The edge value of standard lens in NSC is used as an additional margin on aperture size that can be used/considered for mechincal mounting. ^RtIh-Z.9
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If you assign the lens with a given curvature and compare two situations: 1 set the radial size = edge size and 2 set radial size < edge size, by looking at layout plot you shall notice the difference. Doing so shall make it easier for you to observe the difference.