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(DoN. Nichols) wrote in message ...

That may be the case in the eyepiece focusing on a telescope,
and for *some* camera lenses, but the longer the focal length, and the
more desire for close focusing, the more likely the camera lens is to be
with double helical.

The only ones that I have ever seen with the double helical are the
Macro-Zoom lenses, and the double action was switched out unless using
the macro function.

Yes, multi-lead makes for faster focusing, and is amost always
found in SLRs at least -- and even in the very fine thread on the front
element of a Zeiss 75mm f3.5 Tessar on a folding 120/620 roll film
camera (16 shots per roll, with that short a focal length -- vertical
format by default. That was my first camera with a reasonable quality
lens.

The reason for the multiple lead is that it holds the lens more square
to the body than a single lead of the same lead pitch would. Much
closer tolerances are easily held than would be the case with a
coarser single lead.

The ball and groove one would work well enough for something
light, like an eyepiece, but when you hang the weight of a camera body
-- especially if it has a motor drive as part of it for semi-remote
shooting -- you need something more sturdy, like a real thread.

I'm not aware of any photo/visual being made with a helical focuser,
if a camera is going to be used, it's normall in a rack and pinion
focuser. I don't ever remember seeing a helical in any refractor,
other than in the finder scope, almost always rack and pinion. (Even
the Yerkes 40" Clark, which I've had the priviledge of having a 30
second peek through it. Absolutely, completely, totally spoiled for
life. Open house at their centenial, but only using about 700
diameters magnification. somewhere between 300 to 500 people got to
look through it during the course of the night.)