From what I recall, determining the focal length of a double convex lens just involves an infinite light source and holding the lens close to an image-capturing surface such as a wall and adjusting the distance until the image is at its sharpest...the distance from the lens to the surface is the focal length. ^ Correct me if I'm wrong, I want to double-check ^ What I want to know is, how does one show the calculation/determination of this, as is required by an optics project in school? Do I just draw a ray diagram for the lens, all the necessary light rays converging on a point X distance from the lens? Maybe draw it to scale? Is that all, or is there some kind of formula that must be shown, such as the Thin Lens Equation (which doesn't make sense to me because one would have to use infinity as DO, or object distance)? Please clarify these things for me :$ P.S. I apologize for not using the 3 guidelines to stating a question, I didn't think they applied to my question well...and I searched the forum briefly but didn't find a repeat of my question. Thanks in advance.