How to determine focal length when not marked?

[bill zelinski]

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 t ried 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 wh at do I measure to find out what focal length this lens is? thanks.

[David Grandy]

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.

[Paul Schilliger]

Or, if your bellows's too short, focus something at infinity and measure the out er lensboard side to film distance (provided it is not a telephoto design).

[James Chow]

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.