Focal focus with crop sensor?

[pico]

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OK, here is a simple, clear answer to the specific question originally asked.  No, the depth of field will be different.  The image taken with the larger format sensor will have more depth of field.  This is true regardless of size and viewing distance.

 

The longer answers explain, "why" and "by how much" and "what about if you use lenses with equivalent fields of view."  

 

So, if I make an image on 4x5" film with a 135mm lens at ƒ3.5, subject at 3m it will present on a contact print with the same DOF as an image made on 35mm full-frame with the same lens enlarged to 4x5" (with acceptable trimmed aspect ratio of the 4x5) enlarged to 4x5" Correct?

 

I'm looking for affirmation, not conflict. BTW - Thank you, Jared, for your deep analysis/post.

 

OI! I should delete my post because it contains a fundamental error, however I will leave it to encourage constructive correction.

Edited May 13, 2018 by pico

[Jared]

Assuming you used the same lens and the same aperture for both cameras and didn’t move the lens/camera you would not get the same depth of field since the contact print received no magnification and the 35mm image would be enlarged about 4x (ignoring aspect ratio differences). The lens doesn’t know what size sensor is attached so the blur circles at the imaging plane would be identical, but to get your 4x5” print only the full frame shot requires enlargement. This, the circles of confusion in the print from the full frame shot are larger. The 4”x5” shot would have more depth of field. It would also have a very different field of view, of course. A 135mm on 4x5” gives about the same field of view as a 35mm on full frame, while it’s a moderate telephoto when attached to a 35mm camera.

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[01af]

The answer is quite simple. At the same distance and aperture, the smaller format will yield less depth-of-field when using the same lens (as in the original question), and more depth-of-field when using an equivalent lens (i. e. a lens with shorter focal length). The reason is because depth-of-field is proportional to the linear image size (smaller sensor = less DOF) and inversely proportional to the square of the focal length (shorter focal length = much more DOF).

 

The depth-of-field scale on the barrel of a lens originally meant for 35-mm format (such as an M or SL lens) is valid only when using that lens on a 35-mm-format camera. When using this lens on a smaller-format camera (such as a Leica M8, TL, or CL) then for the same depth-of-field the lens need to be stopped down by approx. one additional stop (1.23 stops for APS-C format, to be precise, or 0.82 stops for the M8). So when shooting at, say, f/8 then read the resulting depth-of-field off the scale at f/5.6.

[gyoung]

This topic generates enormously long threads without much light. The source of confusion is that the lens hasn't changed, and the scene hasn't changed, but the image inside the camera is smaller, and if you are comparing, e.g., an SL with a CL, the pixels are smaller by a factor of 1.5 in linear dimension. So if you want to print or display the resulting picture the same final size, the CL image will have to be enlarged by 1.5x as much. Even though the portion of the image captured by the smaller chip is just as sharp as with the larger chip, you might want to have tighter standards and thus consider the depth of focus to be less with the smaller chip and its closer pixels.

....and the higher magnification of the image when printing. DoF is of course an optical illusion, exploiting the failings of the eye and brain to aee what is actually 'sharp'. This is exacerbated if you peer at images at 100% on a monitor rather than view a print at 'normal viewing distance'.

In practical terms comparing my experience with FF and Fuji aps-c, for a given field of view there is more depth of field at a particular aperture with aps-c than full frame (e.g. with my 14mm at f/4 on Fuji as compared to a 21mm lens on Leica film).

Little point in comparing depth of field for the same focal length as the field of view is different, and thus the use you would be making of the lenses.

 

Gerry