How do straight edges end up curved in photos?

Question

In the standard model for a lens straight lines in the subject of a photograph will always end up straight. However I have seen a variety of photographs in which straight lines end up being curved, e.g. the horizon tapers down at the edges, or a fence near the photo border is mildly curved.

What could cause straight edges to come out curved in a photograph? I have no idea what cameras were used in these cases, but I am certain they are standard consumer grade cameras (so they aren't fisheye type lenses such as equisolid angle lenses). Believe it or not I have to make computations based around objects in photographs and have no way of accounting for why straight things turn out curved in some photographs. From what I know about optics, this shouldn't happen, but it does, so I am missing something. What is it?

Can anyone give authoritative information about this?

It is barrel and pincushion distortion. Wide angle lenses also have perspective distortion. I believe you can find a great deal of information on this site on these topics already. — dpollitt, Jan 18 '12 at 20:39
Like What are Barrel and Pincushion distortion and how are they corrected? — mattdm, Jan 18 '12 at 20:44
Thank you very much. I will look into those. Don't know what to do about an official question answer. — John Robertson, Jan 18 '12 at 20:59
You can leave it unanswered, it can be closed as a duplicate, or you can read the other topics, and answer your own question, if no one else does in the meantime. — MikeW, Jan 19 '12 at 04:36

score 1 · Accepted Answer · edited Jan 19 '12 at 14:08

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You only get completely straight lines using a pinhole camera. As soon as you put a lens element in the path of the light, you get the fisheye-like distortion to some degree.

Modern lenses contain several lens elements which compensate for each other to give straight lines, but you usually get a slight distortion anyway. For really expensive lenses you have a very small distortion, but it's still there.

edited Jan 19 '12 at 14:08

mattdm

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answered Jan 19 '12 at 14:02

Guffa

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Didn't know that. By fisheye like distortion do you mean that a single parabolic lens has barrel distortion? – John Robertson Jan 31 '12 at 00:39

How do straight edges end up curved in photos?

1 Answers1