Compur 1 vs Copal 1 threads

[walter23]

I'm wondering if I can take a lens from a broken compur 1 and plug it directly into any copal 1 without modification.

Are the threads standardized? Are the separations between front & rear when lens groups are fully screwed in standard as well?

Thanks for your help
-Walter

[BradS]

Well, the threads are certainly the same. I'm not sure about the spacing. That said, I've taken the cells from a compur shutter and mounted them in a modern copal #1 and didn't see any dramatic difference in the final prints (well, except for exposre differences). The more difficult problem to overcome is the iris calibration.

[Dan Fromm]

A #1 is a #1 is a #1, position of the diaphragm in press (self-cocking) shutters excepted.

[walter23]

The more difficult problem to overcome is the iris calibration.

If you know your lens is an f/5.6 lens, can you open it up, meter off the ground glass as a base reading, then stop down and meter the glass to calibrate it?

[Jiri Vasina]

Well, I don't think the iris calibration must be that difficult. If I understand it correctly the size of iris is another way of defining f-stop. And f-stop is related to focal length and the diameter of the iris hole. So if you measure the iris hole (exactly enough), divide the focal length by the measures size of iris hole, you get the f-stop.

(I've done precisely that with my Angulon and the "calibration" is good enough for exposure of color negative film - I have yet to develop the diapositives to see, but I think it will work well there too.
Edit: I forgot to say, after the measurement you obviously have to mark the position of the f-stop lever and note the f-stop).

But I may have got it wrong what you mean by calibration, bradS and walter23 :) ...

[walter23]

I admit I don't know much about optics, but I think the focal length / aperture relationship is for very simple ideal lenses of some sort, not compound lenses with multiple elements & groups. Maybe the relationship gets more complicated in those cases. This is speculation, I know nothing really.

In any case, I figure metering the ground glass would get me close enough to a decent aperture scale for most things.

Thanks for your help.

[Jiri Vasina]

No I don't think it depends on lens design. The design is important for amount of aberrations and imperfections correction, but has nothing to do with aperture / focal length relationship. I think (but may be wrong here) that it's even valid for telephoto design lenses.

Walter23, yes, you can do it as you described - metering the ground glass (or with something like Sinar full frame back meter (or whatever they are called)) - that is perfectly valid approach too. But to me it's more difficult and more demanding on the instrumentation. Not better, not worse. Different.

[Ole Tjugen]

If the "iris hole" means the physical size of the aperture opening, it's wrong.

What you need to measure is the effective aperture, i.e. the apparent diameter of the aperture with all lens elements in place.

In most cases the difference is small enough to be tolerable, but with some lens constructions (tele lenses especially, but also "simpler" things like Tessars) the difference can be enough to have a significant impact on exposures.

[Dan Fromm]

If the "iris hole" means the physical size of the aperture opening, it's wrong.

What you need to measure is the effective aperture, i.e. the apparent diameter of the aperture with all lens elements in place.

In most cases the difference is small enough to be tolerable, but with some lens constructions (tele lenses especially, but also "simpler" things like Tessars) the difference can be enough to have a significant impact on exposures.Don't you mean "entrance pupil?" I ask because in closeup work "effective aperture" has quite a different meaning.

Cheers,

Dan

[Glenn Thoreson]

If you have the broken shutter with the aperture scale on it, can't you just measure the openings at the various stops, and tranfer those to the new shutter? If f/** is **mm diameter on the old one, would it not be **mm diameter on the new one, also, for that particular lens?

[erie patsellis]

In theory, absolutely. Older lens/shutter combinations, aging and wide process control windows would probably neccesitate measuring to be absolutly sure, I've run across a few that as mounted were off quite a bit.


erie

[Ole Tjugen]

Don't you mean "entrance pupil?" I ask because in closeup work "effective aperture" has quite a different meaning.


Dan,

Since the f-stop number is defined at infinity the two are interchangeable in this case. I do mean "entrance pupil", but feel that many may find "effective aperture" easier to understand.

[Jiri Vasina]

Thanks Ole for the clarification.

[walter23]

Good point, Glenn. I can't see why that wouldn't work. Actually wait - I can see why - the aperture is jammed on the broken shutter ;)

[walter23]

A #1 is a #1 is a #1, position of the diaphragm in press (self-cocking) shutters excepted.

Well, I just bought a tominon 127 in the polaroid copal 1 (it has an iris) for a steal ($32). Can I mount any copal 1 lens in this shutter or will the iris interfere with some?

I bought it knowing the shutter might not be useful; the tominon will make a great macro lens (or an intermediate between my 90 and my 210 if it covers at infinity - not sure if it does).

[erie patsellis]

it will, barely

[Dan Fromm]

Well, I just bought a tominon 127 in the polaroid copal 1 (it has an iris) for a steal ($32). Can I mount any copal 1 lens in this shutter or will the iris interfere with some?

I bought it knowing the shutter might not be useful; the tominon will make a great macro lens (or an intermediate between my 90 and my 210 if it covers at infinity - not sure if it does).Walter, some lenses made to go in a cock-and-shoot #1 will go into the Polaroid Copal #1 used for the 127/4.7 Tominon, others won't. I, um, stubbed my little toes when I bought a 127/4.7 Tominon in shutter to get a shutter for a pair of 80/2.8 Planar cells I was given. They won't go. Neither will my 95/2.8 Saphir. Both lenses' rear cells hit the diaphragm control plate before they're fully seated. But my 210/6.8 Beryl fits just fine, and so do my Zircons (105/5.6, 150/5.6, 210/5.6).

The Planar won't go in a #1 Prontor Press either, but the Saphir will. Not a Polaroid shutter, either.

Whether a lens will fit a press shutter depends on the lens. The problem is that a press shutter's diaphragm control plate is set farther back than the cock and shoot's. So a deep rear cell can hit it. And note that Prontor and Copal don't put the diaphragm in quite the same position.

FWIW, I use a 127/4.7 Tominon at all distances on 2x3. I wouldn't use it at infinity on 4x5, it is a tessar and short for the format so I expect that the corners will be mush.

Ole, your use of "effective aperture" confused the hell out of me. The term has a well-established meaning and you used it to say something else entirely. Also, whether the aperture is same size as the entrance pupil depends on the lens' design, not on the focused distance.

Cheers,

Dan

[walter23]

Alright... so I should hold off on that broken-shuttered symmar I'm thinking about, eh?

I'll just use it as a macro lens.

[Dan Fromm]

Walter, if I were you I'd wrestle the Tominon's rear cell from its shutter and try the Symmar's rear cell. It might work.

That said, sacrificing a 127/4.7 Tominon to get the shutter it is in seems a little barbaric. I mean, it really is a good lens. After I tried it out I couldn't bring myself to sacrifice the first one I bought. The second one was another matter, though.

Cheers,

Dan

[Richard Kelham]

Ole, your use of "effective aperture" confused the hell out of me. The term has a well-established meaning and you used it to say something else entirely. Also, whether the aperture is same size as the entrance pupil depends on the lens' design, not on the focused distance.



Rather than "effective" aperture, which can have other meanings, I use the term "apparent aperture" I measure what appears to to be the aperture vierwed from the front of the lens which allows for any distortions caused by the lens construction. It worked fine on an Apo-Ronar I remounted in a Prontor Press shutter. At the moment the aperture scale is marked on a strip gaffer tape – not very elegant but cheap!



Richard

[Ole Tjugen]

Good and valid points, Dan and Richard.

I'll try to remember to be precise next time this question comes up (which it will). :)