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01-11-2010, 09:44 AM   #16
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QuoteOriginally posted by PentaxPoke Quote
No. Urban Legend. The only place I have ever heard or read that is on a Pentax forum trying to justfy slow AF.

There's a big a discussion about AF at dpr (again) and it seems like people have been digging up all sorts of AF complains from the canon forum regarding accuracy.

Also, someone linked to this article: Canon AF System (which doesn't seem to be a Pentax fansite)

QuoteQuote:
In one "look," it calculates the distance and direction the lens must be moved to cancel the phase differences. It then commands the lens to move the appropriate distance and direction and stops. It does not "hunt" for a best focus, nor does it take a second look after the lens has moved (it is an "open loop" system).

When the camera determines how far and in what direction the lens must move to cancel the phase difference, it does so within a tolerance of "within the depth of focus" ... Canon's designated circle of confusion is 0.035mm for the 24x36mm format and 0.02mm for the APS-C format. ... There is no guarantee that images enlarged any greater than this will appear sharp.

As a result of this tolerance (within the depth of focus or within 1/3 of the depth of focus), the camera can place the actual plane of focus at random anywhere within the tolerance range, and not necessarily at the same place each time.
From this description, it definitely sounds like Canon skimps on accuracy for speed.

01-11-2010, 09:53 AM   #17
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I think this whole discussion is strange.
Maybe Canon has the same issue as other cameras - in order to get speed up, the focus points cover large areas.
One of the biggest draw backs of the K-x is the HUUUUGE size of the center focus point. It make focusing of a fast lens pointless, because you may focus on this strand of hair or that strand of hair or the eye... It is almost as useless as 11 point AF.

Really need to put a split screen into the K-x.
01-12-2010, 04:47 AM   #18
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QuoteOriginally posted by AdrianN Quote
From this description, it definitely sounds like Canon skimps on accuracy for speed.
I think that this is a question of parameterization.

Don't know for Canon. But I believe to have observed (note my defensive style ) that the K-7 can do one more focus hunt step than the K-x when I once compared with identical lens, light and subject. Both photos turned out to be sharp but the K-7 has more pixels ...
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