View Full Version : Calculating f-stop with different lens elements Copal 1 (or other shutters)

24-Apr-2013, 10:27
I have the 210mm Symmar-s lens elements, which I bought so as to be able to put them in the appropriate shutter. There is no problem for instance putting them in the Copal-1 with which my G-Claron 270mm was supplied.
My question is this: does anyone know a simple method of converting the f-stop markings for the G-Claron 270mm (which go from f9 to f64) to those for the 210mm Symmar-S, whose maximum aperture is f5.6?
Since all Copal 1 shutters are identical, the f-stop markings on each lens-shutter assembly must differ depending on the focal length and maximum aperture of the lens elements actually mounted.
If I put my Symmar-S elements in the Copal-1, and open the diaphragm to its maximum (marked f9), for instance, the Symmar-S is presumably working at an effective f5.6 (the maximum aperture of the Symmar-S in that shutter) so does this mean that the remaining f-stop numbers f11, 16, 22 etc are thus in effect a stop and a half out of kilter? In other words if f9=f5.6, then f11= f6,3, f16 = f9 etc, etc, with the 210 Symmar on the Copal-1. Or is my logic faulty?
If anybody else has dealt with or pondered this question I'd be grateful if they could point me in the right direction.

24-Apr-2013, 10:49
As f stop = focal lenght / diameter it's very easy calculate.

f11 with the 270 is equivalent to 8.55 with the 210.


Bob Salomon
24-Apr-2013, 11:05
Or order a new set of scales from Schneider.

Jim Jones
24-Apr-2013, 20:26
The f/stop = focal length / diameter of the entrance pupil (the apparent diameter of the diaphragm opening as viewed from a distance from the front of the lens). Since measuring this at a distance is difficult for my ancient eyes, I shift my eye from side to side by about the distance of the entrance pupil when making this measurement. Fancy scales can be made, but covering the original scale with white tape and marking the new f/numbers with a fine point Sharpie works for me.

25-Apr-2013, 05:02
I have the same problem with the elements of a SA 210 f8 and a copal 3 marked at 5,6. I hope you can fix it easily.

Bob Salomon
25-Apr-2013, 06:37
I have the same problem with the elements of a SA 210 f8 and a copal 3 marked at 5,6. I hope you can fix it easily.

can't be easier, you unscrew two tiny screws, lift off the wrong scale, place the correct scale in position and replace the two tiny screws. Do the same with the aperture scale on the front of the shutter as well.