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?
Answer
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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