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Forum: Pentax SLR Lens Discussion
03-20-2015, 12:41 AM
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No, that's very wrong. Of course there is a faster way to do it.
Imagine we were playing a game of "guess the number", in the range of -20 to +20. You start guessing with 0. I say that you're too high. So, what's your next guess? It's not -1; that would be silly. Your next guess is -10. And so the game continues: I tell you -10 is too low. Your next guess isn't -9. Your next guess is -5. Then maybe I tell you that's too high now. So, your next guess isn't -6. It's -7 or -8 (technically, it should be -7.5, but let's pretend we're only allowing integers). Eventually you will just guess the right number, and that will be that.
The same procedure works for AFMA, using either the dock or the built-in AFMA in your camera. Higher and lower are nothing more than front and back focus. You can use longitudinal CA to detect front vs. back focus if your lens is susceptible to that sort of thing (a target with a lot of black and white patterns works best), or you can use a ruler or something.
I don't know where everyone is getting this ridiculous idea that you have to go one by one through all possible values until you find the correct one; you are not the first person on the internet I've seen with this misconception. Indeed, it seems that this is perceived as a major downside of the Sigma dock, that you need 41 tries per focal length range (or 164 total, if you have a zoom lens) to properly nail the focus. This is not correct. Simply going from -5 to +5 as you suggested is also not very effective. That's already 11 settings that you need to try, just to decide if you want to try more!
In computer science, the procedure you have described is formally known as a "linear search" and will get you laughed out of most job interviews. It can be proven that the method I describe, which is known as a "binary search", will take at most log(41)/log(2), or approximately 6, tries before finding the correct value. Since it can be a little harder to identify "focus adjustment is correct" vs. just knowing that the number you have chosen matches the number I'm thinking of, you might take 2 extra tests, +/-1 from the final number you settled on. This is still only 8 tries total and it is guaranteed to find you the correct number.
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