On the remainder of this page, we'll find an exact formula for the focal length of a spherical lens, and we'll then find an approximate formula for the focal length at the center of the lens. This problem is somewhat messy, and we're going to break the problem down into easier to handle pieces by cutting the lens in half. We'll find the focal length of each half lens and then put them together to get the focal length of a convex-convex spherical lens.