Stiff focus caused by those two little tabs inside lenses

fatdeeman · Joined: 13 Jun 2009 Posts: 780 Location: UK

First off, I'll post an image of the parts I mean. In this example, it's a Helios 44M but the same applies to various other lenses.

I find every so often I will strip and lubricate a lens and the helicoid will be beautifully smooth and well damped and then when those two damn tabs are put back, I realise THEY are causing the stiffness!

On many lenses, it's a non issue, but I do come across quite a few where no matter how much I try and adjust them, the focus is always stiff or has tight spots.

Take the 44M for example. You might turn the focus and it's semi smooth then hits a tight spot. At this point I loosen the screws on those tabs and re-tighten them again. Now the lens will be smooth and the tight spot gone, but it will be tight somehwere else! So you do the same thing at the new tight spot and the tight spot just seems to end up somehwere else.

I get the same sometimes with a lens such as the Jena 135mm. It feels like you're chasing a moving target!

Anybody got an ideas how to improve such lenses? Do the lines often scribed onto the tabs means anything?

Some people suggest filing the tabs slightly which guess could work but would surely introduce slop if you did it too much?

RokkorDoctor · Joined: 27 Nov 2021 Posts: 1272 Location: Kent, UK

Yes, they can be a pain in the butt.

The tabs are called the helicoid keys, the grooves in which they run are called the helicoid keyways.

Over time the keys tend to "run in" a bit and set themselves to their respective keyways. When servicing it is therefore important that they stay matched to their respective keyways, hence the markings, usually to match the key to the keyways, and to mark back vs. front. The markings will depend on the engineer that worked on the lens, they usually have their own preferred marking methods (cross, arrows, dashes, etc., possibly in conjunction with some notes made during the work).

And yes, re-adjusting the keys can be a bit of a moving target: some work first time, some need trial & error and patience; it all depends on how much clearance the mating helicoids were machined with.
DO NOT FILE the keys down. If necessary the sliding sides of them can be buffed or burnished, but there should be no need for filing; it may only make matters worse: it is not the keys jamming in the keyways that is the problem (assuming the two keys didn't get mixed up), but rather the sub-optimally aligned keys pushing the helicoids slightly out of 100% perfect centration as the helicoid moves in and out, especially if one of the keyways has not been machined 100.00% square to the helicoid. Giving the keys more "slop" can make the de-centering of the helicoids worse during their travel, causing them to bind even more.

Most problematic are lenses with two keys, and a long helicoid (those are usually tele lenses of unit-focus design; wide-angles and normal lenses on the other hand sometimes only have one key.)
Just as you experienced, I often have to work those a bit through their focus range whilst readjusting the keys when I experience the helicoid stiffening up at some points. Patience is key here (no pun intended).

A few further tips:

1) When encountering a tight spot, don't fully loosen up the screws fixing the keys in position. Just take the pressure off the hold-down screws a bit, so the keys are no longer tightly held and can readjust themselves a bit under pressure of the binding helicoid, without them being so loose as to be able to rattle. Then tighten up again. This may take a few tries anyway. If you loosen them up too much, the helicoid will be allowed to center itself too much/perfectly at the current tight spot, possibly creating another new tight spot somewhere else in the focus range.

2) The sliding mating surfaces of the keyways (and keys) require a grease that is quite a bit thicker/stiffer than the grease used in the helicoid itself. Something like Japan Hobby Tool/Pig Iron #1500 or #3000 will do fine, but you can take it all the way up to damping grease if you like such as ROCOL Kilopoise grease (not the oil !!) which is normally used for lubricating things like the sliding surfaces of microscope table/focus dovetails.

3) If the helicoid keeps binding up no matter what you try, consider using a helicoid grease with a molybdenum disulfide dry-lube additive, if you are not already (that is a bit more slippy than micronised PTFE additive, and may help).

BrianSVP · Joined: 09 Jun 2023 Posts: 301 Location: Philadelphia

I use a similar technique - having cleaned and greased the keyways, place the keys/rails and don't fully tighten the screws. Pull the lens through its entire focus travel a couple of times, then stop around the middle of the focus throw. This will typically be the most centered point in the focus travel. Once there, rock the focus back and forth slightly a couple of times, then go a couple degrees one way or the other, and tighten the screws down to firm, but NOT overtight. If you are concerned about the screws loosening over time, use a bit of blue threadlocker prior to inserting them rather than cranking them down.

This should be enough to get it perfectly smooth, as the main trick is to keep the two keys from ending up tilted in opposite directions in their respective keyways. If it doesn't work the first time, loosen, go the other way, and retighten. 99% of the time, everything will be smooth after this procedure. Regular medium consistency grease is almost always all that's ever needed. If you have to resort to fancy additives, either the keyway's not clean, or you have probably reassembled something wrong.

I agree that it's key (pun intended) to make sure the screws and keys are returned to their original positions, particularly on Eastern European lenses, where machining tolerances are poor, but I find that they are almost never pre-marked in their positions from the factory, and its up to the repairer to do so during disassembly (photos help, too). If they are pre-marked, typically it's a sign the lens has been previously repaired.