LTM lens filed down, fixable?

[dexdog]

I found a beat-up Tanack camera at a local thrift shop recently. The camera itself is pretty much a wreck, but the attached 5cm/2 Tanar didn't look too bad, so I bought the camera for $20. After I got home and looked at the lens more closely, I see that a previous owner had filed down a portion of the rear lens barrel that contacts the rangefinder arm on the camera. The filed area is now about 1mm shorter than the un-filed areas. I am trying to figure out how severe the affect of the filing would be on focusing accuracy, and whether or not there was any way to restore the filed areas. Lastly, why the hell would anybody file the lens down in the first place?

 

[Rob-F]

Apparently you are referring to the rangefinder cam. It is possible that the owner didn't file it, but that it came that way from the factory. The portion of the cam that corresponds to close focus is supposed to be shorter than the opposite end, which corresponds to infinity. The cam on many lenses tapers smoothly from infinity focus to close focus. But some lenses, like my 35mm Pre ASPH Summilux, have a slight discontinuity to the cam shape. My 35 Lux also has some surface roughness on some areas of the cam. The surface of my 40mm Rokkor has an even more pronounced discontinuity.

The exact final shape of the cam seems to be made by a grinding process. Grinding is not as pretty as polishing. However, grinding is a precision process, while polishing is not. For example, the film tracks in a Leica, being ground, are not as pretty as the polished ones of some other brands; but they are more accurate! It may be that what looks like filing could really be grinding marks.

So here's what I think you should do: shoot a roll of film and see how the lens performs.

 

[Sonnar Brian]

You would file down a small section of the threads to enable the RF follower of the camera to follow the RF cam of the lens.

I need to see some pictures of the rear of the lens to assess what is going on with this lens.

 

[dexdog]

Initial testing with the lens on a Canon L1 with a known-good rangefinder and a Sony NEX 5. Preliminary indications are that the lens focuses accurately on a camera, although the distance scale is significantly off. I took a series of shots using the rangefinder on the L1, and then measured the distance with a tape measure, and transferred the measured distance to the lens using the scale. As an example, using the L1 to focus, the scale on the lens said distance was 7 meters, while measured distance was about 5 meters. After focusing the lens on the NEX at 7x magnification, the scale on the lens barrel once again said 7 meters. Transferring the measured distance to the scale on the lens resulted in fuzzy images on the NEX. I will have to shoot the rest of the roll and have the film developed to double check focus.

 

[dexdog]

Apparently you are referring to the rangefinder cam. It is possible that the owner didn't file it, but that it came that way from the factory. The portion of the cam that corresponds to close focus is supposed to be shorter than the opposite end, which corresponds to infinity. The cam on many lenses tapers smoothly from infinity focus to close focus.

Yes, there is a gradual taper, and perhaps it s grinding, because it is not a smooth polished surface. The rather rough surface is what made me think that someone had filed it. I will shoot more images tomorrow and see how the lens performs. Also, I focused the lens on a Canon 7sz, and it gives the same distance readings as the L1. The measured distance still does not agree with the scale on the lens barrel, though.

 

[Sonnar Brian]

Typically "indexed Cams" are used on M-Mount lenses, but not screwmount 50's., It is used when the focal length is not quite the Leica 51.6mm standard. The Tanar should be a standard "Leica 50mm" lens, and not have a need for the 1mm taper.

On my Tanar: big back-focus on the M8, and I will need to adjust the shim. It could be possible that this one also needed to be shimmed, but for whatever reason- the person files the RF Cam.