How to determine focal length when not marked?greenspun.com : LUSENET : Large format photography : One Thread
I have taken the lens off of a 1908 Kodak 3a Folder to use on my Speed Graphic. This is a rapid rectilinier type, f4 to f64 in shutter in like new shape and I tried taping it on to a lens board and it seems to cover 4x5 ok (the camera took roll film orig.) There is no marking regarding focal length anywhere. How or what do I measure to find out what focal length this lens is? thanks.
-- bill zelinski (firstname.lastname@example.org), June 13, 2000
If you have enough bellows on your camera you could focus on some thing (say a 4X4 card) until it was life size on your ground glass. Measure the bellows extension and divide it by 2. That's the focal length of the lens.
-- David Grandy (email@example.com), June 13, 2000.
Or, if your bellows's too short, focus something at infinity and measure the outer lensboard side to film distance (provided it is not a telephoto design).
-- Paul Schilliger (firstname.lastname@example.org), June 13, 2000.
Actually, the front side of the lens board to the film isn't necessarily the focal length when focused at infinity or twice the focal length w/ 1:1 magnification. On my 210 apo-symmar, this method yields about 196mm on my Toyo VX, so there's some unknown offset (call it D). However, if you employ both methods (let x=true focal length, x1=film to lens board distance when focused at infinity, x2=film to lens board distance with 1:1 macro), then you have a system of linear equations in (x,D) which can be easily solved.
-- James Chow (email@example.com), June 13, 2000.