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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 1384688" data-attributes="member: 68323">Not to be too picky but ...Distortion and aberration are two rather different things. The important difference is you can easily (completely) undo distortion but you can't (easily) undo aberration.Distortion is introduced deliberately but reduce rolling ball effect but it doesn't change sharpness (or contrast or any other feature of the bin). It just affects the mappings of the angular distribution of the object compared to the angular distribution of the image. Or perhaps to put it more simply it maps a point in the object plane to a point in the image plane. There is a one to one relationship of points between the two planes.This is not true of aberrations that map a point in the object plane to many points (with varying intensity) in the image plane. For example think about coma where a point (a star for instance) is mapped to something that look like a comet with the tail pointing to the edge of the field.<a href="http://en.wikipedia.org/wiki/Lens_(optics)#Aberrations" target="_blank">http://en.wikipedia.org/wiki/Lens_(optics)#Aberrations</a><a href="http://en.wikipedia.org/wiki/Aberration_in_optical_systems" target="_blank">http://en.wikipedia.org/wiki/Aberration_in_optical_systems</a><a href="http://en.wikipedia.org/wiki/Image_distortion" target="_blank">http://en.wikipedia.org/wiki/Image_distortion</a></blockquote>