In article ,
wrote:
On 27 Nov 2004 22:17:10 -0500, (DoN. Nichols)
wrote:



A double helical -- a tube threaded inside and outside -- one
left-hand thread and the other right hand thread, with an inner tube
connected to the camera and the outer tube connected to the telescope,
and some kind of keyway to prevent rotation of the inner tube relative
to the outer tube and you are fine. This is pretty much the way the
focusing ring on SLRs is done.

Nope. Focus ring is a nut, with a retainer to keep it from moving,
the male thread is on the lens barrel. Usually 4 to 6 lead though,
never single lead. Which means there are a lot of ways that it will
go together, only one of which is right.

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. I have disassembled lenses of this sort, so I am
sure that they exist -- even as short as 135mm focal length on a 35mm
camera. The focus ring is tied to the ambisexual ring, with a female
thread attached to the camera body, and a male thread attached to the
lens cell itself, so you can move the lens cell a long distance without
having to have a threaded sleeve the whole length of the travel.

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.

Suppose a helical focuser for a scope could be made that way, but
most of them don't have a real thread as such, it's usually a radiused
groove, and engages a ball indent for rapid focusing. I make mine
like a very shallow acme thread, the groove just wide enough to take
the ball, and deep enough so the ball almost touches the bottom.
Parfocal eyepieces eliminate the need for the rapid, but add quite a
bit to the cost. There are focusers available in just about any
configuration anyone would want, if their wallet is heavy enough.

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.

Enjoy,
DoN.
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