Barrel distortion
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In geometric optics, barrel distortion is a divergence from the rectilinear projection, in which image magnification decreases with increasing distance from the optical axis. The apparent effect is that of an image which has been mapped around a sphere. Fisheye lenses, which take hemispherical views, produce this type of distortion as a result of a hemispherical scene being projected onto a flat surface. Barrel distortion is a common feature of lens elements with a curved surface, which is why in camera lenses such elements are usually paired with a matching inversely-curved element to correct the distortion.
