A 210 is a 210 is a 210...NOT!

[Chauncey Walden]

Just to put some numbers behind the caution that just because two different lenses are the same focal length their shutters may not be interchangeable as regards the f stop markings I did a little testing. I took three 210's, a G-Claron in barrel, an old Symmar Convertible in a Synchro-Compur, and a newish Geronar in a Copal. The G-Claron is often acquired in barrel with the thought of putting it in a shutter. The barrel has an ugly pentagonal aperture since, in as designed use, out of focus effects (the "B" word) were not a consideration. The old Synchro-Compur has a nearly round aperture and the Copal an obviously polygonal one. The 210 Symmar is practically a give away in today's market and the Geronar, in spite of being a really nice little lens and MULTICOATED to boot (as if it needed it) can sell for less than the value of its shutter. Anyway, I dug out the digital calipers and measured the apparent aperture throught the front of the lens with the G-Claron on each of the other shutters. On the Synchro-Compur, a measured f/22 occurred at a point halfway between the marked f/16 and f/22. On the Copal, it occurred at the marked f/16. Just slapping the G-Claron in a shutter and assuming the marked aperture is correct just because there had been another 210 in it obviously won't work; I mean, you might get get away with a half stop under exposure but a full stop under will bite you. You must do the math and the measurements. By the way, the G-Claron ended up in the Synchro-Compur - that lovely round aperture won me over.

[Ted Harris]

That's why, if you can't get an aperture scale that is mated to the lens, you are better off sending ittoSK Grimes to have a scale properly done.

[Ernest Purdum]

Thank you for doing something I'd thought about. Thinking about it and actually getting it done are two different things. (Your procedure is probably better than I would have used, too.)

[Jim Galli]

I question the ability to measure a pentagonal hole for area with a caliper. The way I've measured in the past is quite different. Warm up a light table. Open the barrel aperture to f16 or so. Stick a 1 degree light meter right down in the throat and take a reading. Sure it's meaningless, but you'll notice it changes in tenths (at least with my minolta) with the tiniest adjustment of the aperture, so it is measuring a quantity. Measure the G-Claron barrel first. Then open the Compur and the Copal to indicated f16 and take the same reading. See how much you have to gently move the aperture scales to get the identical reading with the other shutters and see if it's as drastic as what you found so far. I'm saying the barrel and the meter are the "known".

[BradS]

I question the ability to measure a pentagonal hole for area with a caliper. The way I've measured in the past is quite different. Warm up a light table. Open the barrel aperture to f16 or so. Stick a 1 degree light meter right down in the throat and take a reading. Sure it's meaningless, but you'll notice it changes in tenths (at least with my minolta) with the tiniest adjustment of the aperture, so it is measuring a quantity. Measure the G-Claron barrel first. Then open the Compur and the Copal to indicated f16 and take the same reading. See how much you have to gently move the aperture scales to get the identical reading with the other shutters and see if it's as drastic as what you found so far. I'm saying the barrel and the meter are the "known".

Jim, That's a stroke of genius!

[Chauncey Walden]

You're right, Jim. Measuring the area of a pentagon directly would probably require a planimeter. It is fairly easy to measure one side of a pentagon with a caliper and compute an area from that. Then just back into the diameter of a circle and use that to derive an f stop for a particular focal length. Fortunately, I didn't have to go through that exercise as I didn't plan to use the barrel with its pentagonal aperture with another lens:-) As for the meter method, I would have to play with that before I was sure that I wasn't introducing an error just by the way I was using the meter. The easy button for me is that focal length divided by apparent aperture equals f stop. With a nearly round aperture, a good caliper, and a little care that's a no brainer.

[Uli Mayer]

Jim,
your method can also be used to measure the amount of light that is lost in a lens by the glass itself (absorption) and by internal reflections. For this one only needs to fix between light table and exposure meter a sheet of black cardboard that has a hole punched into it.. Through this auxiliary diaphragm you first measure the light without a lens between light table and exposure meter. Then you insert the lens into the light path. The lens should be fully open, and the hole in the cardboard just big enough so that it remains fully illuminated when a lens is inserted into the light path.

Older lenses of identical focal length could vary by almost one stop in transmittance because of light absorption (different glass types, thicknesses, number of air-spaces). A Dagor 1 :6.8 ,for example, was considerably faster than a Dogmar despite the fact that the Dogmar was labelled 1: 6.3.

Cheers
Uli

[Harold_4074]

As a marginally relevant aside, at one time (1970s?) there was a minor fad of having 35mm camera lenses "T stopped"; that is, remarked in equivalent f/ stops according to actual light transmission. The technique supposedly originated in the motion picture industry, so that and different lenses could be used interchangeably without creating awkward "jumps" in film exposure.

The reality of still photography, of course, is that this is pretty much a non-issue, and the idea seems to have largely faded away. It might still make sense for someone with two or three lenses having a stop or more variation in light transmission.

[Ernest Purdum]

Uli's comments remind me of the "T" stops that used to confuse people. I am also reminded of a book on aerial photogrammetry published before coating which went into the subject quite deeply, listing transmittance values for quite a few of the lenses then in use for aerial work. Some of the variations between lenses of the same "f" number were very severe. Coating and multicoating have been wonderful advances.