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Thread: Is there a difference in perspective distortion btw FF and crop?

  1. #1

    Default Is there a difference in perspective distortion btw FF and crop?

    For examples:
    comparing a 18mm lens on a FF cam, with a 12mm lens on 1.5x crop cam, will there be differences in the perspective distortion? (keystone?)

    I've read that telephoto make the background seem closer to the subject (compression), so would this effect be more pronounced if i move a 50mm from a FF onto a 1.5x crop? (getting 75mm)

  2. #2
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    Default Re: Is there a difference in perspective distortion btw FF and crop?

    Quote Originally Posted by ahsosl1e8 View Post
    For examples:
    comparing a 18mm lens on a FF cam, with a 12mm lens on 1.5x crop cam, will there be differences in the perspective distortion? (keystone?)

    I've read that telephoto make the background seem closer to the subject (compression), so would this effect be more pronounced if i move a 50mm from a FF onto a 1.5x crop? (getting 75mm)
    The distortion is the same for a 18mm/FF and a 12mm/1.5x, assuming the same aspect ratio on the sensors. The angle of view is the same in both cases.

    For the second case, the answer is no. The scene from the "75mm" printed 2.25x smaller will superimpose perfectly onto the 50mm picture. They are essentially the same picture, just cropped. The "compression effect" comes from a relative magnification of background objects with respect to the foreground subject, and in this case there is no difference because it's the same lens, same foreground subject distance, same background subject distance.
    Last edited by ST1100; 8th July 2007 at 12:58 AM.

  3. #3

    Default Re: Is there a difference in perspective distortion btw FF and crop?

    Quote Originally Posted by ahsosl1e8 View Post
    For examples:
    comparing a 18mm lens on a FF cam, with a 12mm lens on 1.5x crop cam, will there be differences in the perspective distortion? (keystone?)

    I've read that telephoto make the background seem closer to the subject (compression), so would this effect be more pronounced if i move a 50mm from a FF onto a 1.5x crop? (getting 75mm)
    Perspective distortion is entirely dictated by the field of view so there should be no difference. The only thing that will make them different would be the geometric distortion due to the construction of the lens optics.

  4. #4

    Default Re: Is there a difference in perspective distortion btw FF and crop?

    For the second case, the answer is no. The scene from the "75mm" printed 2.25x smaller will superimpose perfectly onto the 50mm picture. They are essentially the same picture, just cropped. The "compression effect" comes from a relative magnification of background objects with respect to the foreground subject, and in this case there is no difference because it's the same lens, same foreground subject distance, same background subject distance.
    Ok, so in this case, the relative separation between the foreground and background objects are exaggerated in 12 x 1.5crop compared to a FF 18mm then?

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    Default Re: Is there a difference in perspective distortion btw FF and crop?

    Quote Originally Posted by ahsosl1e8 View Post
    Ok, so in this case, the relative separation between the foreground and background objects are exaggerated in 12 x 1.5crop compared to a FF 18mm then?
    No. If the angle of view is the same, camera-subject distances are the same, the two pictures would look the same distortion/compression-wise. The main difference would be the 18mm having a shallower DOF for the same f-number IF they are printed to the same size. You are essentially taking the same picture in two different formats.

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    Default Re: Is there a difference in perspective distortion btw FF and crop?

    Quote Originally Posted by ahsosl1e8 View Post
    For examples:
    comparing a 18mm lens on a FF cam, with a 12mm lens on 1.5x crop cam, will there be differences in the perspective distortion? (keystone?)

    I've read that telephoto make the background seem closer to the subject (compression), so would this effect be more pronounced if i move a 50mm from a FF onto a 1.5x crop? (getting 75mm)
    The perspective depends solely on the position and orientation of the camera relative to the object/scene. In other words, it is completely independent of focal lengths, image format, etc. The only difference would arise from image defects (e.g. pincushion/barrel distortion).

    The erronous belief that perspective depends on focal length probably stems from the fact that wide-angle lenses are frequently used from a close distance, and tele lenses are frequently used from a large distance, in order to "fill the frame" with the subject. But the resulting difference in perspective is entirely due to looking at the subject from different positions, and/or a different tilt of the camera.

    To give an example, if you take a photo of two objects 10 and 20 metres away, respectively, they will obviously be depicted at very different scales. If you take a picture of the same objects from 1 km and 1.01 km away, the difference in scales will be negligible, although they are still separated by the same distance. This is what people (erronously) refer to as "compression".

    You can try it out yourself - take a scene from the same spot at different focal lengths, and crop the results so that you always end up with the same field of view. The result will essentially be identical.

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    Default Re: Is there a difference in perspective distortion btw FF and crop?

    Just look at the difference between a digital compact and a dslr. The digital compact has a much larger crop factor than any dslr. At the same 35mm equivalent focal length, the compression, field of view etc will look essentially the same. So in this respect, there is really no difference in perspective.
    However, if you look at the depth of field, you find that usually the picture in the frame in a digital compact is usually sharp throughout the frame. So essentially, you'd probably see a difference between the depth of field in a FF and a crop, which in some sense, may change the perspective.

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