[SOLVED] Understanding CCTV lenses Hey guys, I'm trying to get my head around the way a lens works... I've found the nodal points, and since the medium surrounding the lens is air constant refractive index (air) then they nodal points coincide with the principle planes, right? Also the Focal length of the lens is 12mm, which I assume since it's a CCTV lens, is measured from the mounting surface rather than the last element of the lens... is that a fair assumption? I've drawn a diagram of what I think represents the path of a ray going through the lens. (see attached) What I'm trying to do is accurately measure the size of something from it's size in pixels in the image.