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1- I know different focal lengths change the depth and flatness of an objects in the picture. For examples longer focal lengths make objects appear flatter so they will be appropriate for portraits. Please see (https://en.wikipedia.org/wiki/File:Focal_length.jpg)

2- I also know that crop sized sensors have a more limited angle of view in comparison to full frame sensors. For example, the angle of view of a 35mm lens on crop sensor camera is almost equivalent to 50mm in full frame.

Given 1 & 2 from above, do crop sensors also change the depth and flatness of objects? I mean, does a 35mm on DX (Nikon) have a different perspective than FX (also Nikon). I've heard that crop sensors only change the angle of view and that they are similar to digital zoom at the center on the picture when in comparison to full frame (so they cannot change the relative depth and perspective). However I also heard that crop sensors also change the depth of field. What does that really mean?

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I think this is covered here - different question, but the answer explains it - Is the angle of view actually different on APS-C?. The accepted answer also points to other questions which further explain. –  MikeW May 16 '13 at 7:45
    
possible duplicate of Does sensor size affect lens distortion? –  mattdm May 16 '13 at 10:10
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Depth of field is also affected, given some assumptions; see this for more. –  mattdm May 16 '13 at 10:13

3 Answers 3

up vote 9 down vote accepted

Foreshorntening (the technical term for the effect of "flattening" objects) is determined by subject distance only, not focal length.

When using a wide angle lens if you are the same distance from your subject as you would be when shooting with a portrait lens, you'll get the same flattering effect, only your subject will take up less of the image.

A crop (DX) sensor will therefore produce exactly the same flatness of objects as a full frame (FX) sensor, provided subject distance remains the same. Furthermore if the full frame lens is 50% longer than the crop lens and has the same size entrance pupil then field of view and depth of field will be the same, thus the two images will be virtually indistinguishable.

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check out this comparison. He doesnt make sure the aperture (absolute size) is equal, so he gets narrower dof on the FF camera, though: kennylovrin.se/post/13455063816/… –  Michael Nielsen May 16 '13 at 8:07
    
Says URL not found for me. Can you double check it? –  BBking Jan 28 at 9:03
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Just to add "evidence" to this answer.. if the opposite was true, every portrait taken with a smartphone/small compact would have impossibly big noses because the focal length on those cameras is on the likes of 2mm. –  Marco Mp Jan 28 at 12:48

This had me scared when I switched to FX from DX. I calculated that 24mm on FX would be the same as 16mm on DX. I spent many thousands, and while waiting for my gear to arrive, I read that the perspective would be different. Needless to say, I was unhappy, but I was able to perform a definitive experiment once the gear arrived.

The left side is FX at 24mm and the right is a DX camera at 16mm. As I had originally surmised, they are the same. 24mm fx vs 16 mm dx

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The field of view is clearly the same from your examples. I know that this means that perspective is the same, but since that is the primary concern of the question, it would be nice to have something that shows that better.... –  mattdm Jan 27 at 19:46

Disclaimer: A Canon 5D MKIII has a Full Frame sensor size. The OLYMPUS E-P2 has a 4/3 (pronounced, "Four Thirds") sensor size and has a crop factor of x2.

Do crop sensors on SLRs changes the depth and flatness of the objects as well?

Yes and no. It depends if you're finding "equivalence" or just simply shooting.

A "crop factor" (eg, x1.5, x1.6, x2) is to find the equivalent field of view to the conventional standard of 35mm (or 135) film.

does a 35mm on DX (Nikon) have a different perspective than FX (also Nikon)

This answer explains that the focal lenth is a property of the lens and the field/angle if view changes when using the same focal length.

Please see below some photos taken with a 50mm f/1.8 OM lens, at 40cm (subject distance):

Canon 5D MKIII Canon 5D MKIII

OLYMPUS E-P2 OLYMPUS E-P2

Notice that the DoF is the same, both the +/- 24 are blurred. Also notice that it's a different perspective. It is kinda "similar to digital zoom at the center on the picture" but not exactly the same.

Now, I want to show you a field of view "equivalence" comparison.

Same lens:

Canon 5D MKIII Canon 5D MKIII, f/5.6 @ 40cm (subject distance)

OLYMPUS E-P2 OLYMPUS E-P2, f/2.8 @ 80cm (subject distance)

Here, the DoF and field of view (just) is the same. As you can see, it has a different perspective. Changing the sensor size, in turn changes the focal length/subject distance but only for the same composition! It also changes background compression (or perspective distortion).

If you're not worried about technicalities, there's nothing to worry about. Shoot in which ever format you like! :)

Note: This slightly falls into the many questions asked about DoF and full frame.

EDIT: As Lee Davison also points out, you will get a similar field of view when using equivalent focal lengths.

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Sorry, I got carried away with DoF equivalence. Edited answer to suit the question more. –  BBking Jan 29 at 13:42

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