Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!decwrl!netcomsv!wolf
From: wolf@netcom.COM (Buckskin Tech.)
Newsgroups: sci.electronics
Subject: Re: Those little autofocus cameras
Message-ID: <1991Jun11.194629.11994@netcom.COM>
Date: 11 Jun 91 19:46:29 GMT
References: <1991Jun8.020128.23216@milton.u.washington.edu> <1990029@hpldsla.sid.hp.com>
Organization: Netcom - Online Communication Services  UNIX System {408 241-9760 guest}
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tonya@hpldsla.sid.hp.com (Tony Arnerich) writes:

>In no case does any imaging take place in the point&shoot's focus innards.
>tonya@sid.hp.com

True of the Point&Shoots, but my Chinon CP9 (as well as many other SLR's)
does indeed use imaging to provide autofocus.  The image in the viewfinder
is projected onto a line array of photodetectors, and comparisons are made
to determine whether there's a sharp edge crossing the line.  If not, the
lens focuses, first one way, then the other, finally getting as sharp a line
as possible focused on the array.  This leads to several interesting 
"idosyncracies" in focusing the CP9.  First, the subject needs to have an
obvious vertical line running through it.  If it doesn't, you have to find
one, request a focus, then lock it down.  Second, the autofocus system doesn't
work at all in low light.  The CP9 has a high-brightness LED mounted above
the lens that kicks in when the autofocus and exposure systems agree it's
too dark to focus.  It lights up the scene a little, usually enough to get
a focus lock.

One nice thing about this system is it can be uses along with the manual
focus lenses.  When using manual focus, you can focus normally with the
CP9 letting you know when it sees a focused image.  I have astigmatism, 
and so far, the CP9's been right when I thought it wasn't.  I've learned to
trust it's opinion.

 - Wolf