This is not a lens repair blog, but as
a) one of the foundational ideas of JAPB was for me to have a place to post stuff for a permanent record (which FB’s algorithms would just bury), and
b) I’ve just today Rewritten the same comment in a Facebook groups for the third time,
it’s time JAPB gets its first lens repair related post.
(BTW, I will add illustrations from my next lens repair)
A helicoid is the mechanism that lenses use for adjusting focus. A Helicoid is made up by two sets of threads on two hollow cylinders (aka pipes or barrels) that mesh. One part of that helicoid is connected to the lens mount (and is hence essentially fixed in place, while another part is turned by the lens’ focusing ring and houses (or otherwise moves) the optics that are used in focusing.
Most legacy lenses are designed to be unit focusing, meaning that the mobile part of the helicoid is designed to move the entire assembly of optics. This leads to that many lenses allow a large number of repairs that do not necessitate separating the helicoid. And that’s a good thing because reassembling helicoids can be a bastardly job (for reasons that will become abundantly clear).
The general recommendation for amateur lens mechanics is a) avoid having to separate the helicoid, and if that is not possible, to make sure to mark the point of separation. See Richard Haw’s brilliant article on working with helicoids.
However, sometimes you might not have those markings, and it may be due to other things than your forgetfulness:
• you might not want to leave any sign that you’ve been there
• the lens might have been opened before, leaving marks that then conflict with your markings, leading to confusion. Either way, you may need to do it the hard way.
Why is it hard?
Face it, the difficult part is not to get the helicoid to thread, but to get it on the right threads. Depending on a combination of focus throw and focusing movement, a helicoid can have anything from (in my experience) 3 to 11 threads, and only one is the correct one. (To be clear: the correct thread is the one where infinity focus lines up with infinity markings.)
How to rethread a helicoid that has no markings or confusing markings?
Whenever I have no clear markings to go by, this is my approach:
1) Install the lens elements
The lens elements need to be in place and correctly mounted for you to be able to (later) ascertain that infinity focus lines up with the infinity markings.
2) Grease the helicoid
Using whatever helicoid grease you would normally use (and after having thoroughly cleaned the helicoid earlier), put some grease on the threads. This is important to make sure you’re even less likely to damage the helicoid in the next steps.
3) Put the heavier part flat on the table with the helicoid facing up.
If the rear end of the lens is the heavy part, the rear cap might be needed to achieve flatness.
4) View the lens from side-on
Depending on your arrangement, you might need to either lower your viewpoint or raise the table, but is important that you will able to comfortably and precisely ascertain horizontals.
5) Gently lower the other part of the helicoid until it rest on the other.
Gently, is the most important part here, because if you dent even one of the helicoid threads, you’ll have trouble to start the threading. Also try to make sure that both parts are centred on the optical axis…
6) Very gingerly start rotating the top part counter-threadwise.
First, note the counter-threadwise, because the point is really to rotate the top part in the wrong direction. Depending on the lens, counter-threadwise might be counter-clockwise or clockwise.
Second, every time you feel a click (and the top drops a bit), you’ve found one of the many threads.
While doing so, keep an eye on that the top part remains horizontal.
Finally, (important) before you reverse and start to go threadwise, ascertain visually (or with a bubble level) that the helicoids are level and not at an angle.
7 ) Start threading.
Stop and reverse if you encounter resistance.
• If you manage to thread to infinity, mount the lens and check whether you’ve found the correct thread (you get infinity focus at infinity markings).
• If you cannot thread all the way to infinity (but are sure you found a thread), check whether the helicoids are obstructed and start over from step 3.
• If you managed to thread to infinity, but infinity focus was off, go to 8)
8) Find the next thread.
Start by unthreading the helicoid, but do not separate the parts. Instead, thread counter-clockwise until the next click. That is the next thread. Jump to step 7.
Repeat steps 7-8 until you’ve found the correct thread.
Obviously some details will differ from lens to lens (removable hard stop, removable helicoid key, infinity adjustable freely by changing position of the focus ring ), but the thread-finding procedure is the same.