### 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 (willy226@yahoo.com), 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 (dgrandy@accesscable.net), 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 (pschilliger@smile.ch), 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 (dr_jchow@yahoo.com), June 13, 2000.