Mel Gross
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burychka wrote:
I think Canon xxD autofocus is better than the firmware allows. That is, it could work in good light with an F8 or F9 maximum aperture. I have the Canon 1.4xII and 2xII converters. I also have the inexpensive Tamron 1.4x (not the "Pro" version). I have experimented with various combinations, and with taping over the extra pins in the Canon converters. Here is what I have found.
Canon 100-400 (F5.6 at 400mm). With the Tamron 1.4x, the autofocus works just fine until late in the day. That converter lacks the extra three contacts, so the camera doesn't know the converter is there. The camera still reports 400mm F5.6. With the Canon 1.4xII, the camera reports the lens as 560mm F8 and accordingly disables AF.
Canon 500mm F4. With the 2xII, the camera reports 1000mm F8 and disables AF. If I tape over the extra three pins in the extender, the camera indicates 500mm F4, but the AF just hunts, rarely getting a focus lock. I don't have a Tamron 2x.
Sigma 50-500 (F6.3 at 500mm, but the lens is designed to "lie" to the camera, that is, it reports itself as being F5.6). The Canon TCs will not fit the Sigma lens, however, the Tamron does, and--lacking the three extra electical contacts--does not report its presence to the camera. This combo, which is is 700mm F9, has no problem with AF in good light. Yes at dusk it is hopeless.
My point in all this is that Canon's lockout of AF at smaller than 5.6 is just being conservative--or maybe it is to support the 1 series cameras? I have had this same AF experience with an original digital Rebel, Rebel XT, 20D, and 40D. Don't know yet about the 5D though, as my local dealer has no information on 5D2's shipping to them. Changing the firmware switch to work up to F8 seems like the most logical and simple of hacks. But then, I'm not a programmer.
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I think what happens here is that while the AF focus will work sometimes, under certain circumstances, but not others, Canon can't say that it does work, so it locks it out.
If Canon didn't, before you could say "class action suit", there would be one over the poor, unreliable AF under those circumstances. They aren't alone here in this.
Unfortunately, liability is something every company must worry about.
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