DOF and Sensor Size

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Archive 2005 · DOF and Sensor Size

Hrow
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p.1 #1 · DOF and Sensor Size

I have read a couple of posts here that indicate that DOF increases when sensor size decreases. If this is true, can someone please explain why?

Aug 12, 2005 at 12:25 PM

Volleybob
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p.1 #2 · DOF and Sensor Size

You can simulate a smaller sensor with your camera. Use Photoshop to crop a photo. Did the DOF change because you truncated some border pixels?

Aug 12, 2005 at 12:44 PM

EnCapture
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p.1 #3 · DOF and Sensor Size

this is a misconception. if you take 2 pictures, 1 with a 1.6x crop factor sensor and one with a FF sensor, using the same lens from the same distance, the image from the 1.6x will appear to have a smaller DOF. however, if you take an equal crop from the full framed image and enlarge it to the same size of the 1.6x image they will look identical (aside from any difference in pixel density). the DOF will be the same.

Edited by EnCapture on Aug 16, 2005 at 06:13 AM GMT

Aug 12, 2005 at 12:45 PM

Hrow
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p.1 #4 · DOF and Sensor Size

Thank you, that's what I thought. For the life of me I couldn't figure out why the sensor size would impact DOF. Film size doesn't. There is a lot of good info on this site but there is also a small percentage that is just bulls**t.

Aug 12, 2005 at 12:53 PM

EnCapture
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p.1 #5 · DOF and Sensor Size

http://www.photo.net/learn/optics/dofdigital/

Edited by EnCapture on Aug 16, 2005 at 06:13 AM GMT

Aug 12, 2005 at 12:56 PM

Sam Bennett
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p.1 #6 · DOF and Sensor Size

EnCapture wrote:
this is a misconception. if you take 2 pictures, 1 with a 1.6x crop factor sensor and one with a FF sensor, using the same lens from the same distance, the image from the 1.6x will appear to have a smaller DOF. however, if you take an equal crop from the full framed image and enlarge it to the same size of the 1.6x image they will look identical (aside from any difference in pixel density). the DOF will be the same.

But this is only part of the picture.

The reason the DoF ends up being different is because you have to use different focal lengths to accomplish the same goal. So, say you want to take a headshot of someone, and you want the FoV of an 85mm lens. With a FF camera, you'd use an 85mm lens, with a 1.6 cropper you'd most likely use a 50mm lens. If you look at the math (I'm going to use a mythical 53mm lens for the 1.6 cropper, just to prevent hair splitting), you'll see that from the same distance you get:

85mm from 10ft @ f/2.8: 8.36 inches DoF
53mm from 10ft @ f/2.8: 13.8 inches DoF

So for all practical purposes (the ones that matter), then yes - FF cameras will produce narrower DoF. If you look at the difference between the two cameras, you'll see that 8.36 * 1.6 = 13.376, so the DoF difference calculation is pretty easy. Use the crop factor as a multiplier to get the DoF equivalent as well.

This also illustrates why most Medium Format lenses don't open up past f/3.5 in general. The DoF at f/3.5 is very narrow already, an 1.4 lens on a Medium Format body would be practically useless.

This also illustrates why FF cameras will in general have a sharpness advantage for certain types of photography. If you're shooting at f/1.8 on a 1.6 crop camera to achieve a certain Depth of Field effect, the same image can be obtained at f/3.6 on a FF camera, which with most lenses is much closer to the "sweet spot" in terms of sharpness.

Edited by Sam Bennett on Aug 12, 2005 at 08:23 AM GMT

Aug 12, 2005 at 01:14 PM

EnCapture
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p.1 #7 · DOF and Sensor Size

those are 2 different lenses so of course the DOF will be different. the lenses have different viewing angles. if however how simply took a few steps back to achieve the same composition, the DOF remains the same.

Edited by EnCapture on Aug 16, 2005 at 06:13 AM GMT

Aug 12, 2005 at 01:20 PM

Sam Bennett
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p.1 #8 · DOF and Sensor Size

EnCapture wrote:
those are 2 different lenses so of course the DOF will be different. the lenses have different viewing angles. if however how simply took a few steps back to achieve the same composition, the DOF remains the same.

Sorry, what do you mean? The example I've given results in the same composition. The point is that the issue surrounds the fact that people don't use the same focal length on each body, that's where the difference comes from. The example you're giving is totally irrelevant. It's true, but it has nothing to do with the users question, ultimately.

Aug 12, 2005 at 01:21 PM

bouch
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p.1 #9 · DOF and Sensor Size

Hrow wrote:
Thank you, that's what I thought. For the life of me I couldn't figure out why the sensor size would impact DOF. Film size doesn't. .

Film size doesn't? Of course it does. Ever notice that 4x5 shooters have to use something like f/64 and movements to get acceptable DOF?

Sam's point above is exactly right - it doesn't make any sense to compare DOF of two camera systems with the same distance to subject and focal length because no one shoots that way. I don't shoot 1.6x tighter on my subject when I'm using my 20D (or 5x tighter when using my digicam). I frame the picture the same way and use a shorter focal length compared to what I would use full-frame.

Aug 12, 2005 at 03:00 PM

nsbca
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p.1 #10 · DOF and Sensor Size

EnCapture wrote:
those are 2 different lenses so of course the DOF will be different. the lenses have different viewing angles. if however how simply took a few steps back to achieve the same composition, the DOF remains the same.

The DOF increases as the fucusing distance increases with any given focal length at any given f/stop. Taking a few steps back will change the DOF.

Aug 12, 2005 at 04:24 PM

Imagemaster
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p.1 #11 · DOF and Sensor Size

Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.

Aug 12, 2005 at 04:35 PM

EnCapture
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p.1 #12 · DOF and Sensor Size

Imagemaster wrote:
Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.

exactly right and that was the initial question

Edited by EnCapture on Aug 16, 2005 at 06:14 AM GMT

Aug 12, 2005 at 04:46 PM

steve_t
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p.1 #13 · DOF and Sensor Size

Imagemaster wrote:
Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.

For a given focal length what effects DOF is two things :-

1) Distance to subject
and
2) Amount of enlargement for printing.

For a fixed FOV, the distance to the subject will vary based on sensor/film size. For example a MF body will be closer to the subject than a 1.6 crop DSLR and thus will have a narrower DOF. This is mitigated slighly by the enlargement factor. Enlarging to make a print enlarges the CoC (Circle of Confusion) and reduces the DOF. The 1.6 crop needs much more enlargement than a MF frame so there is a reduction in comparative DOF because of that. This effect is however much less than that of distance.

Aug 12, 2005 at 04:53 PM

Nathan67
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p.1 #14 · DOF and Sensor Size

Dof is affected... If I shoot a head and shoulders portrait with my FF 1dsmkII and my 24-70 f2.8 at 70mm, then put this same lens on my 1.3 crop factor 1dmkII I will have to step back to get the same compostion at 70mm.. and as "everyone" knows :- depth of field increases with distance from subject.... its as simple as that.

nathan.

Aug 12, 2005 at 04:56 PM

slin100
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p.1 #15 · DOF and Sensor Size

EnCapture wrote:
Imagemaster wrote:
Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.

exactly right and that was the initial question

Exactly wrong. EnCapture, the link you posted, http://www.photo.net/learn/optics/dofdigital/, explains why. Everyone should read the photo.net article. It's very succinct and easy to follow.

Aug 12, 2005 at 04:59 PM

EnCapture
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p.1 #16 · DOF and Sensor Size

the actual measurement of the depth of field is not affect by magnification or sensor size. the relative perception may be but that is it. if a portrait is taken and from the tip of the nose to the start of the ear is in focus, that will not change regardless of magnification. it may become slightly more noticable as magnification increases, but the actual physical distance from the tip of the nose to the start of the ear does not change. for a given lens and at the same distance from the subject, that measurable distance that is in focus will not change regardless of crop factor. it just becomes more appearant with magnification.

Edited by EnCapture on Aug 16, 2005 at 06:14 AM GMT

Aug 12, 2005 at 04:59 PM

firestick
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p.1 #17 · DOF and Sensor Size

Sam Bennett wrote:

85mm from 10ft @ f/2.8: 8.36 inches DoF
53mm from 10ft @ f/2.8: 13.8 inches DoF

Edited by Sam Bennett on Aug 12, 2005 at 08:23 AM GMT

Way to put it Sam. I couldn't have worded it better myself.

Aug 12, 2005 at 05:02 PM

bouch
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p.1 #18 · DOF and Sensor Size

Imagemaster wrote:
Neither sensor size nor film size affects DOF. Cameras and lenses do, but that was not the question.

WRONG

What does this mean? The format size doesn't affect DOF but the camera somehow does? How do you figure? Isn't format size sort of tied to the camera?

I can pretty easily demonstrate that my digicam has more DOF than my 1Ds. The cause of this according to you, is not the format size, but some other inherant propropty of the camera? Do you mind telling me what?

Why don't you just stay out of these discussions if you don't have clue?

Aug 12, 2005 at 05:12 PM

Sam Bennett
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p.1 #19 · DOF and Sensor Size

I can't believe people still argue about this.

Aug 12, 2005 at 05:14 PM

bouch
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p.1 #20 · DOF and Sensor Size

steve_t wrote:
For a fixed FOV, the distance to the subject will vary based on sensor/film size. For example a MF body will be closer to the subject than a 1.6 crop DSLR and thus will have a narrower DOF.

The MF body will not be closer to the subject. The MF body will choose a longer focal length. That's like saying that when I shoot 4x5 I always get a couple inches away from my subject because all I have is ultra-wides.

Aug 12, 2005 at 05:14 PM

Volleybob
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p.1 #21 · DOF and Sensor Size

I gave the perfect answer as the first reply. Why must you clowns further confuse the original poster?

Aug 12, 2005 at 05:21 PM

steve_t
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p.1 #22 · DOF and Sensor Size

bouch wrote:
steve_t wrote:
For a fixed FOV, the distance to the subject will vary based on sensor/film size. For example a MF body will be closer to the subject than a 1.6 crop DSLR and thus will have a narrower DOF.

The MF body will not be closer to the subject. The MF body will choose a longer focal length. That's like saying that when I shoot 4x5 I always get a couple inches away from my subject because all I have is ultra-wides.

READ THE BIT ABOUT "for a fixed focal length"!

In practice you'll use different lenses depending on your body, but assuming you mount, say, an 80mm MF lens on a 4x5, a 1DsII and a 20D and set them up for the same field of view then which will be closest to the subject?

Aug 12, 2005 at 05:21 PM

Sam Bennett
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p.1 #23 · DOF and Sensor Size

Volleybob wrote:
I gave the perfect answer as the first reply. Why must you clowns further confuse the original poster?

Because your reply completely misses the point. The question isn't asking about within one camera. I suspect that the original poster is talking about when people say that a FF camera has narrower DoF than a 1.6 crop camera. You have to look at it from the practical standpoint - for equivalent FoV, taken at the same distance (which means the user would be using different lenses to get equivalent FoV) the answer is yes. This is an incontrovertible fact. It is true when comparing Medium Format to 35mm, 35mm to 1.6 crop digital and 1.6 crop digital to consumer digicams.

Aug 12, 2005 at 05:29 PM

slin100
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p.1 #24 · DOF and Sensor Size

EnCapture wrote:
the actual measurement of the depth of field is not affect by magnification or sensor size. the relative perception may be but that is it. if a portrait is taken and from the tip of the nose to the start of the ear is in focus, that will not change regardless of magnification. it may become slightly more noticable as magnification increases, but the actual physical distance from the tip of the nose to the start of the ear does not change. for a given lens and at the same distance from the subject, that measurable distance that is in focus will not change regardless of crop factor. it just becomes more appearant with magnification.

Encapture, I do not agree with this explanation.

In an image technically only one distance is in perfect focus. All other distances are out of focus by various amounts. The near and far distances at which the blur is perceived as being out of focus is what defines DOF. Therefore, magnification plays a central role in determining DOF because it directly affects the perceptibility of blur.

DOF is not just some fixed quantity inherent to a lens. DOF is entirely a subjective matter. The DOF scale on a lens is set based on the manufacturer's judgement but someone with a more discerning eye might not agree and move the DOF markings closer together.

A practical consequence of the effect of sensor size on DOF is that one should use the DOF markings on a lens approximately 1 1/3 stops wider than the actual aperture when using the lens on a 1.6x crop camera.

Aug 12, 2005 at 05:31 PM

nsbca
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p.1 #25 · DOF and Sensor Size

slin100 wrote:

A practical consequence of the effect of sensor size on DOF is that one should use the DOF markings on a lens approximately 1 1/3 stops wider than the actual aperture when using the lens on a 1.6x crop camera.

I don't buy that. That is the part that doesn't change.

Aug 12, 2005 at 06:35 PM


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