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Message: <blockquote data-quote="henry link" data-source="post: 3316138" data-attributes="member: 6806">Peter,i'm not Kimmo, but you might find your answer in this thread in which the distortions of the SF and SV are discussed:<a href="http://www.birdforum.net/showthread.php?t=313034" target="_blank">http://www.birdforum.net/showthread.php?t=313034</a>I think the short answer is that angular magnification distortion, rather than rectilinear distortion, is the source of "rolling ball" for those who notice it. AMD causes objects to change shape as they near the field edge in a way that mimics the foreshortening of flat shapes painted on a ball as they move around the edge when the ball is rolling. In optical devices AMD begins when  pincushion distortion is too weak to correct it, becomes quite strong at zero rectilinear distortion (perfectly straight lines) and stronger still with increasing barrel distortion. Try moving a small circle or square toward the field edge of your SF and notice how its shape changes. You may not personally experience an RB effect from that shape change, but that's the condition of distortion that causes other people to experience it.</blockquote>