A long story with questions at the end...
The weather was very poor for most of this past long holiday weekend here in Houston, so I took advantage of the storms' keeping me indoors to do some lens testing. I have 3 lens components from Kodak 3A folding cameras:
- a B&L Rapid Rectilinear lens set
- a Kodak Anastigmet 170mm f6.3 lens set,
- a Kodamatic shutter, with speeds 1/2 through 1/150 and marked f/6.3 through f/45.
The Anastigmat lens came mounted in the Kodamatic shutter.
I have used both lens sets with this shutter and in general the negatives were overexposed, by several stops, compared with other exposures I made at the same time with other lenses - - nothing scientific.
My first thought was that the shutter speeds were probably slow, which would cause the overexposure problem. I have a shutter tester and I tested each speed (5 firings each speed, after exercising the shutter well) and found that none of the speeds was off by more than 1/2 stop except for 1/150. So that did not explain the over-exposure I observed.
I decided to take a further step and attempted to calibrate the f-stops against a known source. I mounted each lens set in the shutter on a lensboard, set the shutter to f/6.3, and metered the ground glass with a spot meter, with the camera aimed at a blank wall and the front standard set for infinity focus. I then used an equivalent-length "modern" lens and adjusted the f-stop until I got an equivalent exposure from the ground glass. From the modern lens I could read the f/stop giving the same reading.
Based on this method (which I am certain some of you will be able to identify flaws in), the B&L RR lens actually has a maximum f/ in this shutter of 6.3, which is how the shutter is marked, and the Kodak Anastigmat has an f/ of 4.7. And these results are what concerns me.
I have seen the B&L RR lenses mounted in ball bearing shutters with the US f/ markings of f/4 as the max and I know that that system is about 2-3 stops different from the modern f/stop scale. So, the RR result is not surprising. The Kodak Anastigmat lens is marked f/6.3, and since it was mounted in this shutter originally, that would make sense - - both the lens and the shutter were marked f/6.3. But then how is it possible to get a GG reading equivalent to f/4.7 from an equivalent focal length lens focused at the same point? When I got this result, I repeated my set-up and measurement several times to be sure.
Of course, I am going to confirm what I measured by taking test shots, but I only can get out to shoot once every few weeks and I like to make the best of that time, by working out things here at home when I can.
Does anyone have any ideas? Is my testing strategy all wrong? I appreciate any insight from those of you who stuck it out reading all the way to the end. Thanks.
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