Talk:Perspective distortion (photography)

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This article previously claimed that perspective distortion was caused by lens focal length, and it had the title "Perspective distortion caused by lens focal length".

This is factually inaccurate, since perspective distortion of the sort described here (big noses in portraits, and as shown in the images included in the article) is a function solely of camera to subject distance. Anyone with a camera and lenses of at least two different focal lengths can do an experiment to demonstrate this.

Confusingly, later material in the article (the text describing the photos included in the article) correctly explained that the effect was caused by camera to subject distance, and that reframing identically with a wider angle lens required placing the camera closer to the subject, thus causing this perspective distortion.

I altered the introductory text to correct the factual inaccuracies. I slightly altered the later text describing the images to increase clarity a little. Then I moved the article so that it would not have a title that was factually incorrect. Then I went through the rest of Wikipedia and updated links so that they point to the new location. I've left a redirect at Perspective distortion caused by lens focal length for the usual reasons, and also because the myth that focal length affects perspective distortion is widely believed, and the article might well be searched for under these terms. Jeff Medkeff | Talk 16:11, Jan 31, 2005 (UTC)



Discussion prior to article move follows:

A close up portrait shot with a, say 24 or 28 mm, compared to a portrait of the same person using a 105 is in my opinon the best example of perspective distortion -- all assuming a 35 mm camera ;-). -- Egil 18:52 Jan 29, 2003 (UTC)


nice addition, Egil. could you explain about "pillow" distortion? I've never heard of it. And, yes, it is a 35mm camera. I'll try the portraits when I next get a chance (probably in late february!) That amateur photographer, Koyaanis Qatsi

It is usually called cushion, mea culpa. A perfect wide angle shall never produce curved lines. But even the best of extreme wide angle lenses always produce curved lines to a certain degree, esp. close to the image boundaries. So if you take a picture of a rectangle, it ends up looking like a cushion. A fish-eye is a lens where they have given up correcting this completely. See Aberration in optical systems.-- Egil 19:14 Jan 29, 2003 (UTC)
Right, I knew about the fisheyes. For 35mm cameras, doesn't the designation "fisheye" lens start at 18mm? I had a (one) photography class in June 2002 and then I bought a camera & started taking photos. I'm trying to fact check. Do you think that the bit about curved lines should be removed or rephrased? I take it, it seems misleading. Anyway, I hope I'm not harming the project adding this stuff. Best, and thanks, Koyaanis Qatsi 19:19 Jan 29, 2003 (UTC)
No, no, no! On the contrary, this is extremely useful! In the Wiki world, the initial version of an article is never like the latest - things are developed! I think the distinction wrt. fisheye is the distortion, not the focal length. There are fisheyes > 18 mm and wide angles < 18 mm. But the shorter the focal length, the more difficult it is to get rid of the cushion 80.202.80.14
Interesting stuff. I'd love to see a wide-angle lens >18mm which doesn't cushion the corners. Koyaanis Qatsi

I'm taking the pics back out of the table, as for people with a narrow browser window the table is too wide. By simply sticking them next to each other, we let the browser wrap them into a column -- which is not as satisfying as seeing them in a quad arrangement, but vertical scrolling is much more comfortable for the web reader than horizontal scrolling. --Brion 19:29 Jan 29, 2003 (UTC)


Please note that the link to "Perspective Distortion: Source," added to the bottom of this article points to a new article in which the Conclusion is at variance with the conclusion with this article. Suggest this page is too flawed for minor editing and be replaced with Perspective Distortion, Source. See link at bottom.Pat Kelso 22:57 CST Jan 30, 2004


The title isn't OK this article is about perspective distortion and not lens distorsion (barrel distortion for instance). Ericd 09:51, 7 Mar 2004 (UTC)

Perspective distortion, p e r - s e, has nothing to do with lenses. Please see Perspetive distortion, Conclusion. Thank you. Pat Kelso 18:46, Mar 7, 2004 (UTC)
I will fully disagree, there are several form of perspective distortion, each time perspective is distorded for any cause there is perspective distorsion. The expression clear enough in itself to say what is "THE REAL" perspective distotion. This article deals about the influence of focal length on perpective representation in photography, it's a a important topic in photography, and this kind of distorsion has to do with camera lens. I'm was'nt a great fan of the previous title for the reason exposed before. But I find the present title worse because they are form of distortion (barrel & pillow) caused by the defect of the lens.
BTW what is your new article about ? Seems to be about 3D to 2D projection isn'it ?
If talking photography distortion w/photos, it is not my article ... Pat Kelso 21:09, Mar 7, 2004 (UTC)
And please next time you want to change the title of and article use "Move this page" this will cause less confusion and keep the talk page relevant.
As wikipedia is a collaborative effort can you help me to give accurate titles to the articles as my English is somewhat limited I have to idea how to solve this issue.
Ericd 19:19, 7 Mar 2004 (UTC)

Sorry saw the 'move this page' suggestion too late. Will use in future if my poor brain can retain it that long. But have moved my part of Perspective distortion via cut and paste to Perspective projection distortion ... this to accomodate objection that 'persepctive' can refer to many other things.... Pat Kelso 19:47, Mar 7, 2004 (UTC)

Please in the future use the "Move this page" method.
-- Egil 20:07, 7 Mar 2004 (UTC)
Yes sir. Pat Kelso 21:26, Mar 7, 2004 (UTC)

OK I move this one to "Perspective distortion caused by lens focal lenght" Ericd 19:56, 7 Mar 2004 (UTC)


Could also call it "Perspective compression and extension", since this technique can be applied equally well for paintings (although not seen very often). Perspective distortion should be a pointer to the two articles. Ideally, both should be combined in one article. -- Egil


Or could the article be called "Perspective change"? -- Egil 20:35, 7 Mar 2004 (UTC)

I don't know all this needs some serious brainstroming to give some structure to the question. I remember an interresting article when I was much younger and new to photography. Some paintings have true perpective according to optical laws while other have faked perspective. Aùmong the interresting case in paintings is "La última cena" from Leonardo da Vinci that as an accurate super-wide perspective did Leonardo get this by geometry reasoning or by a "camera obscura" view ?
Ericd 21:32, 7 Mar 2004 (UTC)


________________________

re "Some paintings have true perspective according to optical laws while other have faked perspective. Among the interesting case in paintings is "La última cena" from Leonardo da Vinci that as an accurate super-wide perspective did Leonardo get this by geometry reasoning or by a "camera obscura" view ?"

  • There is no such thing as "true perspective according to optical lays", except as noted in Perspective projection distortion. It has no application here.
  • "Leonardo da Vinci that has an accurate super-wide perspective." It may be an accurately drawn perspective projection - but which has innate distortion. "Camera obscura"?? Please!
  • The "fake" perspective used by Leonardo was used to ameliorate the natural distortion of perspective projection and which becomes more pronounced as the picture widens. Pat Kelso 09:34, Mar 22, 2004 (UTC)

[edit] ==

The focal length of a lens has no dilatory effect on perspective, p e r s e. The perspective is always correct for the focal lengths involved. The focal point of a lens is analogous to the station point of perspective projection - a point that may be moved without changing the correctness of a perspective projection. Will leave this posted for few days for comment before incorporating rationale into the article. Pat Kelso 01:53, Mar 25, 2004 (UTC)

[edit] Need better examples

Wow, no talk in over 2.5 years. This article could use better photo examples, esp. including something forward of the main subject and some more clearly identifiable background objects whose sizes can be easily noticed. A set of only 2 or 3 in more normal layout should be fine. Dicklyon 18:11, 9 December 2006 (UTC)

Also need photo examples of the distortion appearing in many pictures shot with telephoto lenses and a couple of photo examples of the artistic use of perspective distortion that aren't copyrighted. Anoneditor 02:05, 11 December 2006 (UTC)

[edit] Dependence on viewing distance

I'm not sure I like this new treatment. I'm quite familiar with the traditional 19th-century analysis of "correct perspective" based on the viewing distance compared to the focal length times enlargement factor. But it's been presented here without the enlargement factor, so it makes little sense. Even with enlargement factor, it made only theoretical sense. The alternative is to say that perspective is just the relative size of objects in the scene, and that it can be exaggerated by shooting very close, or distorted the other way by shooting from far away. You might still want to invoke a "normal" viewing distance, but of course it will depend on print size if you do. In practice, I don't think you'll find that the perception of perspective distortion will abate much at all by changing your viewing distance. Geometrically, it makes theoretical sense, but the perception of photos doesn't necessarily follow the simple-minded rules that would make it look that way. Is this treatment following some book? If so, need to cite it. If not, let revert it. Dicklyon 07:23, 11 December 2006 (UTC)

Before I wrote this piece, I tried shortening the viewing distance for wide angle pictures with closer objects and telephoto shots of objects at a distance, and it seemed to me that the apparent distortion lessened as stated in the article. There were two problems, however, which might be mentioned: First, if you move back far enough for the telephoto shots, the picture gets awfully small. A practical problem but not a theoretical one. Second, on the wide angle shots, the problem was that with really wide angle lenses, you can't get close enough and still have your eyes focus and be able easily to view the perimeter of the picture. Again, a practical but not theoretical problem.
Exactly. It works in theory, but not so well in practice.
Have you tried it? Though I read about this issue long ago (I was one of the users of Kodachrome 10) I'm not quoting a source on this; I resorted to the experimental method and, as a result, was satisfied that my memory was correct on the point.
I have. I guess I'm a youngster, since Kodachrome II was my thing.
As to the magnification factor, I completely agree that it should be included to have a rigorously correct article, but I felt that what we're doing here is imparting general knowledge. I can't image anyone trying to calculate the distance from which the picture should be viewed to ameliorate or eliminate the distortion in ordinary circumstances, especially if the point of the picture is to create the apparent distortion. However, I certainly won't be offended if you want to include a more mathematical treatment of this issue.
Well, the viewing distance stuff doesn't really work as general knowledge without the theoretical background and the other relevant factor. I'd rather leave it out, which is more conventional in treating perspective. That is, treat perspective as determining relative sizes of objects in images based on their relative distances, and "distortion" in a qualitative way as meaning when those relationships look odd. Those relationships don't change with viewing distance, so that can be safely omitted, as it usually is these days.
Just as an aside question, why do you feel that the geometry isn't particularly relevant to the perception issue?Anoneditor 17:38, 11 December 2006 (UTC)
I think that when people look at a photo they see it as a photo. Viewing it closer or further doesn't change the relative sizes of things, and we're used to allowing the absolute sizes of things in images to vary with viewing distance. People are not perceiving a photo as if they're looking through it at the world, usually. Just my impression. Dicklyon 22:23, 11 December 2006 (UTC)