How is it done? I've done a little research on Wikipedia and found the lensmaker's equation, but one of the values needed to compute focal length is the radius of the lens curvature. Strictly speaking, an aspherical curvature wouldn't have a set radius, would it? The reason I'm asking is because I'm trying to make a computer model of a lens and its fresnel equivalent. I can make a lens with an arbitrary curvature and convert this to a fresnel, but I would like to be able to model the curvature for specific focal points. (Given a fixed lens diameter.) [Edit] For a plano-convex lens, I take it the flat side has infinite radius? Or am I totally screwed up in my thinking? [Edit2] Also, what is the unit for focal length in the equation? Meters? Millimeters?