K-01 and K5-II Contrast AF problems

Started May 5, 2013 | Discussions thread
Kerusker Senior Member • Posts: 1,199
Re: K-01 and K5-II Contrast AF problems


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.

-- hide signature --

we don't see that we don't see (eye's blind spot)

 Kerusker's gear list:Kerusker's gear list
Pentax K-5 II Sony RX10 IV Pentax smc DA 15mm F4 ED AL Limited Pentax smc DA* 16-50mm F2.8 ED AL (IF) SDM Pentax smc DA 55-300mm F4.0-5.8 ED
Post (hide subjects) Posted by
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow