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| p.16 #3 · Dot-Tune: Autofocus Fine Tuning in under 5 minutes |
My point is though, that if a person just sets up their target, focuses, and does a single round or two of tuning, the resulting tune value may not be all that accurate.
I can leave the camera position untouched, simply run the CDF to focus the lens again, and the tune value changes a couple of points. Or I can move the center AF point to a different part of the target and the tune value changes a couple of points. I also found that if your light levels change while doing the tune process, the end points of the tune range change. I had cloud cover come over and reduce my light level a couple of stops, and the tune range shrank several points. While the Dot Tune method does remove a bunch of the mechanical variability, you still have to account for the mechanical variability of the initial focus, and how the AF system responds to changing stimuli, even if you aren't moving the servos.
What's to say the single position on the target you chose, with the single focus setting, is the most accurate tune setting?
Personally, given my experience with this process over the last week, the only way to go is to run the tune process multiple times, refocusing, moving the view to slightly different part of the target, etc. and get the approximate range of tune values. Then take that set of a dozen tune values or whatever, and average it. For instance, using the same checkerboard target, consistent light, no movement of the tripod... only moving the camera's view of the target slightly and refocusing, I got a range of values of +2 to +9.5.
If I setup a target, and no matter where I focused on that target (within reason) it gave me a +/- 1 tune value every time, my argument would be moot, and a single tuning session would be all that's needed.