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Can anybody tell me why the heads of the two women are stretched in the corners? What is the name of this defect and how could one avoid it? I'm not a photographer (though becoming one is one of my dreams) so I may not really understand much of your technical language. Links to further reading will be really appreciated.

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possible duplicate of What does it really mean that telephoto lenses "flatten" scenes? –  mattdm Mar 17 at 22:42
    
I would not have asked this question if I knew the terminology. This is the first time I encounter the term "telephoto lenses". I had a look at that question and honestly it was way above my head. I could not understand the question itself. Let alone the answers. –  Hamid Safdari Mar 18 at 7:06
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Yeah; consider it the further reading you were looking for. :) –  mattdm Mar 18 at 7:54

5 Answers 5

up vote 11 down vote accepted

It's not so much a defect as a property of wide-angle lenses: they distort things on the sides. This distortion is why they are not recommended for portraiture. Of course, the very same distortion is what gives wide-angle its power, making it great for photographing interiors (making them look larger) or when going for an "artsy" effect.

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Wide angle distortion. That was what I was looking for. Your answer helped. –  Hamid Safdari Mar 17 at 7:28

Two things common to Wide angle lenses can do this, barrel distortion and perspective distortion.

For barrel distortion, when pulling in a very wide amount of information from off axis, light gets distorted based on how extreme of an angle it is coming in at because the lens can't completely correct for how far off the lens axis the light is.

On the widest side, you have things like fisheye lenses which make no attempt to correct this or even utilize it to get a wider angle. This is what gives fisheye lenses their distinct look. On the flip side, higher end wide angle lenses reduce the amount of barrel distortion through larger and more complex optics, but come at a cost of price, size and weight.

It is actually possible to get barrel distortion on just about any focal length, but wide angle generally show it the most prominently as the difficulty in correcting depends entirely on how far off the center axis the light is coming from.

Perspective distortion, on the other hand, can not be addressed by even the highest quality lenses. The reason for perspective distortion is simply that, even when projected perfectly to a rectangle, if a camera is pulling in information from far to the side, it will end up looking a bit funny to us since we are seeing things normally outside our range of vision projected on to a flat surface.

As Stan pointed out, it appears that this is probably primarily perspective distortion since the screen itself is rectangular. There is also a possibility that barrel distortion wasn't properly corrected, but most likely it is just perspective distortion.

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That's not barrel distortion (there may be barrel distortion present in the image, but that's not what's affecting the poor women's heads). It's perspective distortion. The difference is that if you get you eye close enough to the picture (so that it mimics the camera position in the scene), perspective distortion goes away. The bulging of barrel distortion doesn't. –  user2719 Mar 17 at 14:27
    
Perspective distortion can introduce curves. An "easy" way to imagine it: imagine a "perfect" lens that has a 180 degree field of view that is taking a picture facing perpendicular to two infinitely long parallel lines. In the center of the image, we see the parallel lines having some separation, but at each edge of the image, at infinity, the parallel lines will be touching each other. There has to be some curvature to these lines to have separation in the middle and be touching at either end. –  Beska Mar 17 at 14:47
    
Barrel distortion would mean that the television would look bulgy with obtuse angles at the corners. It doesn't; it's pretty much perfectly rectangular. If the lens had been creating barrel distortion, it's been corrected out of the image. What's left is perspective distortion. –  user2719 Mar 17 at 16:41
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Actually if the lens has more barrel distortion it will usually have less of this "stretching" (perspective) effect. "Distortion" (while standard terminology) is not the best word for these phenomena, as it implies that the picture doesn't show the subject "true to life", but in a "distorted" way. What actually happens is that the lens needs to project (part of) a sphere (spherical view) onto a plane (the photograph). There are many ways to do this, each with different properties. You can experiment with transforming a photo to various ... –  Szabolcs Mar 17 at 20:49
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Keeping straight lines straight is not the only kind of property one might desire from a projection. Other useful things are: preserving the angles, preserving areas, etc. Projections with these other properties won't keep straight lines (hence they have barrel distortion), but they produce a more natural look in the corners. This is why I said that if the lens has barrel distortion, then it will usually have a less severe stretching in the corners. So your first statement, that barrel distortion can cause this, is not quite accurate. In fact a lens with barrel distortion will stretch less. –  Szabolcs Mar 17 at 20:56

Yes, wide angle lens distortion is the source here, but not the reason why those heads appear stretched so much. If we had the original photo as the lens imaged it on the sensor of the camera, we would see much less stretched heads and more of perspective and barrel distortion.

Correcting for the variety of distortions in post, or in some cases in-camera, stretches the corners of a photo in order to make straight lines straight. The frames of the television screen shown in the photo are very straight, which tells us the photo has been corrected of distortion. You can have either but not both; near-normal heads for the girls, or straight lines for the product displayed in the photo. Given the perspective distortion you would not get really normal looking heads anyway, so it seems better to correct the lines to appear straight and forget about those heads.

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Haha! I don't know if it's just me or otherwise, but I found a little humor in your statement. Anyways I found the photo while reading news and although it was although it was always a bugging question in my mind it never occurred to me I could ask it here. –  Hamid Safdari Mar 18 at 3:24
    
@happyCoder_ My mistake here was that I never thought there are real wide angle lenses without any distortion to speak of. In my mind all wide angle lenses show a variety of distortions which are then corrected after the image is captured. –  Esa Paulasto Mar 18 at 6:46

Like all things in life, you need to make a choice: The TV or the girl(s)?

choice

Also, consider how something polar, like the earth is displayed as a flat map? How warped is that?

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Hopefully this illustration of a pin hole camera will clarify: pin hole camera

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