Summicron 11818 (Rigid) focus quirk

[LCSmith]

I recently picked up another chrome Summicron Rigid (11818). The seller described it as having been CLA'd by DAG. It's a lovely example, near mint, no scratches, and focus is buttery smooth -- 99.9 percent of the time. Every once in a while the focus will halt or get stuck for a moment, but just a moment. This happens quite rarely and I could sit for 10 minutes turning the focus of the lens without it happening. In a full day of shooting it happens once or twice. Any ideas what the problem may be?

 

[raydm6]

I don't know the answer myself but am curious as to the cause:


I have an LTM 50 'Cron and it exhibits similar behavior. It's more like resistance feel. But only 1-way.


I feel it when the lens stop is at the 1 o'clock position (~3.5 feet) and I am focusing back down to Infinity 7 o'clock position. Doesn't do it when I am focusing from Infinity to 3.5 Feet.


Lens was CLA'd by Youxin but I never mentioned it to him when he had the lens apart.

 

[aizan]

The same thing happened on my collapsible Summicron.

I think the helicals get worn down over time, or they weren't quite perfect when they were made. Getting it re-lubed didn't fix it.

 

[LCSmith]

I feel it when the lens stop is at the 1 o'clock position (~3.5 feet) and I am focusing back down to Infinity 7 o'clock position. Doesn't do it when I am focusing from Infinity to 3.5 Feet.

This is exactly how it happens on my lens.

 

[raydm6]

This is exactly how it happens on my lens.

What is odd is that when I point the lens up towards the ceiling or down towards the floor and focus the lens, all the resistance disappears. Strange.


I can't visualize the internal mechanics, but Aizan might have something there about the helical (brass I think?). Perhaps it is shifting somehow and causing some slight interference.