rich_lahrson Posted January 8, 1998 Share Posted January 8, 1998 I've aquired a 3 1/4 X 4 1/4 Graflex reflex camera from about '41that I'll be using. The lens is a Bausch & Lomb Tessar 1 c., f/4.5.Theserial 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, preferablybe finding some information from records of manufacturing. If these records can not be found, what's the best practicaltest for focal length? Is there a formula for finding the focallength if the speed is known. Possibly by carefully measuring thesize of the aperture? Link to comment Share on other sites More sharing options...
ted_brownle Posted January 11, 1998 Share Posted January 11, 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. Link to comment Share on other sites More sharing options...
guy_ulrich1 Posted January 27, 1998 Share Posted January 27, 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 accuracyin making the measurements you can usually get within 2%. However, Ihave heard that the 5x7 B@L tessars are around 180mm focal length. Link to comment Share on other sites More sharing options...
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