These focusing helicoids are reasonably priced and precise. However, one problem I found with the longer helicoids used with this and similar lenses is that the displacement is linear, i.e., a rotation of a given angle produces the same change in helicoid length, along the whole range of extension.

Focusing near infinity usually requires very small changes in extension. In the close-up and macro range, a larger change of extension is usually required (and focusing is best done by moving the lens + camera as a whole after setting the magnification with the helicoid).

If you are satisfied with the focusing precision near infinity with your current setup, no problem. Just keep using your present setup. If you want a better focusing precision (i.e., a wider rotation angle to achieve the same change of extension) in a subset of the focusing range, then the simplest solution is to use a much shorter helicoid, together with a stack of extension tubes to reach the desired length.

All helicoids of this type available on eBay that I have seen (I own about a dozen of different lengths and types) are linear, and all rotate by roughly the same angle (less than one full turn) from minimum to maximum length. None that I am aware of has a nonlinear extension travel. Most well-designed lenses instead have non-linear focusing slots, to make focusing comfortable both near infinity and in the close range.

In practice, with my suggestion you end up with two separate sets of helicoid + extension tubes, one optimized for focusing on distant subjects but not capable of focusing to close range, the other focusing from infinity to close-up but difficult to use for focusing near infinity.

PS - it is even possible to stack two helicoids of very different lengths, to get the "best of both worlds", but then you need to remember which one is best for which focusing range. And not many photographers can boast of having a lens with two focusing rings! But dual coarse and fine focusing knobs are available on most microscopes, so nothing really new.

**Edited by enricosavazzi, 25 August 2017 - 08:58.**