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Old Sep 8, 2005, 11:57 AM   #1
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I know that the Canon dSLRs have a better, higher resolution AF sensor that is enabled on f/2.8 and larger lenses. Is that high-res sensor only active when you're actually using f/2.8? When I stop my f/2.8 lens down to f/8 it reverts to using the standard AF sensor, right?

I'm thinking of trading my Sigma 70-300 APO DG for the Sigma 70-200 f/2.8 EX and just wondered how that worked.
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Old Sep 8, 2005, 12:44 PM   #2
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When you're focusing the lens remains wide opened!

It only closes down to f/8 after the shutter is released and prior to the curtain opening up > the AF sensor focus with the full f/2.8 aperture.


If you stop down the lens with the side button then that's a different story, but then the entire viewfinder would be a lot darker :O
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Old Sep 8, 2005, 12:48 PM   #3
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Ah, well that'll teach me to read up more on these newfangled auto-focus lenses. Guess that makessense, though...otherwise the viewfinder would go all dark when you stopped down far enough.

Excuse my ignorance, I'm still thinking back to my old nikon lenses where the aperture was a manual ring, as well as the focus being manual
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Old Sep 8, 2005, 1:32 PM   #4
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The defocus info on an f/2.8 is greater for the camera to work with...

-> see the illustration in the attached diagram - http://konicaminolta.com/products/co...x7/ope01b.html


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