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The common assumption is that a crop will equal a longer focal length in perspective.

However, rectilinear ultrawides will very visibly distort faces and spherical/cylindrical objects that are well off center. That will remain if you take a crop from these areas.

If a frame equivalent to that crop was shot with, say, a 50mm lens from the same vantage point, would it show the same distortions (if you could force the 50mm to have the same improbable depth of field)?

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    Your "common assumption" is incorrect. Perspective is determined by distance from camera to subject(s). What a crop sensor does is change the angle of view for a given perspective. So taking the same shot from the same position, the crop sensor will give you an angle of view equivalent to c*f, but exactly the same perspective as the full-frame sensor. – twalberg Jan 19 '20 at 14:06
  • I mean a cropped image, not a cropped sensor. And: "same vantage point". Cut a portion of a 14mm image out vs use a 50mm from the same distance. – rackandboneman Jan 19 '20 at 17:09
  • Why not try it - lock down your tripod, use 2 lenses, or a zoom. I've done it for 'standard' zoom lengths, not tried ultrawide, but I don't see how it would be any different. See photo.stackexchange.com/a/97747/57929 – Tetsujin Jan 20 '20 at 8:49
  • @rackandboneman Your assumption makes even less sense with regard to cropping an image... How would cropping the edges off the image somehow change the perspective of the portion of the image section you are keeping? – twalberg Jan 20 '20 at 14:08
  • The assumption is more that focal length DOESN'T change perspective either, just framing.... – rackandboneman Jan 20 '20 at 15:56
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Crop factor gives an equivalent FOV/enlargement, not an equivalent perspective per-se as that is solely dependent on subject distance.

Crop factor cannot account for lens specific distortions... i.e. you cannot force a 50mm to distort like a fisheye lens does regardless of the distance it is used from.

I would have to see an example image to entirely answer the question. Perspective "distortion;" things like larger nose/smaller ears, are due to a short subject distance and a difference in the distance to other things w/in the scene. I.e. if you take an UWA picture of a perpendicular flat wall from 5 ft away, the distance to the wall is only 5 ft along the lens' centerline. If you instead measure a line from the lens to the edge of the FOV the distance is much greater.

Lens distortion tends to be the obvious bending of straight lines; barrel/outward bending or pincushion/inward bending... it does affect everything, but is most apparent in straight lines.

Perspective distortion will be the same regardless of the lens/crop as long as the cropping (in post, with FL, or crop sensor) is towards the center of the composition.

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  • ....so a rectilinear(!!!) ultrawide is supposed to have non-perspective distortions? – rackandboneman Jan 19 '20 at 17:10
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    Not "supposed to"; they are just extremely hard to eliminate. They're basically taking an 8-17mm lens that would other wise be a fisheye and trying to correct it to rectilinear. – Steven Kersting Jan 19 '20 at 20:43
  • Yeah, but are these lens flaws alone responsible for the ultrawide-typical face melting? – rackandboneman Jan 19 '20 at 21:31
  • I would have to see an example image. Perspective "distortion;" things like larger nose/smaller ears, are due to a short subject distance and a difference in the distance to other things w/in the scene. I.e. if you take an UWA picture of a perpendicular flat wall from 5 ft away, the distance to the wall is only 5 ft along the lens' centerline. If you instead measure a line from the lens to the edge of the FOV the distance is much greater. Lens distortion tends to be the obvious bending of straight lines; barrel/outward bending or pincushion/inward bending. – Steven Kersting Jan 19 '20 at 22:08
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    Accepting this because I was indeed confused by a misconception of a crop being a crop even if it is non center... if you think of it, that it is indeed only practically true with two dimensional subjects or very long focal lengths... – rackandboneman Jan 21 '20 at 20:04
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The common assumption is that a crop will equal a longer focal length in perspective.

A center crop.

However, rectilinear ultrawides will very visibly distort faces and spherical/cylindrical objects that are well off center. That will remain if you take a crop from these areas.

Which is not a center crop.

If you have a large frame camera or a tilt-shift lens, you can get exactly the same kind of distortion without cropping by tilting the image plane accordingly. The equivalences only hold when talking about a crop yielding the same projection/framing. Without the ability to shift/tilt lenses, that is a center crop.

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  • You mean shifting it? tilting it, of course, will distort anything :) – rackandboneman Jan 20 '20 at 15:55

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