K-01 and K5-II Contrast AF problems

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

philzucker wrote:

MightyMike wrote:

I've been saying this for months... CDAF isn't the second coming of focusing systems, some lenses just don't work

Sigma 18-125mm F3.5-5.6, Sigma 75-200mm F3.8, Sigma 17-70mm F2.8-4.0, and others, the common issue is it gets worse at the tele end.

If my findings are correct add Tamron 18-250 f3.5-6.3 and Tamron 70-200 f2.8 to that list ... and yes, it's worse at the tele end with those two too ...

Yes my findings are that its always worse at the long end, also it always seems to be front focus

The issues is likely spherical aberrations, all lenses have this to some extent, this causes softening of the image if its really bad and allows for 3 points of focus, of course 2 of those points aren't accurate and one is best focus. What happens is the stupid CDAF system doesn't know which best focus to stop at and just lands on one. there may be a way the CDAF system can be programmed to go past the first point of focus to the best point of focus and this may be occurring in first party lenses.

Hmm, at least with the 70-200/f2.8 it's hard to for me to believe that severe aberrations are the problem here ... it's a very fine lens even wide open ... but of course I don't know for sure.

Spherical aberrations are such that they don't necessarily have a visible affect on image quality, yes bad ones will, however its more of a pattern, you could have poorly corrected SA can lead to focus shift even with PDAF, its still corrected and you can still get great IQ but there is still a difference between 2 inaccurate focus points and the point of best focus. Take for instance the focus shift described on the Samyang 85mm F1.4, below F4 you need to focus wide open, above F4 there is a notable shift and focusing wide open won't necessarily provide accurate focus stopped down, this is all due to SA despite it being a very well regarded lens.

So for all those people who are singing the praises of CDAF being the most accurate system out their, look again, with some lenses and in low light its a dud! its useless!

Then again with other lenses and in good light it does have it's use, doesn't it?

I suppose there are some uses for CDAF, but until its vastly improved i'll stick with my PDAF... on another note the K-5IIs PDAF focuses far far better in low light then the CDAF, according to Pentax the PDAF has a 4 stop advantage over the CDAF

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Mike from Canada
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