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Old Nov 11, 2004, 10:59 PM   #11
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I think that USM does make a difference. I note that in my own testing, the auto-focusing of the 17-40 f/4 was better than a 28 f/2.8. I know that the 2.8 focusing was supposed to be better, but it wasn't in my hands. I had a hard time getting it to perform as well as the zoom, so I returned it.

Of course, these aren't the same lenses, but I think that if you think about it, the quality of the transport can make a difference to the quality of the focusing.

I think that one factor to consider is that more accurate focusing matters more when you have really fast lenses that's wide open. So I personally don't put as much weight into the HP sensors any more. Now f/4.0 is about as slow as I'd like to get, and f/2.8 is certainly brighter. If I wanted to getless DoF, then certainly I would like to have a HP sensor kick in, but I'm not going to choose to get a 2.8 lens just for the HP sensor to kick in, based on my own experience.

Besides, nothing beats manual focusing with the angle finder and the focus indicator in my experience. Yes, a brighter lens would help in lower light, but a nice sharp f/4.0 is usually good enough.
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Old Nov 12, 2004, 7:18 AM   #12
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One can always shoot at f/4 (or higher), but the brighter f/2.8 helps during focus because the lens remains wide open during this time prior to the actual shot!

The high precision sensor is the reason behind why the 1D's focused better than the 10D regardless of a lens USM (or the similar HSM). We are not talking about AF speed here but its accuracy since there's more defocus info for the camera to work with when the diameter of the barrel is larger
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