Re: [Leica] focus change

[Jim Brick <jim@brick.org>]

At 08:28 PM 12/18/97 -0800, you wrote:

>So Jim, if you have a remote setup that you prefocussed at freezing
>temperatures, and are expecting to get a sharp picture after the sun hits
>the lens, good luck! Unless the temperature/focus point curve puts these
>two conditions on the same focus point, you won't get the sharp picture you
>expect. That's a fact of life (and optics) that nobody has yet found a
>solution for for general camera optics. It's akin to demanding that all
>colors focus on the same plane, period. It just doesn't happen.
>

OK, I'll go along with the fact that temperature could indeed cause a focus
shift. It makes sense. It would still piss me off on a $8000 lens. 

But actually, what I really objected to, was that the opinion is that long
lenses focus past infinity just because of the temperature drift. This is
not true. They focus past infinity so that a photographer, knowing the
proper focusing process, can indeed critically focus on very distant
objects with those very long lenses.

My 350 Telyt (not a fluorite based lens) focuses a full half inch past
infinity. This is NOT for temperature drift. Can you imagine the drift
necessary to take up a half inch on the focusing ring? A half inch the
other way is roughly 175 feet. Leica designers are smart enough to know
that to achieve critical focus, you must yo-yo-in. Try yo-yo'ing in on the
moon if your long lens does not have plenty of space past infinity. It came
in very handy when photographing Hale-Bopp.

Jim