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Author Topic: 40D aufocus disappointment  (Read 4006 times)

stever

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40D aufocus disappointment
« on: December 04, 2007, 04:25:30 pm »

just got a 40D.  i had hoped with the "improved" autofocus of the 40D, i would get better focusing with my 100-400 +1.4X with last 3 pins taped.

but autofocus with the combination seems to be unusable (it tries but rarely acheives focus even in good light) whereas the 20D is pretty useable in good light!!?
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JessicaLuchesi

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40D aufocus disappointment
« Reply #1 on: December 04, 2007, 06:45:40 pm »

Quote
just got a 40D.  i had hoped with the "improved" autofocus of the 40D, i would get better focusing with my 100-400 +1.4X with last 3 pins taped.

but autofocus with the combination seems to be unusable (it tries but rarely acheives focus even in good light) whereas the 20D is pretty useable in good light!!?
[a href=\"index.php?act=findpost&pid=158221\"][{POST_SNAPBACK}][/a]

Steve, I'm using my ( also brand new, woohooo ) 40D with the kit lens and a 70mm Macro from sigma. No converter, nor covering the electronic contacts of the autofocus system, and it works perfectly. I even feel the 70mm Sigma lens being more responsive than with other camera bodies I've used it with ( 300D and 20D ).

Why did you feel the need to tape the contacts?
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stever

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40D aufocus disappointment
« Reply #2 on: December 04, 2007, 07:29:12 pm »

Canon does not allow autofocus with certain lens/converter combinations - basically anything over f5.6 -- taping the 3 pins defeats the "lockout" and lets the camera try to autofocus even at f8 - in some cases (doesn't work with the 300 f4 and 2X)
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BJL

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40D aufocus disappointment
« Reply #3 on: December 05, 2007, 11:10:17 am »

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just got a 40D.  i had hoped with the "improved" autofocus of the 40D, i would get better focusing with my 100-400 +1.4X with last 3 pins taped.

but autofocus with the combination seems to be unusable (it tries but rarely acheives focus even in good light)
[a href=\"index.php?act=findpost&pid=158221\"][{POST_SNAPBACK}][/a]
Which shows that Canon disables AF in this situation for a good reason, not just to make life harder for users of its cameras!

AF sensors need a low enough aperture ratio to operate reliably or at all, and typically the limit is about f/5.6 to f/7. A few high end cameras work to f/8, but I believe that this is done by having several AF sensors at some locations, one working to f/8, others working only to f/5.6 or f/4 or even f/2.8.

What not just make all AF sensors work to f/8? Because an AF sensor with lower minimum usable f-stop (say f/4 instead of f/8) is faster and more accurate with lenses having minimum f-stops within its operating range.

(This is one reason why I prefer an SLR system with enough sensor resolution to let me use lenses about f/4 or faster (and certainly f/5.6 or faster) most or all of the time: having to use longer telephoto lenses with higher minimum f-stops like can be bad for AF.
« Last Edit: December 05, 2007, 11:15:05 am by BJL »
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stever

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40D aufocus disappointment
« Reply #4 on: December 05, 2007, 09:00:33 pm »

i still don't understand why the 40D should be noiceably worse than the 20D
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DarkPenguin

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40D aufocus disappointment
« Reply #5 on: December 05, 2007, 09:08:26 pm »

It has a different AF system.  What it does out of spec is undefined.
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BJL

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40D aufocus disappointment
« Reply #6 on: December 06, 2007, 02:30:38 pm »

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i still don't understand why the 40D should be noiceably worse than the 20D
[{POST_SNAPBACK}][/a]
One possibility is the trade-off I mention above, between AF speed and accuracy with "in spec" lenses (say f/5.6 or faster) and having a lower minimum f-stop beyond which AF fails.  Maybe the 40D AF is faster and/or more accurate with lenses f/5.6 or faster in exchange for working worse than the 20D with that f/8 combination.

In fact the press release for the 40D at [a href=\"http://www.usa.canon.com/templatedata/pressrelease/20070820_40d.html]http://www.usa.canon.com/templatedata/pres...070820_40d.html[/url] says:
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Canon has made significant improvements to its speed, precision and functionality ... The EOS 40D camera's completely redesigned nine-point AF sensor provides cross-type AF measurement at all nine focusing points for maximum apertures up to f/5.6, and for the first time in any EOS camera, the central AF point offers enhanced precision for both vertical and horizontal subject contrast when using EF or EF-S lenses featuring maximum apertures of f/2.8 or faster. AF calculation speed with the EOS 40D camera is 30 percent faster than the EOS 30D model.
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stever

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40D aufocus disappointment
« Reply #7 on: December 06, 2007, 03:45:43 pm »

yes, it's really only the central spot, but it seems strange that sensitivity wouldn't be enhanced at all focal lengths

i never had issue with central spot focusing on fast lenses, and obviously would rather have f8 autofocus like my old EOS 3 (or even as good as the 20D)
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BJL

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40D aufocus disappointment
« Reply #8 on: December 07, 2007, 05:07:37 pm »

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yes, it's really only the central spot, but it seems strange that sensitivity wouldn't be enhanced at all focal lengths
[a href=\"index.php?act=findpost&pid=158773\"][{POST_SNAPBACK}][/a]
Focal length is not the issue: minimum f-stop is. (Your AF problem is due to the f/8, not the 560mm).  The extra center point sensitivity is only with lenses f/2.8 or faster. Probably the center AF point has two AF sensors, the normal one like at all other AF points, good with lenses as slow as f/5.6, and a second special one that only works with lenses f/2.8 or faster, but gives better results with those fast lenses than the normal AF sensor does.
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