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the optical centre and the geometric centre can mean two different things actually
the geometric centre is the physical central point of a lens...... however if you
add prism to an optical lens or mirror, that is to have one side of the glass thicker than the opposite side, or "wedge" it, then you have actually moved the optical centre of the system while the geometric centre remains exactly in the middle
so the optical centre and the geometric centre are not in the same spot
pat
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