Thanks for saying this, Dan. I've had the same observation - not once have I seen shims to "correct" lens spacing. The only shims I've ever seen were on rangefinder camera, like the Kodak Retina, where it actually makes sense from a focus perspective.Originally Posted by Dan Fromm
Dan,
I know the difference between nominal and actual focal lenght. E.g. my Rodagon 1:5,6/150mm is marked with 149,7 on the back flange and the Apo Rodagon has three shims, 2 with 0.1mm and 1 with 0,3mm. Both lenses where no special order.
Peter K
I recently purchased three 240mm Apo-Ronars, latest blue-ring version, that had been placed in barrel mounts for use in an optical inspection machine. Each of the three had a thin shim behind the front lens assembly, which I assumed, based on reading other forum postings here and elsewhere, were for correcting optical performance at distances nearer infinity. I placed one of these lenses in a Copal 1 shutter, and tested it (using Fuji 64T with a 69cm rollfilm back on my Technikardan) against a 240mm Fuji A at .1X at several apertures. The results between the lenses were indistinguishable to me. I realized after the fact that this test would have provided an additional comparison if it had also been performed without the shim in place, but that wasn't my goal at the time. My 90mm f/8 Nikkor also has a shim behind the front assembly, and there was an insert in the Nikon literature in the box stressing its importance.
Incidentally, I determined the aperture scale using a back that I have built that accepts a Nikon D70 and other F-mount cameras. Maximum aperture is easily found using the histogram, and 1 stop increments can be subsequently located by changing the ISO or the shutter speed on the camera. I printed an aperture scale in a single-celled table in Microsoft Word, then went to Kinkos and printed this on cardstock with the color inverted, so that it has white numerals on a black background. I applied cellophane tape to the surface for additional protection, and the result looks indistinguishable from an original aperture scale. I'll be happy to post a picture of the lens in shutter tomorrow, when I have time to take it
To sum up part of the discussion, the position of some, not all, fairly modern lenses' cells in the shutter/barrel is tweaked by the maker by shimming. Whether this is the case for older lenses is unclear, but it certainly is possible. That said, many lenses, especially older ones, have no shims.
From which I conclude two things. When taking cells out of a shutter/barrel, one should be alert for the presence of shims but not be surprised if none are there. And broad generalizations aren't safe.
David, thank you very much for presenting more examples of lenses with shims and for giving a photometric procedure for scaling an aperture.
There's still one question on which Peter and I disagree that's wide open. Are LF lenses' aperture scales geometric (f/stops) or photometric (t/stops)? Secondary question, not asked clearly yet. With relatively modern coated lenses, does the difference between the two matter?
Cheers,
Dan
Repro lenses may be shimmed, because optical precision is (was) vital for that kind of work. Wide-angle lenses are more sensitive to cell spacing than most other lenses, so WA lenses are more likely to be shimmed (but don't have to be).
to the best of my knowledge, LF lenses are marked in F-stops - if it were T-stops that would have been marked. With most lenses or fairly recent vintage it makes practically no difference. Even my aging and visibly yellowed Apo-Lanthar has only lost about 1/4 stop. An uncoated dialyte loses about 2/3 of a stop, and that's just about the worst case scenario in LF lenses (8 air/glass surfaces).
My home-made cardstock aperture scales:
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