How to determine focal length when not marked? : 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 (, 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 (, 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 (, 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 (, June 13, 2000.

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