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Message: <blockquote data-quote="Kevin Purcell" data-source="post: 2128516" data-attributes="member: 68323">They're three different things: two fo them affect how sharp things are at the edge of field.Field curvature is literally the focal plane of a lens(es) is not flat. As Steve says you can change the focus to focus out the field curvature. You can also "accommodate" out" the field curavture (depending upon how much accommodation your eyes have) so different people especially at different ages will see different amounts of field curvature."Edge softness" can include astigmatism and coma. Astigmatism occurs when the focal point varies depending upon the orientation of the line in the field: usually measured along the radius of the field (saggital) and at right angles to a radius (tangential). Both are aberrations that increase to the edge of the field.Pincushion distortion is added to reduce "rolling globe or rolling ball" effect. It's a feature not a problem though what amount is the "correct" amount varies with the user.<a href="http://www.birdforum.net/showthread.php?t=161164" target="_blank">http://www.birdforum.net/showthread.php?t=161164</a><a href="http://www.birdforum.net/showthread.php?t=74727" target="_blank">http://www.birdforum.net/showthread.php?t=74727</a><a href="http://www.birdforum.net/showthread.php?t=140202" target="_blank">http://www.birdforum.net/showthread.php?t=140202</a></blockquote>