New AF MA micro adjust method: DotTune

Started Feb 15, 2013 | Discussions thread
Horshack
Veteran MemberPosts: 4,093
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Re: Didn't work for me
In reply to Clem Nichols, Feb 27, 2013

Clem Nichols wrote:

Horshack wrote:

" racking focus in between each photo to the same position on the lens (like infinity). "

Herein lies one of the problems I noticed while trying to MFA my Sigma 30mm f/1.4 lens, which is admittedly not one of the best lenses available. If I manually focused the lens behind the target before pressing the AF button it would focus on a different spot than it would if I started with the lens manually focused in front of the target. It was as if the camera had a satisfactory AF range rather than an exact point, and if it came at that range from opposite directions it would stop as soon as that range was reached. Consequently the results of adjusting the lens were different when starting with the lens manually far-focused vs. near-focused. This problem was not as noticeable with either the Canon 50mm f/1.4 or the Canon 85mm f/1.8, and I was inclined to attribute it to the lens rather than the camera. Regardless of which was at fault, the only way MFA would have helped with this particular lens was if I always remembered to either far or near focus (depending on which way I had racked focus during the test) before taking the shot. In retrospect, probably what I should have done was taken a series of shots both ways and then averaged the MFA adjustment numbers which is obviously what is being done with the Dot Tune method.

I have no experience with the FoCal system, but I know that Michael Tapes recommends that when testing using the LensAlign system the lens always be de-focused in the same direction before the camera is auto-focused

Your experience dovetails with my theories on PDAF operation. MFA fixes only static optical path alignment/tolerance issues, which includes the lens, mount, mirror, AF point, etc; basically, everything in the AF system short of the the AF motor/servo.

AF motor/servo based issues are supposed to be accounted for by the closed-loop iterations of the PDAF system, although that system as you observed is designed to arrive at focus within a given range (for speed), which means lenses with imprecise positioning servos/detectors or nonlinearities in their movements will still produce inconsistent focus even with a perfectly tuned MFA.

The reason multiple PDAF acquisitions must be used in most MFA techniques (including LensAlign/Focal) is to work around the shot-to-shot mechanical variability of the AF motor/servo, so that the (static) AF tune value can calculated in spite of this variability, rather than because of it. As you noted, MFA can't resolve AF motor/servo issues, and based on my experiments and observations it wasn't designed to.

I wrote a post about this here (I'm snapsy on FM), and another related post here.

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