What makes a fisheye fishy?

Started Oct 29, 2012 | Questions thread
Chris Malcolm Senior Member • Posts: 1,890
Re: What makes a fisheye fishy?
1

A linear lens (popularly but incorrectly called a rectilinear lens) tries to preserve straight lines, i.e. a straight line in the world should end up as a straight line in the image. The best lenses manage to do this perfectly. Others fail a little, e.g. giving a slight touch of barrel or mustache distortion to straight lines. But just a little, and easily corrected by editors which know how to correct the perspective geometry of specific lenses.

The so-called "wide angle" distortion isn't really a distortion of the lens at all. It's a distortion caused by you looking at the image with a different viewing angle than the lens did. Move your eyes close to the centre of the wide angle image and look at the edges of the image out of the corner of your eyes. At the right distance the "wide angle" distortion disappears.

But we don't like doing that. So circles and balls near the edges of the image look weirdly distorted. And people at the edges in a wide angle group shot look fatter than they really are. Not a popular kind of distortion for group shots!

Whereas a fish eye lens uses a kind of perspective projection that preserves circularity of shape. So circles and balls near the edges look normal. But at the cost of bending straight lines. The good news is that you can do wide angle group shots with people in a semicircle round the camera -- i.e. all at the same distance -- and everyone looks their normal shape. People at the edges aren't made to look fatter. Good for group shots in small interiors where you can't get far enough back.

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Chris Malcolm

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