optics

Optics

area of the lens without distortion

A spherical lens has an aplanatic point (no spherical aberration) only at a radius equal to the radius of curvature divided by the index of refraction of the lens material. Crown glass typically has an index of refraction of 1.5, so if the radius of curvature of a particular lens is 1.0, then the aplanatic radius is 1.0 / 1.5 = 0.66666..., which is about 67% of the diameter of the lens. That gives an area of only 43% in the middle of the lens that will have no spherical aberration. Images outside this area will show spherical aberration.

Coddington magnifier

I am trying to find information about the calculation of the shape of the groove in a Coddington magnifier. I suspect that it is related to the aplanatic point. I believe the groove may be cut to the aplanatic point in order to block light from outside the useful range of the lens. Unfortunately, I have found little information on how this groove is actually calculated.