the smaller format at the same camera position. Let's say that we use a 50 on the APS camera and an 80 on the 35mm camera. For both, we use f/2.8. The aperture diameter for a 50 at 2.8 is 18mm, and for an 80 at 2.8 is 29mm.

Thus, for 35mm, h=(80)(29)/C, or 2320/C. (We'll set circle of confusion later.)

And for APS, h=(50)(18)/C, or 900/C. Clearly, if the hyperfocal distance is less, so the depth of field will be more.

But, let's adjust the circle of confusion for the same print size. For 35mm, it will be .01mm (1 divided by the product of 5 lines/mm, 10X enlargement, and 2 features in ean line pair), and for APS it will be .006mm.

Thus, for 35mm, h=2320/C=2320/.01=232,000mm or 232 meters.

And for APS, h=900/C=900/.006=150 meters.

Thus, the APS format, when used with a lens that provides the same field of view, and at the same f-stop, will provide more depth of field. Even when enlarging it more, it will still provide more depth of field.

What aperture would we need to get the same depth of field in the smaller camera as the 80 at 2.8 on the larger camera?

At the same degree of enlargement, the 50mm focal length times the aperture diameter must equal 2320. Thus, we need an aperture of 46.4. That means at the same degree of enlargement, we'd need a 50 mm lens at f/1.0 to get the same depth of field as an 80 at 2.8. (a 50mm/46mm = about f/1.) Surprised?

If we judge on the basis of print size, it's not so bad, because the print from the smaller sensor will be enlarged more. Now, the 50(o)/.006 must equal 80(29)/.01. The aperture diameter, o, will be 28mm. 50/28 = approximately f/2. So, to get the same depth of field on an 10x15 print from 35mm with an 80 at 2.8, we'd need a 50 at f/2 using APS.

So, there's no getting around it, if we intend to take the same picture, the shorter focal length necessitated by the smaller format provides more depth of field at the same f-stop. Enlarging to the same print size narrows the gap considerably, but not all the way.

It is not particularly useful to compare them with the same focal length, because it wouldn't be the same picture with different formats. You have to move, changing the perspective, or you get less of the scene in the image--a different picture.

Rick "thinking that arithmetic solves so many problems" Denney

[rent]

wow this thing is still alive?

hi cyber: i think it's necessary to clarify something, before we move on with the whole DOF/x-factor discussion:

there is only one plane that is actually in focus at any given time, namely, that is the plane that is exactly at the focusing distance. anything in front of or behind that plane is technically out of focus.

but we still see the stuff in front of or behind as in focus -- i.e., sharp, because they are not "fuzzy" enough for us to lable them as out of focus. so now the question is... how do we systematically determine if something is sharp enough for us to consider it in focus and if it is fuzzy enough to be labled as out of focus?

this is where we need the concept of circle of confusion. when a point of light is represented in a photo. if it is seen as a single point, we interprete it as in focus, when it appears to us as a small disk or circle, we interprete it as out of focus. the CoC is thus the largest size of this point of light before we consider it no longer a point.

as you can see, this "cut-off" is quite subjective, it all depends on how big the photo is, how far you are looking at it, your eye sight, etc. so customarily, we use a 8x12" print viewed at about 1 ft away as a starting point. and the "average" person in this setting can distinguish a point of 0.250mm, any point of light larger than that is interpreted as out of focus.

if you can buy into what i said above, then you can easily infer why the DOF is dependent on the sensor size (really the CoC, but CoC is different for different sensor sizes) by referring back to my earlier posts and some posts by steven. oh, btw, there is no such thing as an objective DOF, that's why all the DOF scale on those FD lenses are calibrated to a certain CoC.

whew...

-alex

[slin100]

It is not particularly useful to compare them with the same focal length, because it wouldn't be the same picture with different formats. You have to move, changing the perspective, or you get less of the scene in the image--a different picture.

I agree but the original post prompted the discussion along these lines. The inquiry was what happens to DOF when you take a lens from a 35mm camera and use it on an APS format camera. The answer is that the DOF decreases in this special case.

[Citizensmith]

This is IT!!

:D :D :D

Hey IT hit 3 pages.

Didn't we resolve everything a page and a half ago? This post is becoming a mix of bestiality, sadism and necrophilia.

Flogging a dead horse. :)

[PhotosGuy]

" Flogging a dead horse." Since this is a photo site, here's a visual to go with Citizensmith's assessment!

[Tom W]

This post is becoming a mix of bestiality, sadism and necrophilia.
:)

Well, 2 out of 3 ain't bad! :)

[O/confusion]

" Flogging a dead horse." Since this is a photo site, here's a visual to go with Citizensmith's assessment!

That is PERFECT! :lol:

Terry

[Citizensmith]

" Flogging a dead horse." Since this is a photo site, here's a visual to go with Citizensmith's assessment!

"Freakin' sweet" :)

[rdenney]

Hey IT hit 3 pages.

Didn't we resolve everything a page and a half ago? This post is becoming a mix of bestiality, sadism and necrophilia.

Flogging a dead horse. :)

I've always been partial to necrophilia, myself.

Rick "the only good horse is a dead horse" Denney

[CyberDyneSystems]

Well, 2 out of 3 ain't bad! :)

just a differenr kind of "flogging" :shock:

[O/confusion]

I've always been partial to necrophilia, myself.

Rick--guess you're just really lucky nobody's split on you yet! :D

Terry

[rdenney]

Rick--guess you're just really lucky nobody's split on you yet! :D

Terry

I knew I should not have gone down this road.

Rick "who just knew it" Denney

[O/confusion]

I knew I should not have gone down this road.

Rick "who just knew it" Denney

It's O.K., Rick; sometimes these bad old jokes just need an occasional airing--and it's not everyday that a suitable occasion arises! I thank you, my bad old joke thanks you. Now it can go away and sleep again for a few more years.

Cheers,
Terry

[Citizensmith]

I guess the big question is if you were taking a photo of someone flogging a dead horse and it was low light so you had to use a wide appeture, what affect on the depth of field would the type of horse have?

On a more serious note though. This is the first long IT post I've actually had a part in and I've learn't a few specific details that hadn't been clear before. I for one am glad IT got talked about in such depth (of field?).

[O/confusion]

[QUOTE=Citizensmith]I guess the big question is if you were taking a photo of someone flogging a dead horse and it was low light so you had to use a wide appeture, what affect on the depth of field would the type of horse have? QUOTE]

What's its crupper factor?

Terry