I've aquired a 3 1/4 X 4 1/4 Graflex reflex camera from about '41 that I'll be using. The lens is a Bausch & Lomb Tessar 1 c., f/4.5.The serial number is 3161553. No marked focal length, only 5 X 7. It seems to be about 8 inches but I'd like to know exactly, preferably be finding some information from records of manufacturing. If these records can not be found, what's the best practical test for focal length? Is there a formula for finding the focal length if the speed is known. Possibly by carefully measuring the size of the aperture?