Is the 450mm Fujinon-C Truly f/12.5 ??

[Old-N-Feeble]

If I'm not mistaken, the way to calculate maximum f/stop of a lens is focal length divided by maximum aperture diameter. The maximum aperture size of a Copal #1 shutter is 30mm.

So for the 450-C (425mm actual FL) this would be 425/30 = f/14.2.

For the math to work correctly the real focal length would have to be 375mm or the maximum aperture diameter would have to be 34mm.

What am I doing wrong?

[Dan Fromm]

f/# = (diameter of entrance pupil)/(focal length). The entrance pupil is the image of the diaphragm as seen from the front of the lens. The front cell can magnify ...

[xkaes]

The lens has a flange focal length of 425mm, but it is a 450mm lens for computation purposes. The max aperture of the Copal #1 is closer to 39mm. 450mm / 39mm = 11.5. So it looks like 12.5 is probably pretty accurate.

[Old-N-Feeble]

Thanks, folks. I knew I had to be doing something wrong. :)

[Dan Fromm]

The lens has a flange focal length of 425mm, but it is a 450mm lens for computation purposes. The max aperture of the Copal #1 is closer to 39mm. 450mm / 39mm = 11.5. So it looks like 12.5 is probably pretty accurate.

The relationship between flange-to-film-distance and focal length is somewhat tenuous.

[ic-racer]

The 'flange-focal-length' is no more related to the lens focal length than the front filter size is related to the focal length. They are independent measurements.

[Mark Sawyer]

I'd agree that the filter size has nothing to do with focal length, but the flange on a Copal 3 is close to the iris (about 15mm) and consistent from lens to lens in a Copal 3, so I'd say it can be a useful measurement to know.

[Dan Fromm]

I'd agree that the filter size has nothing to do with focal length, but the flange on a Copal 3 is close to the iris (about 15mm) and consistent from lens to lens in a Copal 3, so I'd say it can be a useful measurement to know.

Useful to know, absolutely. Equal to focal length, not.

[Old-N-Feeble]

The 425mm focal length I referenced is from the Fujinon information page HERE (http://www.subclub.org/fujinon/byseries.htm). I'm not educated or intelligent enough to go by any other standard so I tend to keep things simple.:) I appreciate everyone's help and I have my answer. The 450mm Fuji-C is a f/12.5 (or close enough).:D

You folks are the best. Please be kind to one another.:)

[Dan Fromm]

Look, OnF, xkaes' list of Fuji lenses is a valuable resource. I'm very glad he took the trouble to compile the information in it and to post it.

His English is at times a little idiosyncratic. The idea "distance from the mounting flange to the film when the camera is focused at infinity" is usually expressed in English as "flange focal distance." I have no idea why xkaes chose to call it "flange focal length." This is a trap for the unwary. That's you.

[ic-racer]

I'd agree that the filter size has nothing to do with focal length, but the flange on a Copal 3 is close to the iris (about 15mm) and consistent from lens to lens in a Copal 3, so I'd say it can be a useful measurement to know.

Flange focal length is an essential measurement to know, but it is not related to focal length by any formula or equation. You have to get it off the spec sheet or measure it.

[xkaes]

It's not my English. Fuji calls it "flange focal length", so I do. "flange focal length", "flange focal distance", let's talk semantics. I call it what Fuji calls it. You can call it whatever you want, but it is not an idiosyncracy of mine!


Look, OnF, xkaes' list of Fuji lenses is a valuable resource. I'm very glad he took the trouble to compile the information in it and to post it.

His English is at times a little idiosyncratic. The idea "distance from the mounting flange to the film when the camera is focused at infinity" is usually expressed in English as "flange focal distance." I have no idea why xkaes chose to call it "flange focal length." This is a trap for the unwary. That's you.

[Dan Fromm]

So (http://www.cameraeccentric.com/html/info/fujinon_1.html) they do. Apologies.

Still a snare for the unwary, especially those who don't know that there's more than one way to express an idea.

Nikon, Rodenstock and Schneider all say "flange focal distance."

A quick and sketchy run through lens catalogs on cameraeccentric.com finds that pre-WWII lens catalogs don't mention FFD at all. A few mention back focus; some catalogs mention it only for telephoto lenses. This gives the impression, possibly mistaken, that mentioning FFD became important after modern WA lenses with FFD longer than FL came in.