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Could you guys expound upon that? This is the first I've heard of the 40D being particularly bad at f/8.
I think you've misunderstood. The 40D is not 'bad' at f8, it has one of the best AF systems in its class and works well at any aperture .
The autofocus of the 40D, like the 5D and other xxD and xxxD bodies, is designed to work with lenses that when opened to their maximum aperture are at f/5.6 or wider - which is all canon ef and ef-s lenses and most after market lenses.
It is designed to not AF with lenses that have a maximum aperture smaller than f/5.6 (in other words, it won't work with an f/8 lens - this is different from an f/5.6 lens stopped down - for which AF WILL work). This means that it theoretically won't work with a f/5.6 lens that has a 1.4 adapter, making the total lens maximum aperture f/8. Neither will the AF of any canon body except the 1 series.
You can get around this limitation by fooling the body that you're using a f5/6 lens, by using a tamron teleconverter with no electronics, or by taping the pins on a kenko or canon teleconverter. This trick works with the outer focus points of the 40D, but not so well (if at all) with the highly sensitve centre point. The taping trick should also work with the 5D, 30D, 20D, 10D, and xxxD series.
Edited on Jan 25, 2008 at 02:07 AM