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View Full Version : Swapping lenses in a Copal 3?



Neal Chaves
13-Mar-2012, 17:40
There is a 360 Tele Xenar in a barrel advertised on the site. It interests me because the seller says it will fit in a Copal 3 and I have a 360 lens in a Copal 3 now, but I can't figure this out. My Fujinon W 360 is a 6.3 and come to think of it I don't know of any 360 W types that are 5.6. They all seem to be 6.3 max.

The Tele Xenar is 5.5. Given that the effective focal length is 360mm wouldn't the aperture scale be accurate with the Tele Xenar cells swapped in for the Fujinons? The throat of the Copal is approx 55mm, with front and rear threads both approx 58mm.

I also had an old 360 5.5 Tele Xenar once that came in a Compound and I had a machine shop make a barrel mount for it so I could use it wide open on a Graflex. I still have that barrel and the threads are approx 55mm, the same as the Compound which opened to 5.5.

I think I'm learning something here but correct me if I'm wrong. The size of the aperture for given f stop on a tele lens is based on the flange focus distance, not on the effective focal length and if I swap out the Fujinon W cells for the Tele Xenar in the Copal 3, even though they are both 360s, the aperture scale will not be accurate.

Leigh
13-Mar-2012, 19:43
I think I'm learning something here but correct me if I'm wrong. The size of the aperture for given f stop on a tele lens is based on the flange focus distance, not on the effective focal length...
Nope. The aperture f-number is based on the optical focal length of the lens.

Specifically, the f-number equals the optical focal length divided by the diameter of the entrance pupil.

Conversely, the f-number N defines the diameter of the entrance pupil D by D = FL / N.

A note regarding the aperture scale...
Since the f-number depends on the diameter of the entrance pupil, which varies with the lens design,
the scale on a shutter for one lens may not be correct for a different lens of the same nominal focal length.

- Leigh

Neal Chaves
14-Mar-2012, 07:18
Thanks Leigh,

So I guess this means that the Tele Xenar has a larger entrance pupil than the Fujinon W of the same effective focal length, so its maximum aperture will produce a larger f stop in a shutter of the same size.

Leigh
14-Mar-2012, 15:51
So I guess this means that the Tele Xenar has a larger entrance pupil than the Fujinon W of the same effective focal length...
I don't know if that's true or not.

Best to leave it to somebody who does this for a living, like SK Grimes.
They have the proper gauges and the right information to do the job properly.

- Leigh