Bausch & Lomb Tessar "5X7" lensgreenspun.com : LUSENET : Large format photography : One Thread
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?
-- Rich Lahrson (firstname.lastname@example.org), January 08, 1998
The best way of estimating the focal length of a large format lens is to focus on something at "infinity" ~ usually beyond 100 yards. The lens will have a "back focus" dimension approximately the same as the focal length. Measure from the plane of the aperture or shutter blades in the lens (as near as you can determine) to the inner (ground) surface of the ground glass. Most ground glass is approximately 1/16" thick. If the aperture scale is missing, you can measure the diameter of the aperturee at any given point. You will have to remove one of the lens cells to do this. You run the risk of damaging the iris assembly, so be extremely careful. The formula for determining aperture is: A=Fcoal Length/Aperture diameter Be sure to keep measuring units the same for both values (ie, millimeter or inches). A normal lens for 5 x 7 is the same as the length of the film diagonal. This is about 8.6" or 218mm.
-- Ted Brownlee (email@example.com), January 11, 1998.
The most accurate way to measure focal length is the lifesize method. All lenses, whether telephoto or normal, reproduce a 1:1 copy when ex- tended there exact optical focal length. Depending on your accuracy in making the measurements you can usually get within 2%. However, I have heard that the 5x7 B@L tessars are around 180mm focal length.
-- Guy Wayne Ulrich (firstname.lastname@example.org), January 27, 1998.