K-01 and K5-II Contrast AF problems

Started May 5, 2013 | Discussions thread
philzucker
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Re: K-01 and K5-II Contrast AF problems
In reply to Kerusker, May 5, 2013

Kerusker wrote:

Phil,

some time ago I observed a similar behavior with a SIGMA Zoom at the long end i.e. PD-AF is spot on and CD-AF is off in almost every shot.  I tried to get some info from Pentax but they blamed Sigma (of course).

There are at least three possible reasons:

A) The contrast of the lens is too low for contrast detection by the CD-AF system.

B) Lens aberration like CA, field curvature etc. are too high for CD-AF.

C) The CD-AF algorithm has to go past the focus plane and it may use a return value R to return to the distance of optimal contrast. Some lenses may not be able to return correctly or the value R may be wrong.

Case C) is most likely to me but I don't have enough information (or experience) to be sure.

As I already said in my answer to Mike, I don't think inferior contrast or to severe aberrations are real issues with the Tamron 70-200/2.8. So I'd also settle for "C", whatever it means!

Phil

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