jlw
Rangefinder camera pedant
This isn't really R-D 1 specific, but I'm posting it here because there isn't a general "technical" forum...
...Even though I had thought my R-D 1's RF alignment was fine, after reading the thread about adjusting it I decided to check again and decided I should do a little more tweaking of the vertical alignment to try to get it "perfect." Reaching that gray screw was a bit scary because of the angle involved, but I did get it and was pleased to see the difference when I had the vertical bang-on rather than just "close."
Having done this, I immediately started checking all my lenses for infinity focus, using a distant but contrasty TV antenna tower, to make sure the vertical adjustment hadn't disturbed that (I know, it shouldn't, but when you're poking around with screwdrivers you never know...)
The first lens I checked was my Canon 50/1.4, and it lined up perfectly at infinity. Then I tried my 35/1.7 Ultron, and was annoyed to find that it did NOT quite reach infinity. I bought this lens new and have never messed with its innards, so I didn't see why it should be "off."
Then, on a whim, I switched the two lenses' screw-to-bayo adapters. Sure enough, now the Ultron focused to infinity and the Canon would not -- it HAD to be the adapter!
They were having a sale on them at the local Harbor Freight Tool store last week, so I had bought a digital caliper with an LCD readout switchable for inches or millimeters. I put this on mm and measured the thickness of each adapter at several points around its rim.
The "good" adapter measured 0.98 or 0.99 mm all the way around. Makes sense: I knew that the flange-to-film distance of an M body is exactly 1mm less than a screwmount body, so making the adapter slightly thinner than that would allow 0.01mm margin at both the body-to-adapter surface and the adapter-to-lens surface to allow clearance for removal.
Then I measured the "suspect" adapter. Its thickness was 0.98mm about halfway around -- but on the other half, it measured as much as 1.02 to 1.03mm thick! Obviously either it hadn't been machined perfectly flat, or had been warped slightly during the subsequent finishing... and apparently, this slight additional thickness is enough to keep the RF from reaching infinity.
Now not being able to reach infinity is annoying enough -- but going from theory (and some theorist correct me if I'm wrong) I am guessing that this extra thickness would also throw off focusing for non-50mm lenses only.
I say that because this type of rangefinder is calibrated so the follower arm moves in and out by the same distance a 50mm lens moves in and out when focused; because of this, a 50mm lens doesn't need a differential cam to compensate for its focusing. So, even if the adapter is too thick, it will move BOTH the lens and the cam out by the same amount -- so correct focus will be maintained even though the lens won't reach infinity.
But a shorter lens requires less extension to focus at a given close distance than a 50mm lens requires -- so a too-thick adapter would move the lens disproportionately vs. the RF cam, causing the lens to "front-focus" (focus on a plane closer than the one intended.)
I hadn't noticed this behavior with the Ultron, possibly because of its relatively generous DOF and/or because the disparity isn't too great.
But a lens with a shorter focal length would have a greater disparity, and of course a longer-focal-length lens would be "off" in the opposite direction (back-focus rather than front-focus) because the too-thick adapter would be providing proportionately LESS extension than indicated by the RF cam.
Sorry if that's too complicated, but the moral of the story is that non-spec screw-to-bayo adapters apparently ARE out there, and they can affect both infinity focus (of all lenses) and RF accuracy (of non-50mm lenses.)
I'm now trying to decide whether I should try to lap 0.05mm off the too-thick area of the adapter, or just say the heck with it and try my luck with a new one.
If anyone else has a metric caliper, it might be interesting to measure your adapters and report if you've got any "deviants"...
...Even though I had thought my R-D 1's RF alignment was fine, after reading the thread about adjusting it I decided to check again and decided I should do a little more tweaking of the vertical alignment to try to get it "perfect." Reaching that gray screw was a bit scary because of the angle involved, but I did get it and was pleased to see the difference when I had the vertical bang-on rather than just "close."
Having done this, I immediately started checking all my lenses for infinity focus, using a distant but contrasty TV antenna tower, to make sure the vertical adjustment hadn't disturbed that (I know, it shouldn't, but when you're poking around with screwdrivers you never know...)
The first lens I checked was my Canon 50/1.4, and it lined up perfectly at infinity. Then I tried my 35/1.7 Ultron, and was annoyed to find that it did NOT quite reach infinity. I bought this lens new and have never messed with its innards, so I didn't see why it should be "off."
Then, on a whim, I switched the two lenses' screw-to-bayo adapters. Sure enough, now the Ultron focused to infinity and the Canon would not -- it HAD to be the adapter!
They were having a sale on them at the local Harbor Freight Tool store last week, so I had bought a digital caliper with an LCD readout switchable for inches or millimeters. I put this on mm and measured the thickness of each adapter at several points around its rim.
The "good" adapter measured 0.98 or 0.99 mm all the way around. Makes sense: I knew that the flange-to-film distance of an M body is exactly 1mm less than a screwmount body, so making the adapter slightly thinner than that would allow 0.01mm margin at both the body-to-adapter surface and the adapter-to-lens surface to allow clearance for removal.
Then I measured the "suspect" adapter. Its thickness was 0.98mm about halfway around -- but on the other half, it measured as much as 1.02 to 1.03mm thick! Obviously either it hadn't been machined perfectly flat, or had been warped slightly during the subsequent finishing... and apparently, this slight additional thickness is enough to keep the RF from reaching infinity.
Now not being able to reach infinity is annoying enough -- but going from theory (and some theorist correct me if I'm wrong) I am guessing that this extra thickness would also throw off focusing for non-50mm lenses only.
I say that because this type of rangefinder is calibrated so the follower arm moves in and out by the same distance a 50mm lens moves in and out when focused; because of this, a 50mm lens doesn't need a differential cam to compensate for its focusing. So, even if the adapter is too thick, it will move BOTH the lens and the cam out by the same amount -- so correct focus will be maintained even though the lens won't reach infinity.
But a shorter lens requires less extension to focus at a given close distance than a 50mm lens requires -- so a too-thick adapter would move the lens disproportionately vs. the RF cam, causing the lens to "front-focus" (focus on a plane closer than the one intended.)
I hadn't noticed this behavior with the Ultron, possibly because of its relatively generous DOF and/or because the disparity isn't too great.
But a lens with a shorter focal length would have a greater disparity, and of course a longer-focal-length lens would be "off" in the opposite direction (back-focus rather than front-focus) because the too-thick adapter would be providing proportionately LESS extension than indicated by the RF cam.
Sorry if that's too complicated, but the moral of the story is that non-spec screw-to-bayo adapters apparently ARE out there, and they can affect both infinity focus (of all lenses) and RF accuracy (of non-50mm lenses.)
I'm now trying to decide whether I should try to lap 0.05mm off the too-thick area of the adapter, or just say the heck with it and try my luck with a new one.
If anyone else has a metric caliper, it might be interesting to measure your adapters and report if you've got any "deviants"...
