> Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass. So why not make the entire lens from fluorite? B

> Fluorite just has low dispersion and therefore a lens made from it has less chromatic aberration than glass. So why not make the entire lens from fluorite? Because pairing an element with one with different refractive index can lower the overall dispersion. Typically you'll see compound lenses are made of pairs of elements where one is positive (convex) and the other negative (concave): instead of making one lens with power of, say, +4, the group is made from one that's +5 and one that's -1 of different type of glass so the refractive indexes cancel-out dispersion and other aberrations.