Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!valid!caber!lou
From: lou@caber.valid.com (Louis K. Scheffer)
Newsgroups: sci.electronics
Subject: Re: Those little autofocus cameras
Message-ID: <564@valid.valid.com>
Date: 14 Jun 91 02:38:53 GMT
References: <1991Jun8.020128.23216@milton.u.washington.edu>
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wiml@milton.u.washington.edu (William Lewis) writes:


>   You know those cheap little autofocus ("point and shoot") cameras
>(usually with a sort of sliding thing over he lens): I've been
>wondering how the focus mechanism works. I looked at one and the only
>things that looksed likely were a pair of lenses behind a plastic
>window.

   In at least some of these models, there are two lenses in the classic
rangefinder configuration.  By looking at the scene from two slightly
different angles (hence the two lenses) you can figure out the distance
to the target, at least to the accuracy required by these cameras.  On
my (waterproof) point and shoot, this causes an interesting side effect-
the autofocus does not work under water, since the angles are screwed up
by refraction.  I think they calculate the angle by doing a correlation
of the images cast onto two linear arrays of photo-diodes.

   The polaroid point and shoot uses ultrasound (or did anyway).  This
works poorly when shooting through windows.

   Some fancier autofocus cameras work by maximizing the high frequency
content of the image.  This works poorly on scenes with low high frequency
content, such as clouds.

-Lou Scheffer-