Activating 35mm framelines for 40mm lenses without filing the flange: a bit additional info about focus accuracy

[Greenhilltony]

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Every user of Summicron-C 40 / M-Rokkor 40 / Voigtlander Nokton Classic 40 should have known the 35mm framelines can be activated by not fully rotate the lens to click when mounting, because there is a little bit of extrusion on the part of the flange for frameline activation.

The main finding and conclusion from me is: this method works if the lens focus cam is the parallel style, which means the CLE version M-Rokkor (with serial numbers on barrel) and Voigtlander 40/1.4 benefit from this trick.

When the focus cam is parallel, it couples with the lever in body by purely back and forth motions. So rotating the lens a bit backward does not affect the focus cam positions. On the other hand, the sloped focus cam used on Summicron-C 40 and the early M-Rokkor 40 made for CL brings trouble when you do the incomplete mounting trick. The coupling lever does not stay in place once you rotate the lens: as the focus cam is sloped, the lever will contact with the lower part of the cam, indicating a closer position to the rangefinder. It means the lens now is unable to be focused at infinity position, most obviously, and always focus a bit closer through out the 0.8m-infinity range.

Edited September 21, 2023 by Greenhilltony

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[dpitt]

Ages ago, when I used my unmodified 40mm Summicron C. I posted a hack, where I locked the frame line lever into place by inserting a small piece of plastic under the lever. This way I could use this lens with 35 mm frames.

I now have a modified version that brings up 35mm framelines in stead of the standard 50mm lines. These framelines are quite accurate for the 40mm when you stay inside the lines.

[dpitt]

I found it: