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Author Topic: 20D Focussing  (Read 1909 times)

BJL

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20D Focussing
« on: September 14, 2004, 09:31:44 pm »

[font color=\'#000000\']Since I believe that no-one has a 20D yet except reviewers, let me pass on what I have read.
a) as with all but a few high end dSLR's, the auto-focus system requires lenses of f/5.6 or faster. How badly the performance falls of beyond that, I do not know. (I believe that so many modern zoom lenses are f/5.6 at the telephot end, and so few are slower, because of this widespread requirement of AF systems.)
 the center AF point is cross style, but reportedly only has that full functionality with lenses f/2.8 or faster (e.g. not with any EF-S lens).[/font]
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jabberwocky

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20D Focussing
« Reply #1 on: September 15, 2004, 10:48:45 am »

[font color=\'#000000\']I posted this question on Rob Galbraith's forum and got the answer from Chuck Westfall of Canon USA. It is f5.6.[/font]
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jabberwocky

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20D Focussing
« Reply #2 on: September 12, 2004, 02:07:21 pm »

[font color=\'#000000\']I've never actually seen or held a 20D but am considering one as a backup camera to my Mark II. What I need to know is whether or not the 20D will autofocus (center point only is OK) with an effective aperture of f8. For example when using a 500 f4 with a 2x convertor. Anyone out there have one of these cameras that can answer this?

Thanks
Ed[/font]
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meyerweb

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20D Focussing
« Reply #3 on: September 14, 2004, 11:22:09 pm »

[font color=\'#000000\']Not quite right.

Yes, the AF sensors require lenses of f5.6 of faster.  If the maximum aperture reported is less than f5.6 (as with a 100-400 IS and EF 2X TC), autofocus doesn't work at all.  With a 3rd party TC that doesn't accurately report the aperture, AF may work, but probably not very reliably.

The center cross sensor is actually 2 sensors.  One spaced for optimum performance with 2.8 or faster lenses, the other used for lenses slower than 2.8.  But both are cross sensors.  From the Canon USA press release:

"The sophisticated 9-point autofocus system has a high-precision cross-type sensor in the center position. This new design seen for the first time in the 20D provides full cross-type performance with maximum apertures as small as f/5.6, yet it achieves up to 3 times the standard focusing precision when used with EF lenses featuring maximum apertures larger than or equal to f/2.8."[/font]
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