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hal

In a lens, we often get chromatic aberrations, so different wavelengths of light focus at different distances, but by putting a concave and convex lens together, they can cancel out the chromatic aberrations.

foobar

What is the V-Number, and how does it relate to solving this problem of canceling out those chromatic aberrations? I took a look in the book Modern Lens Design (which had a lot of additional plots) and I am still confused about the meaning.

hanrahan

The v-no refers to the change in the index of refraction with wavelength. If two types of glasses have the same v-no then the both refract in the same way. If they have different v-nos, then the effect can cancel out.