So, how do you do it? I have some old lenses I'd like to better understand.
Bruce, you can calculate the focal-lenght from image size and angle of view, use a collimator telescope or Abbe's "Focometer".
But much more easy is to align the camera in such a way a glass-ruler or a transparant graph-paper is shown acurately on the ground-glass in the ratio 1:1. Than focus on a realy distant point. The difference in bellows-extension is the focal-lengt of the lens.
Mount lens to camera, focus on something at infinity, and measure from the film plane (inside of ground glass) to.... that's the hard part. If it looks like an old lens that is still in its original form, then I measure to the aperture if it has one or the waterhouse stops if it has those, otherwise to the flange. Or try and find some original specs on the lens from the various antique lens folks on this forum.
The only trouble with doin' nothing is you can't tell when you get caught up
As Paul mentions, the difficulty with old lenses is to figure out if you have to calculate from the aperture or flange plane.
I have tried the following calculator (see link). It has come pretty close to the actual focal length of some old barrel lenses I have.
If you focus the camera to infinity and measure from the focal plane (ground glass) to the rear element of the lens, you get the back focal length.
The flange-to-focal-plane distance is irrelevant, as flange location is arbitrary. A lot of old barrel lenses have focusing tracks built in that let one change the lens position in relation to the flange.
What I think you want is "effective focal length" which is aperture-location-to-focal-plane. The aperture is usually at or very close to the rear nodal point. If you don't have an aperture, you're kinda screwed and just have to guess where it would be based on where it is on similar lens designs, (which should actually get you pretty close).
Personally, I think it's time we all started referring to our focal lengths in "35mm-equivalents", like the digital people do.
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
Thanks, all, for the information. This was prompted because I purchased a lens on the big auction site that is marked at a certain focal length, but doesn't seem to be that length.
It is a Cooke-Luxor Anastigment, 5- 3/4 inches, f/4.5. The back focus at infinity is less than an inch from rear element to image, placing the nodal point about 2 inches in front of the lens.
Is that possible?
The barrel of the rear element is slightly darker than the rest of the lens, leading me to the conclusion that someone switched rear elements. The lens was purchased from a certain aussie dealer before I read the reports on this website.
Cry foul. The Luxor is a garden variety Cooke Triplet. These lenses are in no sense telephotos.
Here is one on e-bay. Does the rear element look like that one? (or is this the one you have?)