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Author Topic: How (and why) does sensor size change DOF?  (Read 23802 times)

Etienne

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Re: How (and why) does sensor size change DOF?
« Reply #30 on: July 15, 2013, 03:56:27 PM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


Good concise explanation!

I think of it like this:
A photographer must frame a shot. To get the same shot as a FF using the same lens on a crop camera, the photographer has to move backwards (by a factor of 1.6). Instead of 10 feet away, he must be 16 feet away. This increased focal distance increases the DOF of the shot.  Do the calculation here:   http://dofmaster.com/dofjs.html

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Re: How (and why) does sensor size change DOF?
« Reply #30 on: July 15, 2013, 03:56:27 PM »

Etienne

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Re: How (and why) does sensor size change DOF?
« Reply #31 on: July 15, 2013, 04:20:32 PM »
This has been confusing for me for a long time thank you everyone for clarifying it.

Here's my new understanding.
• Physically the DOF does not change because your not changing the lens (ie. set up a shot,  switch a crop to a FF, and you'll just get a wider field of view but same DOF).
• But in practice it essentially does change (ie set up a shot, switch bodies, now move the came to have the same field of view, now the DOF of the crop camera photo will be larger. Same goes for not moving the camera and changing focal length).


Yes.

Photographers frame shots. None of the ultra-technical mumbo-jumbo about "circle of confusion" and "DOF is a function of the lens" is helpful.

Using a 50mm lens:

Framing on a FF cam at 10 feet = Framing on a crop at 16 feet

BUT the extra focal distance necessary to frame the same shot on a crop camera causes the DOF to increase. In order to get the same shallow depth of field on the crop camera, the aperture has to be opened by 1.3 stops in comparison to the FF camera.

Prove it for yourself. Go to http://dofmaster.com/dofjs.html and calculate the DOF for ( these two examples will give the same fov framing of the scene):

Crop camera with 50mm lens at f=1.8 and distance 16 feet
FF camera with 50mm lens at f=2.8 and distance of 10 feet

These two examples result in the same DOF (2.1 feet total). The crop camera needs to increase the aperture by 1.3 stops in order to get the same DOF as the FF camera.

PS ... the effect of "circle of confusion" calculations for different sensors is completely swamped by the overwhelming effect of the need to stand further back to compose with a crop camera.

privatebydesign

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Re: How (and why) does sensor size change DOF?
« Reply #32 on: July 15, 2013, 04:26:34 PM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


Good concise explanation!

I think of it like this:
A photographer must frame a shot. To get the same shot as a FF using the same lens on a crop camera, the photographer has to move backwards (by a factor of 1.6). Instead of 10 feet away, he must be 16 feet away. This increased focal distance increases the DOF of the shot.  Do the calculation here:   http://dofmaster.com/dofjs.html


It might be a concise explanation, but it is not good or accurate.

DOF, or CoC (they are entirely interchangeable), is a function of apparent aperture and subject magnification. Just those two. You can work out dof figures from just those two numbers. Subject magnification is a function of focal length and subject distance, as well as viewing distance and reproduction (print or screen) size.

When you put focal length and subject distance into a dof calculator all it is working out is the subject magnification on the sensor; it then needs the sensor size to work out the CoC for a given standard output, often an 8"x10" print viewed at 12".
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Etienne

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Re: How (and why) does sensor size change DOF?
« Reply #33 on: July 15, 2013, 04:38:48 PM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


Good concise explanation!

I think of it like this:
A photographer must frame a shot. To get the same shot as a FF using the same lens on a crop camera, the photographer has to move backwards (by a factor of 1.6). Instead of 10 feet away, he must be 16 feet away. This increased focal distance increases the DOF of the shot.  Do the calculation here:   http://dofmaster.com/dofjs.html


It might be a concise explanation, but it is not good or accurate.

DOF, or CoC (they are entirely interchangeable), is a function of apparent aperture and subject magnification. Just those two. You can work out dof figures from just those two numbers. Subject magnification is a function of focal length and subject distance, as well as viewing distance and reproduction (print or screen) size.

When you put focal length and subject distance into a dof calculator all it is working out is the subject magnification on the sensor; it then needs the sensor size to work out the CoC for a given standard output, often an 8"x10" print viewed at 12".


Wrong: the dof calculator also calculates the circle of confusion. http://dofmaster.com/dofjs.html

The technical mumbo-jumbo can be ignored. Crop factor cameras used to collect the same image as a FF camera will need to open the aperture by 1.3 stops in order to get the same DOF in the final image.

This is true regardless of whether you achieve the same field of view by moving the camera back by a factor of 1.6, or whether you use a shorter focal length (by a factor of 1.6, for Canon APS-C).

Circle of Confusion calculations won't make any difference at all. Partly because the effect is so small when comparing one real-world camera to another.

neuroanatomist

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Re: How (and why) does sensor size change DOF?
« Reply #34 on: July 15, 2013, 05:23:53 PM »
Wrong: the dof calculator also calculates the circle of confusion. http://dofmaster.com/dofjs.html

The technical mumbo-jumbo can be ignored.


Sorry, but then I suggest that you just ignore it, instead of calling others' correct statements wrong based on your misunderstanding.

FYI, DoFMaster doesn't calculate CoC, it assigns a standard value based on sensor size. It uses that assigned value to calculate DoF.
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Re: How (and why) does sensor size change DOF?
« Reply #35 on: July 15, 2013, 05:31:19 PM »
Last not least, note that if people rave about thin dof they often mean "strong background blur" except if you like the "only the nose in focus" type shots, and same thing here: bokeh also strongly depends on focal length and object/background distance relationship.
Except that bokeh really refers to the quality of the OOF blur, independent of quantity.  But maybe that's a story for another day.  ;)

Thanks for the hint, you are correct, even trusty Wikipedia says so (though you've probably written the article there :-)).

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Re: How (and why) does sensor size change DOF?
« Reply #36 on: July 15, 2013, 08:10:02 PM »

The technical mumbo-jumbo can be ignored. ..................

Circle of Confusion calculations won't make any difference at all. Partly because the effect is so small when comparing one real-world camera to another.

A common CoC figure for 135 format sensor is 0.029mm, divide that by 1.6 and you get? Voila, 0.018mm, the CoC for APS-C. if you wanted to ignore the difference in CoC, that is the same magnitude as focal length, aperture and noise, then you might as well ignore all of it!

You can't ignore the "technical mumbo-jumbo" in answering a technical question.

Now if you did want to ignore CoC differences you could, but your crop camera print (or on screen) could only be 40% as big as the ff print. If you were looking at an 8"x10" ff print you could only compare that to a 5"x6.25" crop print. Or a 12"x16" crop print would have the same CoC characteristics as a ff print 19"x26", quite a difference to my mind.
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Re: How (and why) does sensor size change DOF?
« Reply #36 on: July 15, 2013, 08:10:02 PM »

MrFotoFool

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Re: How (and why) does sensor size change DOF?
« Reply #37 on: July 15, 2013, 10:22:50 PM »
I was hoping this thread would answer the question for me too, but I would have been better off not reading it.  Now I am in the circle of confusion.

If I am understanding the consensus, the only difference is due to the need to back up or use a wider angle lens in APS-C to get the same coverage.  If you have a FF and APS-C camera at the same position and same lens and aperture, the depth of field will be the same (even though the APS-C shot will be cropped tighter).

I am not sure I agree with this (but I very well could be wrong).  My experience is based on shooting through zoo fences with my cousin on APS-C and me on full frame.  I could swear there were times we had the same lens length and aperture and yet I could blur out the fence and he could not.  I have heard that full frame cameras have a shallower depth of field and this experience seems to confirm that.  Am I just imaging things?

neuroanatomist

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Re: How (and why) does sensor size change DOF?
« Reply #38 on: July 15, 2013, 10:42:30 PM »
I am not sure I agree with this (but I very well could be wrong).  My experience is based on shooting through zoo fences with my cousin on APS-C and me on full frame.  I could swear there were times we had the same lens length and aperture and yet I could blur out the fence and he could not.  I have heard that full frame cameras have a shallower depth of field and this experience seems to confirm that.  Am I just imaging things?

Sorry, but probably, yes. If you were, in fact, using the same lens and aperture and not changing magnification (distance), and were the same distance from the fence, the blur would be equivalent.
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bdunbar79

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Re: How (and why) does sensor size change DOF?
« Reply #39 on: July 15, 2013, 11:22:18 PM »
DoF ~ focal length * aperture * subject distance
If you want to shoot the same picture using the same lens with both FF and crop sensor cameras, you need to be closer to the subject with FF camera to get the same framing and that's the only difference.


Good concise explanation!

I think of it like this:
A photographer must frame a shot. To get the same shot as a FF using the same lens on a crop camera, the photographer has to move backwards (by a factor of 1.6). Instead of 10 feet away, he must be 16 feet away. This increased focal distance increases the DOF of the shot.  Do the calculation here:   http://dofmaster.com/dofjs.html


It might be a concise explanation, but it is not good or accurate.

DOF, or CoC (they are entirely interchangeable), is a function of apparent aperture and subject magnification. Just those two. You can work out dof figures from just those two numbers. Subject magnification is a function of focal length and subject distance, as well as viewing distance and reproduction (print or screen) size.

When you put focal length and subject distance into a dof calculator all it is working out is the subject magnification on the sensor; it then needs the sensor size to work out the CoC for a given standard output, often an 8"x10" print viewed at 12".


Wrong: the dof calculator also calculates the circle of confusion. http://dofmaster.com/dofjs.html

The technical mumbo-jumbo can be ignored. Crop factor cameras used to collect the same image as a FF camera will need to open the aperture by 1.3 stops in order to get the same DOF in the final image.

This is true regardless of whether you achieve the same field of view by moving the camera back by a factor of 1.6, or whether you use a shorter focal length (by a factor of 1.6, for Canon APS-C).

Circle of Confusion calculations won't make any difference at all. Partly because the effect is so small when comparing one real-world camera to another.


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Re: How (and why) does sensor size change DOF?
« Reply #40 on: July 15, 2013, 11:41:47 PM »
As per usual: "full of sound and fury..."  ;D No need to get your panties in a twist, but do you CR people actually take pictures too, or just blablabla your days away???  8) How about posting some (good!) shots to prove your point? Especially you Neuro, you are always acting like you are the most knowledgable photographer on CR, but we haven't seen much of your great volume of work now haven't we? How about accepting my challenge and go out somewhere, shoot a number of pictures to prove your theory (what ever that was) and impress us with your work instead of your words...?
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Re: How (and why) does sensor size change DOF?
« Reply #41 on: July 15, 2013, 11:49:09 PM »
As per usual: "full of sound and fury..."  ;D No need to get your panties in a twist, but do you CR people actually take pictures too, or just blablabla your days away???  8) How about posting some (good!) shots to prove your point? Especially you Neuro, you are always acting like you are the most knowledgable photographer on CR, but we haven't seen much of your great volume of work now haven't we? How about accepting my challenge and go out somewhere, shoot a number of pictures to prove your theory (what ever that was) and impress us with your work instead of your words...?

It's actually not neuro's theory.  The question was, "does sensor size affect DOF?" (admittedly highly evolved since the OP's question).  There is only one correct and objective answer, and the answer is YES.  What more needs to be said?  At least he is trying to educate and offer an explanation.  This concept is not some unknown or magical theory.  Anyone knows sensor size affects DOF.

What is highly ironic, is that you refer to the thread as "you CR people" wasting time when in fact you yourself have contributed absolutely nothing to the thread.  I highly doubt neuro, or anyone really for that matter, cares about your challenge and owes absolutely nothing to you.

With that aside, DOF is affected by sensor size.  Ok?
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Re: How (and why) does sensor size change DOF?
« Reply #42 on: July 16, 2013, 12:43:19 AM »
How do you define 'DOF'?

Q1. If you shoot photo A using FF+50, then photo B using APS+35, at same F number DoF of photo B > photo A

Q2. If you shoot a photo A, then crop the border out leaving a smaller photo B, magnify it back to original size, DoF of photo B < A, but since the framing has changed, it's not the same photo anymore.

And that's it. DoF on two different format has two trends fighting against each other. One tends to decrease DoF (entrance pupil size), another tends to increase it (magnification or CoC). So here's the answer:

Answer 1. When comparing using the same Angle of View (effective focal length), larger sensor always has less DoF.

Answer 2. When comparing using different Angle of View, like comparing two different photos, it DOESN'T MAKE SENSE.

It seems most of the arguments and 'confusions' are around point 2, which is not a valid argument at all. Different Angle of View can never give you the same photo, you are comparing apple to orange.

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Re: How (and why) does sensor size change DOF?
« Reply #42 on: July 16, 2013, 12:43:19 AM »

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Re: How (and why) does sensor size change DOF?
« Reply #43 on: July 16, 2013, 02:05:24 AM »
The question was, "does sensor size affect DOF?" (admittedly highly evolved since the OP's question).  There is only one correct and objective answer, and the answer is YES.

Actually what I hear people saying is the answer is NO.

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Re: How (and why) does sensor size change DOF?
« Reply #44 on: July 16, 2013, 02:38:11 AM »
Here's an explanation on DOF and sensor size.  There's also a calculator for this.

http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm

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Re: How (and why) does sensor size change DOF?
« Reply #44 on: July 16, 2013, 02:38:11 AM »