Formula for determining Chromatic aberration in achromats and apochromats.?
I heard that if you square the diameter of a refractor in inches you can determin what focal ratio it would need to provide a color free image. This seems okay for a 3 inch (f9) or a 4 inch (f16) but a 2 inch could be an f4? a 6 inch would need f36? doesn't seem right.
Oh yeah and that supposed formula was for achromats. I really with they would let you Edit a question once you posted it