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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX
Hi BC:

Thank you for your clarification. So, if I understand it correctly, the PDAF is fast but crude, compared to CDAF. And, for the last step of lens movement the camera does not check it again. Then, the speed of PDAF relative to CDAF is mainly due to the algorithm, the CDAF is slow because it uses more iterations of calculation and movement, right?

PS. Now I see that the focus problem talked about in my question is not called "focus shift"--"focus shift" should refer to the shift in focus when the lens is stopped down. I don't remember what other people call it (the focus is incorrect when the lens is wide open), perhaps "front/back focus problem" (?).

Best regards,


[quote name='Brightcolours' timestamp='1343112966' post='19579']

Yes, it is correct that PD Af works in iterations. On internet it has been a long belief that it was a look once move once affair, but lots of evidence makes clear it is a process done in steps. The reason why PD AF can get it so wrong still, at times, is because the steps are quite crude for speed reasons, and the last step is not verified again, also for speed reasons. If the camera is certain it is close to focus, it will tell the lens to move a certain distance compared to the last step the lens did. If the lens goofs up this step or the camera calculation for whatever reason is incorrect, you end up with a more or less missfocussed image. Also, some lenses just are missfocussing very constantly on some cameras, even though their lens movements are calibrated. This can have to do with certain lens aberrations, especially if the AF system looks at just a certain range of the spectrum.

Focus shift has nothing to do with how PD AF works though. It is called focus shift when the lens wide open (AF is determined when the lens is wide open) has a different focal plane than when you close the lens down a bit. The focal plane shifts, that is focus shift.

Some cameras can take that into consideration, only when the camera knows about that certain lens of course, and if their AF system is advanced enough and has such considerations implemented.


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Next PZ lens test report: Tokina AF 50-135mm f/2.8 AT-X Pro DX - by frank - 07-26-2012, 04:15 AM

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