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Message: <blockquote data-quote="henry link" data-source="post: 1558249" data-attributes="member: 6806">Dennis,When I said Ed didn't measure field curvature I meant that he didn't present measurements in diopters. The diopter is the standard unit of measurement for change in focus. A loss of resolution is not the same thing as a change in focus. I imagine you and I see mostly the same thing through the SE, but I believe you're confusing low astigmatism with low field curvature. The SE has very low astigmatism at the field edge, less than 1 diopter difference between the sagittal and tangential foci. That's why the image can be brought to such a sharp focus at the edge. Henry</blockquote>