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AF is out

Started Dec 9, 2013 | Questions thread
WilbaW
WilbaW Forum Pro • Posts: 11,643
Re: AF is out
1

Mike Tansey wrote:

It doesn't matter if I'm in live view, as I'm using quick-af which snaps out of live view for a second for "proper" focusing using the centre AF point.

It seems a bit hit and miss too, as sometimes it gets it right, other times not so much.

Sounds like the classic behaviour for which that lens is notorious. Typically, focus will be confirmed in two distinct places, which at f/2.8 near MFD can put the subject randomly outside the DOF. It's not because that lens has a crude focus motor/mechanism as is often erroneously claimed, it seems to be about how the light it delivers can be interpreted by PD AF sensors.

I know using a wide aperture gives a very narrow focal plane, so the margin for error is large, but surely a wide aperture also means the "circle of confusion" either side of in-focus should help the AF determine it's not yet in focus and try again, rather than accepting a blurred image as in focus.

No, that's another myth. PD AF sensors see the scene like two telescopes at a window - as long as the window is wide enough that they both have a clear view, making the window wider doesn't make any difference.

I can't upload pics right now as I'm at work.

No worries, I know what they look like. 

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