Nikkor shutters

[Ole Tjugen]

... With a complex lens, the aperture diameter should really be measured as the entrance pupil, i.e., by viewing the aperture through the lens, which might magnify the aperture and change its apparent diameter. ...

Here's what Michael wrote, and it's absolutely correct. It's the entrance pupil which must be measured, as the "magnification" of the front cell affects the light-gathering ability.

If you object to that, take a close look at a telephoto lens - something like an old Tele-Xenar 360/5.6. The entrance pupil of that lens is 64.3mm, yet it's in a Compound #3 with a 55.69mm cell thread. That's almost 8.6mm too small! And the max physical aperture opening is even smaller than that. Clearly the glass of the front cell makes a difference.

[JW Dewdney]

Here's what Michael wrote, and it's absolutely correct. It's the entrance pupil which must be measured, as the "magnification" of the front cell affects the light-gathering ability.

If you object to that, take a close look at a telephoto lens - something like an old Tele-Xenar 360/5.6. The entrance pupil of that lens is 64.3mm, yet it's in a Compound #3 with a 55.69mm cell thread. That's almost 8.6mm too small! And the max physical aperture opening is even smaller than that. Clearly the glass of the front cell makes a difference.

Well - I really would have to look into that to see and confirm for myself. But I think that's a pretty far-out assumption that you are making. Your example with the Tele-Xenar, I think, is something erroneous - unusual. Are you sure it's properly shuttered and set up? I've measured apertures of lenses a long time ago - just when I was first getting into it and in my 'techy' period - and found them fairly consistent to what you think even the most elementary theory would dictate. That's like saying 'all black people are theives' because you once saw a black person take something that you assumed did not belong to them. All I'm trying to say is that I'm not convinced of that at all - you'd have to present a far more compelling argument.

[Ole Tjugen]

The Tele-Xenar was chosen as an example precisely because it's unusual: LF lenses are generally rather short and more or less symmetrical, so they don't tend to show the difference this clearly.

At http://www.schneiderkreuznach.de/archiv/archiv.htm you can find techical descriptions of Tele-Xenars - a bit more complete than on the US Schneider site.

Then check the specifications for a Compound III/7 shutter. I can only find it in one place on the net, unfortunately - and I wrote that entry: http://www.apug.org/forums/article.p...=83#comment243

But next time you see a Tele-Something in LF, measure the entrance pupil, the exit pupil, then unscrew either cell and measure the physical aperture opening. Try to calculate the aperture for the lens from all three measurements, then compare those results with the specifications of the lens in question.

I'm not "assuming" anything at all.

[Dan Fromm]

JW, "there are exceptions" means what it says.

Here are two more: 210/6.3 Prinz (made by Yamazaki, identical to 210/6.3 Commercial Congo) in Copal #1; 100/2.8 Planar, also in #1. Do the arithmetic, look up the shutter's specifications, cogitate for a while.

Cheers,

Dan

[JW Dewdney]

Well - I suppose I can look at this just for my own curiosity... but are you quite certain you guys aren't getting thrown off by the measured focal length being significantly different than the 'official' focal length - ? I mean, call me crazy, but I'd think that you'd derive apertures as a clear 'fraction' of the focal length, and not have to correct for magnifications of optical assemblies. If I'm understanding you correctly (which I hope I'm not), the opening diameter for f/8 on a 75mm lens would NOT be close to f/16 on a 150mm lens.

[Dan Fromm]

Well - I suppose I can look at this just for my own curiosity... but are you quite certain you guys aren't getting thrown off by the measured focal length being significantly different than the 'official' focal length - ? I mean, call me crazy, but I'd think that you'd derive apertures as a clear 'fraction' of the focal length, and not have to correct for magnifications of optical assemblies. If I'm understanding you correctly (which I hope I'm not), the opening diameter for f/8 on a 75mm lens would NOT be close to f/16 on a 150mm lens.

Typical focal length tolerances are on the order of 3%. And if you visit Schneider's site you'll find that true FL isn't always the same as nominal; for example they say that the lens sold as the 105/5.6 Componon is in fact 108.5. Nice try, but the discrepancies aren't large enough to explain, e.g., 210/5.6 plasmats in #1 shutters.

Yes you're understanding us correctly. There's no guarantee that a hole that gives f/8 with cells of a 75 mm lens around it will give f/16 with the cells of a 150 mm lens around it. Sometimes it will, sometimes it won't.

Practical implication is, always measure entrance pupil when swapping a lens into a shutter whose diaphragm is scaled for another lens. Sometimes, as in the case of a 135/5.6 Symmar I put into a Compur #0 with diaphragm scaled for a 135/4.8 Lustrar, it works. Other times it doesn't.

Cheers,

Dan