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Old February 27, 2013, 09:48 PM   #38
Bart B.
Senior Member
Join Date: February 15, 2009
Posts: 6,318
Abberation, I understand what you're referring to:
Even at the mean focus, you are going to have a bell curve of that focus depending on the size and position in the field of view, and of each color.
Which is why different glass types (different refraction indexes), shapes and coatings are used in each element of doublets and singlets, even triplets, so each color band's focused closer together on the optical axis and eliminate as much of field curvature as possible at the edge of the field of view..

But there's still only one point among all those different colors focused at different distances from the lens that's the "focus" point as seen by the eye.
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