More about adapters [warning: long!]
More about adapters [warning: long!]
Will said:
Great insight, thanks a lot.
Regarding plating, they should only be 1~3 micrometer, but that's for the watch industry, so maybe it is not as important as the thickness of the metal...
My thinking is that if the metal is already on the thick end of the tolerance band, plating on top of it will only make the situation worse. (Note that I've never gotten a too-thin adapter, only too-thick ones, so I suspect that too-thick is the more common error.)
It seems to me that it would be more accurate to plate the adapter, then mill the rear mounting face to the proper final thickness. I believe this is how the Leitz-branded adapters were made. However, I have had some good adapters with plating on the rear face, so as you say, it shouldn't be a problem if the process is well-controlled. I'm just more suspicious of adapters with plating on the back, as it suggests at least the RISK of a short-cut manufacturing process.
Read on some DIY large format website stating that, the mount register should be +/- 0.007mm, is it true? In the case of using adapters, we have two variables, the camera mount, then the thickness of the adapters...Will
I don't know exactly where they would have gotten that figure, although it doesn't sound implausible. (I have to wonder how many large-format film holders really locate the film surface within 0.007mm, though!)
My problem with it is that the equipment I have only reads out to the nearest 0.01mm -- so even if more precision would theoretically be desirable, in practice there's no way to get there!
All I'm really trying to do by measuring is separate bad adapters from good ones, and for this purpose a measurement to the nearest 0.01mm seems to be close enough. The worst adapter I ever got (1.03mm thick) was off enough that I could see the difference in the R-D 1 rangefinder: a lens that focused to infinity with a known-good adapter would show a rangefinder image that was slightly "off" with the bad adapter.
However, other adapters that were less drastically inaccurate did not show an apparent difference in infinity focus via the rangefinder -- but still created problems with "near-focusing" when used with long lenses (i.e., the lens' actual focus plane would be closer than the rangefinder indicated.) I started measuring adapters to find a way of detecting these. Before I started doing this, I wasted a lot of time and effort thinking that my near-focusing problems indicated that my R-D 1's rangefinder was out of calibration -- when in fact all along the problem was out-of-tolerance adapters!
Incidentally, one subtlety about adapter thickness is that it will have NO effect on close-distance focus accuracy when a 50mm lens is used. This is because the rangefinder's coupling mechanism is based on the distance a 50mm lens moves outward as you focus it closer. If you fit a too-thick adapter, the lens won't quite focus to infinity -- but for close distances focusing via rangefinder WILL be accurate. The extra thickness of the adapter will displace the RF coupling cam forward by the same amount as the optics, so the correct relationship between lens extension and RF-cam extension is maintained. Unless you closely scrutinize pictures taken at the infinity distance setting, you may not even realize the adapter is out of whack.
When you mount a shorter or longer lens, though, an out-of-tolerance adapter will produce added problems. Compared to a 50mm lens, these lenses require different amounts of extension to focus to the same close distance: a longer lens requires more extension, while a wider lens requires less. Such lenses contain a mechanism that "translates" the amount of
lens extension required to focus at a particular distance to the amount of
rangefinder-cam extension that a
50mm lens would require to focus at the same distance.
If your adapter is too thick or too thin, this relationship is thrown off -- the cam compensates for how much the thickness change would affect a 50mm lens, but the actual focus distance changes by a different amount. Confusing!
