I'm designing an eyepiece for fun and want to be accurate. I just realized my physics book equations are approximated for thin lenses only. Suppose I have a convex convex lens or a plano convex lens, lateral parallel rays coming from one side or the other, and a focal point on the opposite side, and the lens is thick enough that the vertical center of the lens is at least 40% closer to the focus laterally than edges are. If the focal length is, say 10mm, and the lens maybe 5mm thick and 12mm in diameter, from what point on the lens is it measured? The lateral center at the vertical center? The vertical center at the surface? At the edges? At the back surface? Please be specific so I'll know how to calculate it with two thick lenses in a row touching each other. I think this is why the eye relief of an eyepiece is not necessarily equal to the focal length. I thought about defining some ideal thick lenses in a spreadsheet via their conic section equations and derivatives, and then ray tracing them to their focus, and finding relationships in order to find the best focal length measuring method, but I think that would take a lot of work to do, especially with two lenses.