distortion

A geometric third-order optical aberration where lateral magnification either increases (pincushion distortion) or decreases (barrel distortion) with increasing distance from the optic axis.

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Geometrical Distortion

Distortion is an aberration commonly seen in stereoscopic microscopes, and is manifested by changes in the shape of an image rather than the sharpness or color spectrum. The two most prevalent types of distortion, positive and negative (often termed pincushion and barrel, respectively), can often be present in very sharp images that are otherwise corrected for spherical, chromatic, comatic, and astigmatic aberrations. In this case, the true geometry of an object is no longer maintained in the image.