Vunderblochen SLRs typically use passive AF and, as you mentioned, need some
kind of contrast for best results. Focus assist lights have very limited
range and would be useless on subjects like planes flying overhead.
Oddly, some P&S cameras have more sophisticated AF systems. Pentax, and
possibly others, have combined passive and active systems for reliability.
But, again, the range is limited.
The trick is to find a surface the AF can lock onto, then recompose along
the same focus "plane." ;) The Olympus ZLRs seem to be very good at this.
My mom's iS-2 can focus on a blank white wall out to reasonable distances by
projecting a red beam of vertical and horizontal lines from the focus assist
light.
Lex
===
From: "Gary Edwards" <garyetx@xxxxxxxx>
Subject: Re: [OM] Airplanes -Reply
Date: Wed, 2 Aug 2000 19:07:24 -0500
Some years ago several of us were photographing one of the first public
appearances of the B-2. My friends mostly had AF wonderbricks and several
experienced difficulty focusing on the bomber as it made low passes over
NAS
Dallas - their cameras would constantly hunt for focus. There are several
explanations possible, including all of the normal problems of AF, but we
speculated on two others. One, the dark charcoal grey airplane offered no
visual contrast across its airframe. Two, I've heard that some AF systems
operate in an IR band - and the airplane is intentionally an IR "hole."
Handy if the bad guys are using late model all-aspect IR-homing missiles,
which don't require a hot exhaust plume. Anybody know if the IR AF theory
is true?
OM content - the two of us shooting OM cameras had no problem at all.
Unfortunately, the B-2 in flight is mostly an undifferentiated grey profile
against the sky. Absolutely elegant to the eye, however.
Gary Edwards
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