Canon LTM Backfocusing Canon 50mm f/0.95 need help

Canon M39 M39 screw mount bodies/lenses
johnastovall

johnastovall

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I just got back a Canon 50/0.95 which was converted to an M-mount for use on my R-D1. The person who did the conversion had a problem getting it set up due to 'problem with the R-D1's rangefinder."

I have tested the lens with both my new M8 and the R-D1 and it does front focus badly.

Where can I find some one to get both the R-D1 and the Canon 0.95 to play together?


 
johnastovall said:
I just got back a Canon 50/0.95 which was converted to an M-mount for use on my R-D1. The person who did the conversion had a problem getting it set up due to 'problem with the R-D1's rangefinder."

I have tested the lens with both my new M8 and the R-D1 and it does front focus badly.
Click to expand...


By "front focus" you mean it focuses in front of the intended focus point?

If so, that means the collimating shim in the lens is too thick for your specific camera(s.) In other words, for a given rangefinder-indicated distance, the too-thick shim makes the lens extend slightly too far. More extension gives closer focus, so the lens focuses slightly in front of the intended point.


(Speculation: I've encountered a number of Canon RF lenses that front-focus slightly on non-Canon cameras, and I suspect that Canon set up its body dimensions assuming slightly less film thickness and/or curliness than, e.g., Leica. Since Canon thought the film would be slightly closer to the lens, they spaced the lens slightly farther from the body. The result is a lens that focuses perfectly on a Canon, but front-focuses slightly on a camera set up to Leica specs.)​

Ideally you'd get this addressed by the same person who did your lens conversion, but he'd probably need to have your camera body with it so he could match them specifically. Note that the focus point on these lenses is critical enough that there's probably no point in sending just the lens to any technician -- whoever does the work is going to need both the lens and the body, so they can be matched as closely as possible. (If you want the lens matched to two bodies this probably will mean a compromise setting.)

Or if you're brave and reasonably dextrous, you might be able to do this yourself. The idea is to remove the lens' optical section from the focusing mount, by unscrewing the large slotted ring around the rear element (hidden behind the breech-lock flange on a standard 50/0.95, probably under the M mount on a converted lens.) The optical section lifts out of the focusing mount in one piece, quite easily and safely, and the collimation shim is a brass spacer ring that fits between the optical section and the focusing mount.

The idea is that you extract this shim and then thin it down very slightly, by rubbing it against a sheet of wet, very-fine-grade abrasive paper laid on a flat surface. Turn the shim frequently as you rub it, to reduce the risk of getting the two faces out of parallel.

Bear in mind that you're trying to remove a very small amount -- for example, if you set up a test shot at an intended distance of 1 meter, and your actual point of best focus is 994 mm (6mm of front focus) then you need to aim to remove no more than about 0.02mm -- two one-hundredths of a millimeter. That doesn't take much rubbing, even with fine-grade paper. An inexpensive digital caliper might be a good investment to make sure you're not removing too much material too quickly!

In practice, you don't want to rely on measurements at all; instead you want to rub a bit, then very carefully wipe off any metal grit from the collimation shim, reassemble the lens, put it back on the camera, and make more test shots. Do this until you "sneak up" on the correct amount of thinning to match the lens to your specific camera.

Yes, it's a bit tedious (and unless you wear gloves you will get bloody fingers, because the edges of the shim are sharp!) but several of us here have done it with excellent results.

Also, make a note of how much material you remove. That way, if you ever want to return your lens to its original spec for a Canon camera, it should be fairly easy to do by going to a hardware store and buying some brass shim stock of the proper thickness. Brass this thin can be trimmed easily with a hobby knife, so you should be able to cut it using the original collimation shim as a pattern, then sandwich the two shims together to "re-Canonize" your lens.
 
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