1. A focusing method, comprising: focusing a microscope optic on a plurality of points of a specimen to determine a focal length for each point; fitting a normalized surface to the points at the focal lengths; capturing a plurality of microscopic images of portions of the specimen while focusing on each point based on the fitted surface; selecting an additional point; determining another focal length for the additional point; and fitting another surface to the plurality of points at the focal lengths and the additional point at the other focal length.