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Thread started 17 Dec 2008 (Wednesday) 16:02
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D.O.F./sensor size Question

 
Cheesey
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Dec 17, 2008 16:02 |  #1

If you take a fast zoom lens, open it up all the way and photograph an object using a crop camera, then do the same thing zooming in that lens with a full-frame camera so the object is the same size, is the depth of field the same? Or does the full-frame camera have a shallower or deeper depth of field?

Asked another way, is the long-distance advantage that a 1.6 crop camera enjoys balanced off with a disadvantage if one is looking to blur a background?


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gazcoyle
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Dec 17, 2008 16:10 |  #2

Yes, full frame = shallower DOF, crop cameras = more DOF




  
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JeffreyG
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Dec 17, 2008 16:12 |  #3

If I understand this part of what you wrote:

so the object is the same size,

Meaning so that the object is the same size relative to the frame (i.e. prints the same size if both are made into 8x10 prints or whatever) then the answer to your question is 'yes'.

Given equal framing (the quick way of stating 'equal size') the FF sensor will deliver the same DOF as a 1.6X sensor that is shot 1.3 stops faster, e.g. f/4 on FF is equal in DOF to f/2.5 on 1.6X.


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Dec 17, 2008 16:39 |  #4

Cheesey wrote in post #6897846 (external link)
If you take a fast zoom lens, open it up all the way and photograph an object using a crop camera, then do the same thing zooming in that lens with a full-frame camera so the object is the same size, is the depth of field the same? Or does the full-frame camera have a shallower or deeper depth of field?

Asked another way, is the long-distance advantage that a 1.6 crop camera enjoys balanced off with a disadvantage if one is looking to blur a background?

Inherently, the FF sensor gives the deeper DoF as less magnification is needed to reach the "standard" size at which DoF is judged.

In the situation you describe you will however achieve a shallower DoF because this effect is being swamped by the opposing and much greater effect of increasing focal length.

There is no reach advantage for croppers, unless you subscribe to the theory that capturing a smaller portion of an image somehow makes it larger.

There IS a reach advantage with higher density sensors as greater magnification is possible with greater resolution. These tend to appear first on APS-C bodies which have the faster product-release cycle.




  
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Dec 17, 2008 16:49 |  #5

Cheesey wrote in post #6897846 (external link)
If you take a fast zoom lens, open it up all the way and photograph an object using a crop camera, then do the same thing zooming in that lens with a full-frame camera so the object is the same size, is the depth of field the same? Or does the full-frame camera have a shallower or deeper depth of field?

Asked another way, is the long-distance advantage that a 1.6 crop camera enjoys balanced off with a disadvantage if one is looking to blur a background?

This has been discussed to death you know :rolleyes:

Answer (read it all carefully before judging it's validity):

At the same focal length and distance, crop bodies have LESS depth of field at 100% crop than a comparable full frame, since the pixel density is greater in the crop body. Hence, at 100% crop the density determines the depth of field not FF vs crop.

However, at the same field of view, the crop sensor has more DOF than the FF at 100% due to the increased distance between the camera and the subject, and even the increased resolution of the crop body doesn't make up for it.

Now, when zoomed to fit (see the entire image at once, not changing focal length), the DOF for the first case will be the same, but the DOF in the second will say that the FF has much smaller DOF.

Now, when printing, DOF is the same as for zoomed to fit condition. You can try to do the math yourself (and reach the same conclusions after a few min to a few hours depending on your level of math), or just believe me.


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Panopeeper
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Dec 17, 2008 18:10 |  #6

It is not nice to answer a question this way, but sometimes one has to say, that the question itself is nonsensical. In this case the question ignores several aspects and tries to jump over some facts. Thus, the question should be rephrased.

The problem with the question is, that it ignores the very basics of DoF: It is not a function of sensor size, nor of pixel (sensel) size, not of cropping.

It is a combination of the above PLUS more. Thus it is nonsensical to discuss this subject on the basis of the question.


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JeffreyG
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Dec 17, 2008 18:16 |  #7

Panopeeper wrote in post #6898570 (external link)
It is a combination of the above PLUS more. Thus it is nonsensical to discuss this subject on the basis of the question.

DOF is fundamentally a function of the size of the blur disk radius relative to the acceptable blur disc radius that we will judge as sharp (not be able to detect as a disc).

So the OP's question really does make sense simply by having us assume that he wishes to understand the difference with some relatively straightforward assumptions.

1. He obviously wants us to compare for like print sizes and like qualifications for acceptable sharpness....why else would he be asking about this relative to sensor size?
2. He abviously wants to understand the effect on DOF for like perspective and framing...what one would get shooting the same scene from he same position while varying sensor size and focal length.

Given these two assumptions a confident and meaningful answer can be given. This is not squaring the circle here.


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Cheesey
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Dec 17, 2008 20:03 |  #8

Panopeeper wrote in post #6898570 (external link)
It is not nice to answer a question this way, but sometimes one has to say, that the question itself is nonsensical. In this case the question ignores several aspects and tries to jump over some facts. Thus, the question should be rephrased.

The problem with the question is, that it ignores the very basics of DoF: It is not a function of sensor size, nor of pixel (sensel) size, not of cropping.

It is a combination of the above PLUS more. Thus it is nonsensical to discuss this subject on the basis of the question.

Panopeeper, I must say that your technicalities totally miss my point. There is nothing "nonsensical" about a wish to determine what the effect of a lens might be on a camera that one does not have access to. It may be ignorant, or simplistic, or any number of things, but not nonsensical.

You are right, however, about your answer not being "nice".


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Panopeeper
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Dec 17, 2008 20:45 |  #9

Cheesey wrote in post #6899167 (external link)
Panopeeper, I must say that your technicalities totally miss my point. There is nothing "nonsensical" about a wish to determine what the effect of a lens might be on a camera that one does not have access to

The issue is, that you were not referring to a specific camera but to one cropping vs full frame. The lack of further specification makes the question nonsensical.

You need to understand, that digital photography is NOT about sensor size, nor about print size or printing density. No digital camera creates a print or a monitor display. There is no point to talk about a "cropping camera". The two most important factors in this relation, the pixel size and the image quality vary more than any other consideration.

For a few years ago it was accepted, that FF sensors have large pixels compared to cropping cameras. Well, compare the cameras now: some FF sensors have smaller pixels than some cropping cameras; some other FF sensors have much larger pixels. Do you think it is reasonable to throw a 50D with a 350D together, simply saying that both are cropping cameras?

Beside the number of pixels, the image quality (how clean the pixels are) is important. This decides, how much they can be enlarged or have to be reduced for a certain type of presentation. Don't you see a difference between the pixels of a 10D and a 40D in this relation?

So, what is the point of categorizing the cameras in FF and cropping? There is only useful aspect, namely that the cropping cameras are using the "cream" of the projected image, i.e. the sharpest part with the least aberrations. This aspect did not come up here (and this too can not be assigned to cameras; this comes from lenses).

Ultimately, the unit of digital photography is the PIXEL. In order to compare two images of whatever camera for whatever purpose, the easiest way is to express them in terms of number and quality of pixels; the result can be converted in whatever presentation you want to see.


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JeffreyG
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Dec 17, 2008 20:59 |  #10

Panopeeper wrote in post #6899409 (external link)
The two most important factors in this relation, the pixel size and the image quality vary more than any other consideration.

I disagree with this pretty much in totality.

Do you think it is reasonable to throw a 50D with a 350D together, simply saying that both are cropping cameras?

Yes I do. 100% so for a simple discussion on DOF which is what the OP was asking for.

And beyond that.....the difference in IQ between all of the various 1.6X sensors made by Canon have been more alike than different.

I think you would be hard pressed to show a huge difference in IQ between those two bodies....let alone showing a difference in DOF!

Are you really suggesting that the 50D will deliver a significantly different DOF given normal (say 8x10) print sizes?

Beside the number of pixels, the image quality (how clean the pixels are) is important. This decides, how much they can be enlarged or have to be reduced for a certain type of presentation. Don't you see a difference between the pixels of a 10D and a 40D in this relation?

No, I never have. I've never seen a difference in DOF between them to be very clear and on the point of this thread.

Wandering off topic and to be quite honest I never saw an IQ difference at all between the XT, 30D and 40D bodies I owned.

So, what is the point of categorizing the cameras in FF and cropping? There is only useful aspect, namely that the cropping cameras are using the "cream" of the projected image, i.e. the sharpest part with the least aberrations. This aspect did not come up here (and this too can not be assigned to cameras; this comes from lenses).

1. What does this have to do with relative DOF?
2. What happens to that 'cream' of the image when you enlarge the image 1.6 times as much to make the same print?

Ultimately, the unit of digital photography is the PIXEL. In order to compare two images of whatever camera for whatever purpose, the easiest way is to express them in terms of number and quality of pixels; the result can be converted in whatever presentation you want to see.

Incredibly incorrect.


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GCGuy
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Dec 17, 2008 21:05 as a reply to  @ Panopeeper's post |  #11

Cheesey wrote in post #6897846 (external link)
If you take a fast zoom lens, open it up all the way and photograph an object using a crop camera, then do the same thing zooming in that lens with a full-frame camera so the object is the same size, is the depth of field the same? Or does the full-frame camera have a shallower or deeper depth of field?

Asked another way, is the long-distance advantage that a 1.6 crop camera enjoys balanced off with a disadvantage if one is looking to blur a background?

Most of the techno-oriented responses I've been reading seem like they missed the original simple question?? Am I right to understand your questions like this:

70-200 f/2.8 lens on a 40D body at 100mm and f/2.8
versus
70-200 f/2.8 lens on a 5D body at 160mm and f/2.8

Maybe my math is wrong with the mm and such, but I'd think that the DOF would be even shallower on the full frame due to the longer focal length of the lens needed to achieve the same result as with the 40D. Is that kinda what you meant? ;)

And I liked Jeffrey G's first response, nice simple rule of thumb for me to keep in mind when I get my 5D eventually :D


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Cheesey
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Dec 17, 2008 21:18 |  #12

GCGuy wrote in post #6899506 (external link)
Most of the techno-oriented responses I've been reading seem like they missed the original simple question?? Am I right to understand your questions like this:

70-200 f/2.8 lens on a 40D body at 100mm and f/2.8
versus
70-200 f/2.8 lens on a 5D body at 160mm and f/2.8

Maybe my math is wrong with the mm and such, but I'd think that the DOF would be even shallower on the full frame due to the longer focal length of the lens needed to achieve the same result as with the 40D. Is that kinda what you meant? ;)

And I liked Jeffrey G's first response, nice simple rule of thumb for me to keep in mind when I get my 5D eventually :D

Yes GCGuy, this is pretty much what I was asking, and I too had accepted Jeffrey G's first response as the answer to my question.

I was simply thinking back to when I was a kid, using a 35mm film camera and a 1.8 50mm lens. I was able to open it up and blur the background pretty much at will. Over the years I moved to slower zooms on film cameras, and now to a digital XT, and I feel I've lost some creative control in regard to depth of field. I was contemplating whether a fast prime or faster zoom might address this limitation, or if my 1.6 crop factor was playing a role as well. If I understand correctly now, I'd need an even faster lens than my old 1.8 to achieve the shallow depth of field I used to be able to obtain with my old film camera.

Thanks to all for bearing with this thread.


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basroil
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Dec 17, 2008 21:28 |  #13

JeffreyG wrote in post #6899473 (external link)
I disagree with this pretty much in totality.



Yes I do. 100% so for a simple discussion on DOF which is what the OP was asking for.

And beyond that.....the difference in IQ between all of the various 1.6X sensors made by Canon have been more alike than different.

I think you would be hard pressed to show a huge difference in IQ between those two bodies....let alone showing a difference in DOF!

Are you really suggesting that the 50D will deliver a significantly different DOF given normal (say 8x10) print sizes?



No, I never have. I've never seen a difference in DOF between them to be very clear and on the point of this thread.

Wandering off topic and to be quite honest I never saw an IQ difference at all between the XT, 30D and 40D bodies I owned.



1. What does this have to do with relative DOF?
2. What happens to that 'cream' of the image when you enlarge the image 1.6 times as much to make the same print?



Incredibly incorrect.

Lets not get into this again... what is it, the fifth time already?

Lets recap:
For prints and full image views, Jeff is absolutely correct. DOF is simply a factor of focal length, distance, and enlargements.

For 100% views (like the good pixel peepers do), my thing is correct. DOF is a function of pixel density, focal length, and distance.

For resolution limited photography, pano is mostly correct, but we don't tend to get into resolution limited stuff anymore, since after 4-6MP the human eye can't really tell the difference at standard viewing distances.


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Dec 18, 2008 02:19 as a reply to  @ basroil's post |  #14

Without disagreeing in any way with the previous post, I'd like to point out that DOF has traditionally been understood in terms of a common print size and is still, quite rightly, understood that way in general. If someone asks about DOF, it is absolutely normal and sensible to assume that is what they are talking about - the OP's question was completely reasonable.

Considering the visibility of DOF effects at the 100% view on screen is of theoretical interest, but no more than that to most people. It is not as if DOF is a new concept that was invented for digital cameras. To say that pixel density affects DOF is the same as saying that diffraction effects are worse on a 50D than on a 40D, or that lenses are softer on a 50D.


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JeffreyG
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Dec 18, 2008 05:41 |  #15

gcogger wrote in post #6901121 (external link)
Without disagreeing in any way with the previous post, I'd like to point out that DOF has traditionally been understood in terms of a common print size and is still, quite rightly, understood that way in general. If someone asks about DOF, it is absolutely normal and sensible to assume that is what they are talking about - the OP's question was completely reasonable.

Considering the visibility of DOF effects at the 100% view on screen is of theoretical interest, but no more than that to most people. It is not as if DOF is a new concept that was invented for digital cameras. To say that pixel density affects DOF is the same as saying that diffraction effects are worse on a 50D than on a 40D, or that lenses are softer on a 50D.

Thank you, yes. People take photographs to view the entire image, usually on a monitor or a print. It is just common sense to assume that any question on DOF is asked relative to how a person will normally view a photograph.

I cannot understand dumping a DOF definition on an ususpecting poster that assumes 100% crops.....let alone whatever bizarre definition Panopeeper is coming from.

Isn't it reasonable to assume that the question in a photography forum is based on humans viewing photographs?


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