Hello. I have a question about DSLR lenses that has always bothered me, and it's about the focal length millimeter values. I know that the focal length is the length in millimeters between your sensor and the optical center of your lens, that is, a piont where the light rays collimate. But I don't understand how this works for ultra wide-angle lenses, for example a 10mm lens. Since the optical center is (usually) inside the lens, 10mm is way too short to fit a DSLR mirror between it and the sensor, isn't it? Since I'm obviously not good at optical concepts, can someone explain me what I'm missing here? Thanks in advance.