Depth of field: Full-frame lens vs. APS-C lens, both on APS-C sensor

elemental · Oct 13, 2014

Here's an odd question I've been struggling to work out in my head - if I'm missing an obvious answer, please feel free to point me to it - using Nikon terminology because it's easier (for me, at least):

Given the same focal length and aperture and the same sensor, will a larger-format lens produce a shallower depth of field than a smaller-format lens? For example, would a 35mm DX Nikon lens at f/4 on a DX-format camera produce the same depth field characteristics as a 35mm FX lens at f/4 on the same camera body? My brain says it has to, but when I sketch out the way I think it should work, my sketch says the larger-format lens is shallower. And let's say I'm just asking about the depth of field here, I know the actual appearance of out-of-focus areas is a much subtler phenomenon.

Also, good to see this place is still here! It's been a while.

Derrel · Oct 14, 2014

IDENTICAL depth of field regardless if the lens is a DX-Nikkor, or a regular Nikkor. Just as a point of fact there have been a few lenses labeled as DX Nikkors, but there has never been any lens labeled as an "FX" lens. Not that you implied there had been, just wanted to point out that 99.9% of all lenses Nikon has made since 1959 have been Nikkors in F-mount.

Anyway, say you have the 35mm f/1.8 DX-Nikkor AKA "the cheap $199 one", and the newest 35mm Nikkor, the new 35mm f/1.8 G, AKA the "expensive one". EACH lens will give the same, exact depth of field at each and every f/stop when used on any DX-format Nikon camera body.

Same with the 55-200mm DX-Nikkor lenses, and corresponding focal lengths found on regular Nikkor zooms: at 85mm and f/8, the DOF will be the same on a 55-200 DX lens as it will be at f/8 on the 70-200 VR-II.

Also, since we're on the subject, a 35mm DX Nikkor has a focal length of 35 millimeters; the new 35/1.8 G-series Nikkor also has a 35 millimeter focal length, as does the older 35mm f/2 AF-D model. And the same is true of the 35mm f/1.4 AF-S G lens--it too is a 35 millimeter focal length lens. Your brain telling you that a DX lens and an FX lens of the same focal length on the same camera body will produce different depth of field characteristics is imaginary, and incorrect.

elemental · Oct 14, 2014

Thanks Derrel – I knew in my head this had to be true, and I think I figured out the mistake in how I was sketching it.

Nikon 35mm F-mount lenses on DX was the simplest example I could think of to illustrate the question, but I'm actually thinking about 35mm SLR lenses on Micro Four-Thirds. Manual-focus lenses with focus peaking on a compact body seems like a great setup for travel, at least for me, and some of the mirrorless bodies remind me of the size of my KR-5 Super II, which I think is just about perfect for carrying around.

Braineack · Oct 14, 2014

So you actually want to know if f/4 @ 35mm on a APC-S is the same as f/4 @ 35mm on a 4/3?

then, no. No, it's not.

The 4/3 will have more DOF than the APC-S, and more than that of the FX, all other things being equal (including framing--which means distance to the subject is not the same).

example:

With a FX using a 55mm @ f1/6 with the distance to subject at 10ft, the total DOF is around 12ft.
With a DX (1.5x) using a 55mm @ f1/6 with the distance to subject at 15ft, the total DOF is around 18.5ft.
With a 4/3 (2.0x) using a 55mm @ f1/6 with the distance to subject at 20ft, the total DOF is around 25ft.

Ido · Oct 14, 2014

Then on Micro Four Thirds, an adapted 35mm lens at f/4 and a native 35mm lens at f/4, from the same spot, focusing to the same distance, will give the same depth of field.

snowbear · Oct 14, 2014

Correct. DoF is determined by sensor or film size, aperture, distance to the subject and magnification. All things being equal, the DoF will be the same.

KmH · Oct 14, 2014

If the lens magnifies more or less but the sensor size and lens aperture does not change, the point of focus distance has to change to keep subject scale constant if you want the DoF to stay constant.

So if one shot is made using a 50 mm lens, as sensor size and f-stop stay the same, and a second shot is made using a 200 mm lens the focus distance has to increase 4x to have the same DoF with the 200 mm lens the 50 mm lens had.

Braineack · Oct 14, 2014

Ido said:
Then on Micro Four Thirds, an adapted 35mm lens at f/4 and a native 35mm lens at f/4, from the same spot, focusing to the same distance, will give the same depth of field.

35mm is 35mm regardless of sensor size. (It's technically the distance of the glass from sensor plane when focused at infinity.)
f/4 is a function of diameter size to focal length.

So yes, the DOF will always be the same between two different 35mm lenses on a 4/3.

ruifo · Oct 14, 2014

Read this article. It will help you on this:
What is equivalence and why should I care?: Digital Photography Review

elemental · Oct 15, 2014

Thanks for all the replies. I'm quite comfortable with differences in depth of field and effective/equivalent focal length across format (I've been shooting 35mm and APS-C side-by-side for as long as I've had serious cameras), for some reason I got tripped up by the idea of a different size projected image (which, as we've established, isn't relevant).

I actually chose FX/DX initially because of how many nonsensical MFT vs. APS-C arguments I found trying to answer this question on Google. As I guess I should've expected, people are passionate about this stuff.

JerryLove · Oct 16, 2014

Braineack said:
With a FX using a 55mm @ f1/6 with the distance to subject at 10ft, the total DOF is around 12ft.
With a DX (1.5x) using a 55mm @ f1/6 with the distance to subject at 15ft, the total DOF is around 18.5ft.
With a 4/3 (2.0x) using a 55mm @ f1/6 with the distance to subject at 20ft, the total DOF is around 25ft.

With distance, not sensor size, being the deciding factor.

DoF is not directly affected by sensor size. DoF is affected by distance to subject, and sensor size may influence the distance at which you shoot (since your angle of view is different).

Braineack · Oct 16, 2014

however, according to fun online calculators:

let's say the two of us are standing 20ft from a beautiful subject.

I'm using my D600, you: a D7100.

I have my on body a 105mm 2.8 prime, and you: a 70-200 2.8--set to 70mm.

We both shoot from the same spot (distance of 20ft) and both using f/8.

I'll still only have 5.3ft of DOF, while you'll have 8.2ft...

The images will look more or less exactly the same (since 70mm is 105mm is 1.5x.), but for DOF.

CameraClicker · Oct 16, 2014

Braineack said:
however, according to fun online calculators:

let's say the two of us are standing 20ft from a beautiful subject.

I'm using my D600, you: a D7100.

I have my on body a 105mm 2.8 prime, and you: a 70-200 2.8--set to 70mm.

We both shoot from the same spot (distance of 20ft) and both using f/8.

I'll still only have 5.3ft of DOF, while you'll have 8.2ft...

The images will look more or less exactly the same (since 70mm is 105mm is 1.5x.), but for DOF.

Many of the on-line calculators have a problem. They suggest the same prime lens on different bodies FF and APS-C for instance will have different DOF if the camera and subject do not move and aperture is the same.

I have full frame and APS-C bodies, and prime lenses, so I got out a tripod, a target and a tape measure. Sensor size has no effect on DOF.

Sensor size lets you move the camera to frame the scene to match the other body... which changes the parameters that affect DOF. That's a totally different experiment. You can put a 10 mm lens on an APS-C Canon body and a 16 mm lens on a Canon full frame body, and the framing will look the same. View both at 100% and the differences become evident. If you have two APS-C bodies with different photo site densities, you can play other games too. Much attention has to be paid to how the question is framed, and how the reply is framed, or the results can be misleading.

Alexr25 · Oct 16, 2014

CameraClicker said:
Many of the on-line calculators have a problem. They suggest the same prime lens on different bodies FF and APS-C for instance will have different DOF if the camera and subject do not move and aperture is the same.

The major factors influencing depth of field are focal length, aperture and subject distance but there will indeed be a different DoF for different sensor sizes because the circle of confusion is a function of film or sensor size. The difference will not be great and will tend to slightly lessen the increased DoF due the increased subject to lens distance when shooting with a crop sensor.
So I would have serious reservations about any DoF program that DID give the same DoF with the same lens, f stop and distance setting but different sensor sizes.

Braineack · Oct 16, 2014

But that factors in a diamter limit of the sensor. The APS-C is litterally a crop, so that vaule should match that of an FX sensor should it not?

if I took a pictures, and literally swapped out the FX sensor for a DX, and using the exact same settings, took another picture the DOF should be identical--I just don't collect all the image information that went beyond the sensor diameter.

if

where c = CoC
then c, regardless of DX or FX, should be identical.

Which is the case since 5.3ft of DOF is exactly 65% of 8.2ft.

and the CoC of a DX sensor is 0.019mm vs 0.029mm of an FX, or that same 65% difference...