Really? Huh. When I was taught/told about focal length by the builder of my cameras, he said the focal length represents the diameter of flat of field for the lens. For example a 780mm lens divided by 25.4 gives just shy of 30". So I can safely use this lens to receive on film or PMT an image up to 21"sq with zero edge distortion, or print from an image 21"sq with zero distortion. I wonder if that explanation is because my cameras can't see to infinity. The frame work limits the lenses from getting close enough to the film plane. For those of you who just went, HUH!?. I am not a photographer, my cameras are reproduction cameras in a darkroom.