Evrim Icoz pointed out

> http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=001spa

which describes the EOS-3 having difficulty in focussing on periodic
patterns.

My eyes would have a similar difficulty when focussing on a bunch of
periodic vertical lines with the old central split-screen system where
you focus by aligning the upper and lower image. When looking only at
the split-screen part, it is difficult to see which upper line belongs
to which lower line.

The camera has a similar problem. It tries to figure out how to
position the lens to make two light patterns on the AF sensors match
up. The sensors see only part of the periodic image and if it chooses
the wrong lines to match up it will turn the lens to the wrong
distance.

You can probably help the camera by manually focussing sufficiently
close to the correct distance and then starting AF.

Hope this helps,

Peter Wagemans
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