Rangefinder adjustment-dancing angels?

[aad]

So I pointed my Elmar at the moon last night and noticed a small misalignment when focused at infinity.

I dove in to all the information on adjustment-then I stopped.

At great distance, the rangefinder mechanism is at its least accurate-fortunate, because focus is least critical. At short distance, it is at its highest accuracy.

(Note that by accurate I mean repeatable)

When we adjust the rangefinder at close distance, however, we can only verify that the lens marking is accurate.

I also have read several instruction sets explaining how to adjust at both (near) infinity and at 4 feet-but there is only one screw. If you're set at one point, the rest of the focus accuracy is determined by the lens cam.

So-wouldn't it be best to measure precision at a close distance and let infinity determine itself? Or even better, check by photograph, aligning the rangefinder on a given object then chcking for focus-and ignore markings altogether?

For what it's worth, I've never had a question about focus-even wide open with a Nokton, it seems ok.

Comments?

 

[VinceC]

I've been adjusting my RFs using infinity for a couple of decades now. The moon's actually a little difficult. I prefer a bright star or planet, like Venus. Infinity is accurate because infinity is just as precise a distance as 4 feet. Infinity is marked on the lens and is simpler to calibrate than measuring 4 feet from the flim plane. If RF is accurate at infinity, and the arm-travel is accurate, then 4 feet will also be in focus.

 

[clintock]

I also have read several instruction sets explaining how to adjust at both (near) infinity and at 4 feet-but there is only one screw. If you're set at one point, the rest of the focus accuracy is determined by the lens cam.

So-wouldn't it be best to measure precision at a close distance and let infinity determine itself?

Comments?

There is always more than one screw-(sometimes three?) one for the infinity and one for the close, and moving one affects the other a little, so it takes some doing to get them both right at the same time. One screw is often under a cover plate or screw plug on the top shell of the camera, the other is sometimes part of the RF arm, in the case of leica and some CV, it's in the center of the roller on the end of the RF arm. Wouldn't it be wonderful to have service literature for CV!!!!

I suppose one could verify the close focus distance vs lens marking using groundglass and loupe, and prehaps re-mark the real 1m or 4' mark on the lens with tape or pencil or something before using the lens as a reference.

 

[aad]

Well, today I tried again-lo and behold, if the RF patch is centered in the window, infinity seems to be, um, infinite. Amazing how much movement there is if you don't keep that patch centered.