Question about replacing a Compur shutter with a Copal

I bought a nice Sironar W 150mm/f5.6 (Sinaron branded) on the devil bay last year. The lens came on a Compur 1 shutter which worked fine since my last hiking in the New Zealand mountain. All the iris blade moved away from their axis. I thought it would be easy to fix (I already fixed many shutter problems) but it looks like a gorilla already put his hands in the shutter before me. Most of the threads are damaged. I did my best to fix it but I can not keep the shutter leafs popping out of their axis due to the loose screws. As I have spent enough time trying to fix the problem, I decided to replace the Compur 1 with a modern black Copal 1 harvested from a Fujinon W 180mm/f5.6 I bought on ebay (the lowest price lens available I could get on ebay with a f5.6-f64 copal 1 shutter).


The Sironar lens cells fit perfectly well on the Copal 1. The picture on the ground glass is sharp as it could be however I'm a bit concerned about the iris. The Compur 1 has a scale which goes from f5.6 to f64, same for the Copal 1 but does the Copal f22 is really equivalent to the f22 of the Compur 1?

The Copal 1 seems to be a model 47 (if it makes sense)?
The Compur 1 has the letter K written on the diaphragm scale.

Both lens seems to have the same front lens diameter (+/-2mm)...

Quote:

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Originally Posted by stormpetrel

but does the Copal f22 is really equivalent to the f22 of the Compur 1?

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No, not necessarily, but not due to the difference in shutter.

The size of the aperture hole at f/5.6 for your original lens should be 150/5.6 or about 26.786mm. This would be measured by the entrance pupil as seen through the front of the lens, not the bare blades. For the 180mm lens, at f/5.6, the size will be slightly bigger (32.143mm). Therefore, your shutter markings are about 1/2 a stop off. You would have to stop down an extra 1/2 stop to get the equivalent setting (stop down to f/22 and a half to get f/22).

I think I got that math right.

One way to check is to move the aperture blades till you can just barely see them, and then open them up slightly so they disappear and take note of that setting. I'm betting it's about halfway between f/5.6 and f/8 on the new shutter. That's your f/5.6 aperture. You can also just measure the exact opening set at f/5.6 on your Compur 1 and set the Copal 1 to that exact opening and make a note of the setting. You'd probably have to have a caliper to get it just right.

Thank you Corran for your inputs

Compur 1 + 150mm lens: Apparent diameter ~25.1mm as seen through the front lens cell @ f5.6
Copal 1 + 150mm: Apparent diameter ~24.9mm as seen through the front lens cell @ f5.6
Copal 1 + 180mm lens: Apparent diameter~35.4mm as seen through the front lens cell @ f5.6
Measuring the apparent diameter is not easy but all the measurements have been done from the same distance & perspective (one arm length above the lens)

Compur 1: bare blade diameter: 28.9mm
Copal 1:: bare blade diameter: 29.9mm

I'm not an optical engineer so I can only hazard to guess that the apparent magnification of the aperture pupil is an optical trick on the 180mm lens. If set at f/5.6 and the apparent diameter was practically the same for the 150mm on both shutters, I say leave it. The small difference would be insignificant, not even a quarter of a stop.

Have you shot any test shots?