[Leica] focus on infinity

["Alan Shils" <ashils@bestweb.net>]

Bob (etc.)

Decades ago I read a technical paper about (camera) lenses that stated that
infinity for a lens was defined as 1,000 (or, if I do not remember
correctly, perhaps 2,000) times the focal length of the lens. (Maybe,
2,000.)  But - it doesn't matter. It is a mathematical and
optical-practical definition. (It may even change or be meaningless for
certain optics - i.e. flat-field rather than spherical lenses.) To a
photographer it is just the distance after which it makes no sense to be
concerned about the focus setting. 'Focus on infinity' is word-play, but
the lense has to be 'someplace'.  Thus, when a lens is serviced the tech
should set the lens at the distance from the film plane when the
focus-marking is set on infinity so the lens is at the proper distance
(from the film) using collimated light. You can think of such light as
'comming from infinity'.  We could never afford the shipping costs to send
something from (or to) infinity unless -1- photons had no weight and -2-
colimated just meant parallel. So, whether you are looking at stars or the
tech's focus-lamp e.g., collimated light source,  save the shipping charges
on your photons. (This is because of very technical stuff I read ages ago
and mostly forgot. See an optics engineer for the latest lecture.)  It is
all due to the design of the lens and the criteria  for its resolution
(circle of confusion and such not-so-confusing stuff and it is somewhat
arbitrary).  In short - don't worry about it.

The definition(s) of 'infinity' tend to give people other than
mathematicians and engineers the shivers involking all sorts of
metaphysical garbage. Perhaps the simple truth(s) of science are too easy
for those people. (Wanna buy a ticket to ride a UFO?)  First, infinity is
the division of any non-zero number by zero. (Don't start with 0 / 0. That
is a special problem.) Big deal. Just math you snicker. Ok, infinity has a
lot of practical applications. To a radio engineer, infinity can represent
resonance in a tuned circuit. To a math-cat it can be a limit in
limit-theory or a value you will not see as a variable approaches a value
but never (as in NEVER) reaches it. (As in asimtotic) (?spell?).

In photography, (not of the documentry, medical, forensic, etc. types)
forget about infinity and just be rather careful in focusing in general -
including being careful about what you want out of focus. I suggest being
concerned whether the picture you are thinking about making will be worth
looking at tomorrow, next week, next year or next generation or more. If it
isn't any of that, it is a waste of materials. (Yes, I have wasted my share
of materials.) (But also caught some biggies.)

Hope this doesn't confuse things even more. Words can more often get in the
way and mess things up more rather than clarify an item. (Anyone who
doesn't believe that should hang around a courtroom for a while.) That's
why math is so refreshing - the clarity - like a sunny day over a burning
city.

Your comments, please.

..........Alan