Which CV 35 2.5

[wilsonlaidlaw]

Hi Wilson,

I am not sure that would work - because adding thickness to the lens mount will only shift the scale focus of the lens and the offset between the RF and the actual focus will remain the same. If the lens is back focussing then the RF cam needs to be made longer with respect to the optical cell.

Think I have got that right.

David,

I am not sure you have this right.

Let's say for argument that it is back focusing by 10cm at 2 meters. That means that the point of most accurate focus is at 2.10 meters, when the lens is set on scale to 2 M or focusing on the RF to 2 meters, assuming that the scale and RF cam are in sync. So in order to get the lens to focus at 2 meters, you would have to focus on scale to 1.90 meters. This would involve moving the lens cell (and RF cam) forward, so that adding a shim between lens and mount would have the same correcting effect.

Now has my brain become scrambled (not unlikely with the new anti arthritis regime I have been put on) or is this correct?

The above was why I could not get my Summicron 50 corrected, because the sync between the RF cam and optical cell seemed to vary between infinity and 2 meters and the extent of back focus varied as well. In theory if a lens is correct at 2 meters, as long as the focus thread is the correct pitch, the lens should be correct at infinity.

Wilson

 

[Braeside]

Wilson, This is hurting my brain too. But perhaps we are slightly at cross purposes.

In the 2 cases I have dealt with, I had 2 different problems to fix.

I felt the M8's RF was adjusted fine as it worked well with other Leica lenses.

1. CV 75/2.5 Front focus at infinity and other distances - when the rangefinder patch was aligned, the lens was focussed slightly in front of where it should be. This I could cure by shortening the RF cam length relative to the optical cell, by lapping. NOTE: This lens was accurate on the scale - infinity marking on lens was actually infinity OK, before and after the RF cam length was reduced.

2. CV 35/2.5 Infinity marking on lens not infinity focus, but Rangefinder focus accurate. In my case infinity was a little bit short of the infinity end stop, so I shimmed the lens mount which moves both RF and optical cell forward by the same amount. This brought the scale into agreement with the actual distance and the rangefinder still focussed accurately.

Hopefully from the above you can work out which of the two problems you have, it may you have both - the lens needs shimmed to bring the scale into alignment and the length of the RF cam needs adjusted to get the RF to agree with true focus.