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[OM] Re: lenses and AF

Subject: [OM] Re: lenses and AF
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Wed, 28 Sep 2005 13:49:13 -0400
This link <http://www.fredmiranda.com/forum/topic/241524> has some 
extensive discussion on Canon autofocusing and some revealing data on 
the limitations of its accuracy.  To quote a piece:

--------------------------------------------------------------
When the camera determines how far and in what direction the lens must 
move to cancel the phase difference, it does so within a tolerance of 
"within the depth of focus" of lenses slower than f2.8 (down to f5.6) or 
"within 1/3 of the depth of focus" of lenses f2.8 and faster. The depth 
of focus is the range at the sensor plane within which the image of a 
point will be reproduced as a blur smaller than the manufacturer's 
designated "circle of confusion" (CoC). Canon's designated circle of 
confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C 
format. The CoC is based on maintaining the appearance of sharpness in a 
6x9 inch print at about an 10 inch viewing distance (as revealed by the 
Euro-Canon web site). There is no guarantee that images enlarged any 
greater than this will appear sharp.
--------------------------------------------------------------

Sobering numbers from an autofocus system that is considered very good. 
  Buy those KatzEye screens and touch up the focus.

Chuck Norcutt

ScottGee1 wrote:
> I'd have to dig through my archives to find it, but Popular
> Photography here in the U.S. published a report about AF focusing
> accuracy a number of years ago.  The results were less than pretty.  I
> wrote Jason Schneider a while back suggesting they run tests on
> current models, but I wouldn't be surprised if their advertisers
> raised hell after the first article and Pop is gun shy now.
> 
> They published 'real life' in camera metering tests a couple of times
> with similarly ugly results and we haven't seen one of those recently
> either.
> 
> FWIW/ScottGee1


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