AF Adjustment

How does this explain my Pentax DFA 100 F2.8 WR Macro lens John that requires +9 adjustment of my K-5iis and +8 on my K-3? The factory tolerances didn't work too well on that one did they?

Regards
David

AF adjustment is necessary because of tiny variances in the tolerances of the body and the lens. It's just not feasible to manufacture such equipment to absolute perfection so AF adjustment serves to neutralise the differences.

I understand at least some Oly bodies read the serial number of the lens so they can even have different AF adjustments for different copies of the same model. I think that's the ideal setup.

I'm not sure that is the entire story. When I (try to) focus adjust I do find the f/b focus variance changes from lens to lens, which would be accounted for by variances in lens mount thickness. However I also find the same lens can vary according to the direction the lens is moving in (i.e. infinity -> min focus or the reverse). I also find variance according to how far out of focus the lens started i.e. how fast the lens is moving at the time it achieves focus.

This makes me think that as well as mount thickness there is a dependency on the lens's focusing characteristics as well. Which doesn't give me confidence that it won't change with lens (or body focus motor) wear either. And I wonder if it will vary if my battery is fully charged or nearly flat and whether I have a battery grip on as well - but I can't be bothered testing that.

I think in practice you can't do any better than pick a value that seems reasonably optimum for a variety of different focus distances and lens travel speed shots and then use that without worrying any further. It's going to be pretty good for most shots - and you can manual focus or use LV for a check if you have a shot that is particularly critical. Until Pentax automated focus adjust facility and profile focus adjust requirements rather than just store a single calibration per lens I don't think we can do any better.

The calibration is to make sure the path of the focussed light is exactly the same to the main sensor as it is to the AF sensor. I'm sure there a number of tolerences contributing to such an inaccuracy not just lens mount thickness.

Lens mount thickness or anything else doesn't affect contrast AF on the main sensor itself, so the main issue has to be how various lenses affect he accuracy of the lightpath in the AF Module.

Falk Lumo did a lot tests on the K-5 becuase of it's initial bad AF in low light ( My first K-5 body was so bad SRS replaced it for me )


http://www.falklumo.com/lumolabs/articles/k5focus/

See section 6

Well mine did have the secret service menu but with no entry for focus compensation/adjustment. So, I just put it back to normal and will have to leave it as it is.

Reading this whole thread to date, I'm going to calibrate my primes at least, when I get my K-30 back...

The only way to do it on the K10D was in debug mode. I actually left my K10D in debug mode and changed the value when I changed lenses. yes it was a PITA, but better to have images in focus