Transverse linear magnification

Distortion, similar to coma, is not a primary aberration but results from other aberrations. In distortion, the transverse linear magnification in the image varies with the distance from the optical axis, which results in distortion and causes a square object to look like a barrel or a pincushion, as shown in Figure 2.11. Distortion cannot change the resolution of the image, but it changes the locations of image points. For a sufficiently thin lens there is no distortion. 

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