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!!?

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?

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)

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.

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

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

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 http://www.usa.canon.com/templatedata/pres...070820_40d.html says:

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)

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.