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Thread started 16 Aug 2015 (Sunday) 15:24
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Sensor size affect DOF? Methinks not.

 
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Aug 17, 2015 06:55 |  #46

sploo wrote in post #17671949 (external link)
Very roughly; if you used 50mm on the APS-C and 85mm on the FF (for the same framing, at the same subject distance), f/1.8 on the FF would require approx f/1.1 on the APS-C to match the DOF. Just about doable with a 50mm f/1.2L I guess.

That's not the point. If you want to start down that road you could just get a faster 85. If you have a bag full of the fastest canon primes, focal length and camera subject distance are going to be the primary factors in determining DOF and the extent of OOF area blur.


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Aug 17, 2015 07:17 |  #47

Sirrith wrote in post #17671924 (external link)
You're cropping one photo. That invalidates your point. Try to reproduce the results by obtaining the same framing WITHOUT cropping either photo, only by changing lens/moving forward or backwards, and see what happens.

Careful with that. Moving forward or backward has the same effect on DoF relative to the main subject. BUT, in moving, the perspective changes so the foreground, subject and background have different a relationship to each other. You can only compare like with like by changing focal length.




  
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Aug 17, 2015 07:41 |  #48

Azathoth wrote in post #17671929 (external link)
DOF depends only of three things:
- distance to subject;
- focal length of the lens;
- Aperture.

That's it.

That's not "it" by a long shot.

The camera format (size of the film frame or digital sensor) also applies.

The size of the test image that's being viewed matters a lot even though DOF calculators typically assume the standard size of the test image being viewed, the viewing distance, and the visual acuity of the person viewing the test image.


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Aug 17, 2015 07:43 |  #49

sploo wrote in post #17671944 (external link)
I feel that's slightly orthogonal to the main discussion though; that DOF is the range of focus distances where the resulting print goes from perceptually sharp to perceptually soft.

Yes, it is....but I'm trying to explain terminology and trying to settle a middle ground with old film formats vs digital imaging:) The traditional method of determining DOF is finding a CoC value from enlarging a print at a certain viewing distance, and considering a person's visual accuity. With digital imaging, where we can dial in absolute resolution, most all those factors are part of an image's perceptual resolution. If you want to go by the old method of finding a camera's CoC value, that's fine: it yields the same result. It's just not a coincidence that CoC values also fall in line proportionally with a camera's sensor size. I don't seem to be arguing fundamentals...only definition of terms: IMO, it's best to think of digital photography like the rest of graphics: perceived sharpness first comes from an image's resolution. DOF is the range of focus within an optical system.

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Aug 17, 2015 09:11 |  #50

Azathoth wrote in post #17671929 (external link)
DOF depends only of three things:
- distance to subject;
- focal length of the lens;
- Aperture.

That's it.

Make it two:
- Aperture
- Scale of reproduction

And make that last one include everything that has to do with how big you think the print is, i.e. including print size and viewing distance, and you are home.


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Aug 17, 2015 09:44 |  #51

AJSJones wrote in post #17671423 (external link)
Your example is the same as setting up the camera, keeping the FL and aperture the same but "cutting out a small piece of the negative" from the 35 mm negative that is the same size as the 110 negative and printing them up the same size - of course they will be the same D'oh! No-one is quibbling with that experiment's outcome - only its relevance! That simply tells us that when you use a crop sized piece of film and a given FL and aperture you get the same results each timevmad We could have cut it out of an 8x10 sheet of film - once you crop the capture area to the same sizes, it doesn't matter what the bigger size sheet was...

The whole point of getting a camera with a bigger sensor is to actually use it, not just the 13x17 bit in the middle:D When you do, you have to use a different lens - a different lens - a different lens - to get the same FoV to land on the bigger sensor. If you use the same lens, you won't get the same FoV. And we've explained about the "entrance pupil/physical aperture" being different from f stop.

My 2 cents to help explain the phenomena...
I am only on Page 2 reading the thread. The above reply I agree is true. Geokrass' assertion is almost true and loses site of the circle of confusion (CoC) in both the calculation and per edition (equalized print). The different sensors have different circles of confusion when printed for same FOV onto same size print. So with same lens/aperture/distance scenario, and you change camera formats And digitally zoom onto common print size, you are actually modifying the perceived CoC to plug into an equation. This explains how they begin to look the same. . However, if you didn't digitally zoom for the print and just looked at the ruler around the focal plane in each, the larger format sensor would have visually larger (on the ruler in the frame) DOF since you can't discern the details as well and they still look sharp "longer". (I corrected my smaller typo).

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Aug 17, 2015 09:48 as a reply to  @ gschlact's post |  #52

according to all online calculators, same aperture, focal length, distance...and just changing the cameras, leads to the FF camera having a larger DOF than the crop camera

the reason everyone talks about shallower DOF with FF cameras is because they are matching the framing by changing the focal length, or distance...none of that applies to this discussion though


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Aug 17, 2015 10:09 |  #53

davesrose wrote in post #17672018 (external link)
Yes, it is....but I'm trying to explain terminology and trying to settle a middle ground with old film formats vs digital imaging:) The traditional method of determining DOF is finding a CoC value from enlarging a print at a certain viewing distance, and considering a person's visual accuity. With digital imaging, where we can dial in absolute resolution, most all those factors are part of an image's perceptual resolution. If you want to go by the old method of finding a camera's CoC value, that's fine: it yields the same result. It's just not a coincidence that CoC values also fall in line proportionally with a camera's sensor size. I don't seem to be arguing fundamentals...only definition of terms: IMO, it's best to think of digital photography like the rest of graphics: perceived sharpness first comes from an image's resolution. DOF is the range of focus within an optical system.

CoC values (external link)

Indeed , it seems to be terms again :D
You are focusing (ahem) on perceived sharpness and what affects that - and presumably needing to set a rendering resolution so the viewer perceives the image as sharp under the intended viewing conditions. This is driven by the (variable) acuity of the viewer's vision and the image resolution and the viewing distance. Something with a lower resolution will be perceived as less sharp than one with a higher resolution when compared side by side at some viewing distances. If we take a picture of a (flat) test chart (where the whole subject is on the focus plane and the whole captured image is therefore within the DoF) with a "good" lens and a "poor" lens and compare same size prints from a suitable distance, we perceive that one is "sharper" than the other. We conclude that lens A is perceptibly sharper than lens B. Alternatively we can take either image and print it on a high rsolution printer or a low resolution printer and similarly conclude one print is sharper than the other. This is all about "perceived sharpness" and DoF does not play any role in that.

When we discuss DoF here, we are discussing the factors that influence how we perceive the transition between the in focus parts of the viewed image and the out of focus parts of the image. This is not affected by whether the image was created wholly digitally in a computer or by analog or digital acquisition of an analog optical image. Our viewing process is (effectively) analog. Let's say we use lens A and B to capture an image with some parts in focus and some out of focus - any 3D scene where the only "truly" in focus parts are those located at the distance of the focus plane. In each case, the objects in the plane of focus will be the sharpest parts of the image and we perceive them as "in focus" (and if we compare between the prints we will still be able to perceive the established difference in maximum perceived sharpness). However, there are parts in each image that are perceived as "not as sharp" as the sharp parts in the same image. If we look along the ruler we see there is the zone that is seen as sharp, and we can read off the distance where we can begin to see the markings as not quite as sharp: that transition marks the edge of the DoF - and it does so for each lens. And it will do so whether the image was created in a computer or captured from real life. (The slight differences between lenses in maximum captured sharpness plays relatively little role in where we discern the transitions; however, the resolution at which we present the images for assessment can make a big difference if we contrive the resolution and viewing distance conditions, we can make the blur from the lost resolution comparable to or even much greater than the blur at the edges of the DoF - but that is a big "contriving":( It would be like the billboard viewed from a foot!).

From the point closest to us that seems sharp, to the point furtherst from us that seems sharp is the zone called the "depth of field".

Now, since these transitions are based on an assessment of whether the objects are "in focus", I can see why you might (in the computer graphics world) want to refer to the range as "depth of focus (external link)" - but I hope that's not the case. It would be very confusing, because that term has been in use for much longer than computers, to decscribe the range of distances in front of and behind the film plane where the plane (out in the "field" of view) of the focus on the subject is recorded on the film as "sharp". Depth of field is on the object side of the lens and depth of focus is on the image/film/sensor side of the lens.


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Aug 17, 2015 10:27 |  #54

DreDaze wrote in post #17672138 (external link)
according to all online calculators, same aperture, focal length, distance...and just changing the cameras, leads to the FF camera having a larger DOF than the crop camera

the reason everyone talks about shallower DOF with FF cameras is because they are matching the framing by changing the focal length, or distance...none of that applies to this discussion though

If you change the distance, you change the perspective and you get a different image. There's no point in that comparison - or, if there is, what's the point?
If you don't change the FL, you get a different image - the crop camera, well, crops the image. There's no point in that comparison - or, if there is, what's the point?
The DoF calculators are based on the standardizd (but arbitrary) condition of making an ~8x10 or 8x12 inch print and viewing it from ~12-15 inches. So the degree of enlargement from the original optical image to the print is different for each sensor size.

Now, it seems that the discussion started about using lenses with a given FL in a fixed place, setting the aperture and capturing the image with sensors of different sizes and then printing (or putting on screen) the same part of the image at the same size. Who does this? With a P&S sensor you'll capture a tiny portion of the view in front of the camera while using an 8x10 you'll capture a huge part of the view. That discussion is comparing the tiny part cropped from each image to the dimensions of the P&S - but it's the same optical image enlarged to the same extent so of course the DoF is going to be the same.....


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Aug 17, 2015 10:41 |  #55

AJSJones wrote in post #17672214 (external link)
If you change the distance, you change the perspective and you get a different image. There's no point in that comparison - or, if there is, what's the point?
If you don't change the FL, you get a different image - the crop camera, well, crops the image. There's no point in that comparison - or, if there is, what's the point?
The DoF calculators are based on the standardizd (but arbitrary) condition of making an ~8x10 or 8x12 inch print and viewing it from ~12-15 inches. So the degree of enlargement from the original optical image to the print is different for each sensor size.

Now, it seems that the discussion started about using lenses with a given FL in a fixed place, setting the aperture and capturing the image with sensors of different sizes and then printing (or putting on screen) the same part of the image at the same size. Who does this? With a P&S sensor you'll capture a tiny portion of the view in front of the camera while using an 8x10 you'll capture a huge part of the view. That discussion is comparing the tiny part cropped from each image to the dimensions of the P&S - but it's the same optical image enlarged to the same extent so of course the DoF is going to be the same.....

Who does the?
Answer: people who are focal length limited will attempt to make same print/ FOV of their target no matter which body they use.




  
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Aug 17, 2015 11:00 |  #56

AJSJones wrote in post #17672214 (external link)
If you change the distance, you change the perspective and you get a different image. There's no point in that comparison - or, if there is, what's the point?
If you don't change the FL, you get a different image - the crop camera, well, crops the image. There's no point in that comparison - or, if there is, what's the point?
The DoF calculators are based on the standardizd (but arbitrary) condition of making an ~8x10 or 8x12 inch print and viewing it from ~12-15 inches. So the degree of enlargement from the original optical image to the print is different for each sensor size.

Now, it seems that the discussion started about using lenses with a given FL in a fixed place, setting the aperture and capturing the image with sensors of different sizes and then printing (or putting on screen) the same part of the image at the same size. Who does this? With a P&S sensor you'll capture a tiny portion of the view in front of the camera while using an 8x10 you'll capture a huge part of the view. That discussion is comparing the tiny part cropped from each image to the dimensions of the P&S - but it's the same optical image enlarged to the same extent so of course the DoF is going to be the same.....

It's not about cropping the same area. It's a simple question, will a shot taken at the same distance, focal length, and aperture on two different sensors give the same DOF(ignore that they won't be the same framing)....the OP says they will, online calculators say it will be different


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Aug 17, 2015 11:15 |  #57

DreDaze wrote in post #17672251 (external link)
It's not about cropping the same area. It's a simple question, will a shot taken at the same distance, focal length, and aperture on two different sensors give the same DOF(ignore that they won't be the same framing)....the OP says they will, online calculators say it will be different

Well, aside from the fact that the images will be different(:eek: ), they will have been enlarged to a different extent by going from optical image capture size to the standard conditions (used by DoF calculators - see above, taking the whole image from the sensor and going to 8x10 etc), so they won't have the same DoF. (OP had cropped and enlarged the same area of the optical by the same amount - rather than the whole image from the sensor - so he showed that when you do that, you get the same DoF)


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Aug 17, 2015 11:46 |  #58

For calculating DoF for any fixed set of viewing conditions (print size, viewing distance, visual acuity) there are only TWO more things you absolutely must need to know. The first is the Total Magnification ratio, that is the ratio of the size of the subject in real life (at the real world side plane of focus of the lens system), compared to the size of the subject in the final image. So a subject 20" high reproduced 10" high would have a reproduction ratio of 2:1 or 0.5×. The second thing required to be known is the absolute diameter of the aperture, not the f/number. Although it is possible to calculate DoF from these variables, what happens is that it will allow you to calculate a range of other variables that will produce the same results. As you decide on one of the calculated values to use, so the other choices of values become more constrained. Eventually it comes down to picking a subject distance a focal length (which defines f/number), and will also affect the final enlargement ratio.

Of course in practical photography one is usually limited by the equipment available. This will limit the size of the imaging area, perspective requirements will also limit the camera to subject distance. So we are now also limited to what focal length lens we can choose to frame the subject. Generally these will be overriding considerations, and it may well be that the DoF ends up being limited either wider or narrower than one would ideally like for artistic reasons thanks to limits in available equipment.

This applies the same whether we are looking at analogue or digital imaging, although both mediums have limits. For analogue imaging the limits are the size of the grains in the emulsion, both on the film and the printing paper, although the film grain is most likely to be the limiting factor. In digital imaging the limit is Nyquist. It will require that both the taking sensor, and the output device can operate within the relevant Nyquist limits for the required final viewing conditions. As long as one is operating within those limits the rules for DoF apply equally.

Alan


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Aug 17, 2015 11:51 |  #59

gschlact wrote in post #17672229 (external link)
Who does the?
Answer: people who are focal length limited will attempt to make same print/ FOV of their target no matter which body they use.

I was asking "Who does the comparison?" - not "Who crops and enlarges because they are FL limited in their lenses?" Whenever that is done, standard DoF calculators don't apply because the user is altering the reproduction ratio upon which the "standardized DoF" is calculated. Cropping the central 1.6 crop area from a FF and making the standard 8x10 print from it will yield the same DoF as when the same print was made from the whole of the image from a crop camera (same lens, position, aperture etc). So, yes, in general, cropping alters DoF.


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Aug 17, 2015 12:14 |  #60
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Sensor size affect DOF? Methinks not.
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