Pentax-L 43mm f1.9 - Focus Problem

[Chris Bail]

That makes sense to me. If the correlation between where the RF cam thinks the lens is focused at and where the lens actually is focused at is off, you can affect the focus by messing with the adapter but the place the RF thinks the focus at will shift, too, and things will still be off.

That's my conclusion as well. I'm going to probably have to send it off for adjustment.

Anyone know if this sort of thing would be worked on during a CLA? For that matter, what exactly is done during a lens CLA, besides cleaning and replacing the lubricants?

 

[John Shriver]

Well, sometimes a CLA involves correcting incorrect assembly. Both the threads for the focusing helical, and for the rangefinder helical (used on lenses wider than 51.8mm) have multiple starts, put the lens on the wrong start and it won't focus right.

There's two things you have to adjust on Leica RF lens. One is collimation, when set to infinity, it should focus on the film/sensor accurately. The other is the setting of the rangefinder cam, on an LTM lens it should protrude exactly 7.5mm from the lens mounting flange when focused to infinity. This is something you may be able to measure with your lens.

There's also the issue that the rangefinder cam moves at the right "speed" or "rate" as you focus closely, but this is a matter of the rate being correct for the true focual length of the lens. Leica made focusing cams with different thread pitches, and picked one that matched the actual focal length of the lens. But I suspect that Pentax just made all the lenses close enough in focal length to use only one thread pitch on their rangefinder cams. (They probably selected ones of the right focal length from the SLR production.)