[Leica] Focusing long lenses (long)

[Jim Brick <jim@brick.org>]

At 11:59 PM 12/17/97 -0500, you wrote:
>At 12:06 AM 12/17/97 -0800, you wrote:
>
>Jim:
>
>So perhaps my practice is just making the most of DOF. Thanks.
>
>Chuck Westphal of Canon explained to me that longer lenses with flourite
>and other rare earth glass are made to focus past infinity because the
>glass can expand in very warm situations, shifting the focus points. I'm
>not sure I buy it.
>
>Tom

Tom,

I personally am skeptical about a lens changing focus points based on
temperature. Imagine this... you are in Alaska. It's very cold so you leave
your camera and long rare earth lens in the car (or porch) so moisture
doesn't condense on it and it'll be ready the next morning. Just before
dawn, the next morning, you set-up your camera and lens in a tree blind to
photograph Eagles in their nests. You have, perhaps, a long remote trigger
cable, or radio trigger. You critically focus on the nest and then move
away, part way down the tree. Dawn comes, the Eagle and perhaps chicks,
start moving. You make a few exposures. The Eagle flies away. The sun
appears and shines on your lens. The sun's radiation warms your lens. The
Eagle comes back. You continue to expose frames. So as the lens warms, it
changes focus. Now I don't know about you... but I would be plenty pissed
if my $5,000 or $10,000 lens changed focus just because it changed
temperature. Unless Erwin shows us that this temperature focus change
really happens to the extent that the lens would have to focus past
"infinity" to compensate for it, I DON'T BUY IT! Not for a minute! The
lenses that I'm familiar with that focus past infinity aren't all rare
glass lenses. They are all long telephotos though. So this is how I see it:


Long lenses, of course, will focus on far away objects. With a WA lens, you
can barely see these objects. Like the Moon. When you focus on something,
you turn the lens focus ring until the object comes into focus and continue
past perfect focus. You then reverse the direction and come back past
perfect focus, only by a little. Then back to the forward direction slowly
stopping on "perfect focus." This is a normal focusing pattern. Stopping on
both sides of perfect focus, closing in on perfect focus. You may repeat
this a few times to "just make sure."

You use long lenses, quite often, to photograph far away objects. Think of
photographing that far far away mountain peak. Or better yet, the full Moon
on a very clear night, away from the city. With your 800mm lens. Or even
your 350mm lens. It works exactly the same. Where is infinity with the
800mm or 350mm lens and the Moon? These kinds of lenses will focus past the
Moon. Way past the moon. The moon is 239,000 miles away. But through the
Leica R viewfinder, with your long lens, it looks quite large. With long
lenses, you can focus on the Moon as if it were house, or a tree, something
close. You focus past the moon, in front of the moon, then on the moon. But
remember, it's 239,000 miles away.

I personally believe that the reason long lenses focus past distant
objects, is so you *can* focus on distant objects. In order to easily focus
on very very distant objects, such as the moon, the lens will have to focus
beyond what we perceive as infinity. If it didn't, critical focusing on
these very distant objects would be mechanically impaired. The normal human
focusing cycle is retained, with long lenses, by allowing the mechanics to
take the focus beyond (well beyond) infinity.

Another note. My 350mm Telyt focuses past very distant objects, such as the
moon. Way way past infinity. Just look at the lens barrel focusing marks
and you can see just how far the 350 Telyt goes past the infinity mark. A
LONG WAY! I've used this lens in below zero weather (winter in the Colorado
Rockys) and in very very warm places (summer in Death Valley CA). It
focused past "infinity" whether freezing cold, or boiling hot. If the extra
focus room was supposed to be taken up by "focus expansion" it didn't
happen. And it won't ever happen.

Remember, this is just my humble opinion, based on my fifty years of
experience. But so far, it hasn't let me down. I'm always looking forward
to learning something though.

I apologize for being so long winded. I get that way when it's late.

Jim