Originally posted by jonby In other words, no matter what the focusing distance, a turn of 1 degree from the point of perfect focus will give the same apparent change in sharpness, all else being equal. So I'm not sure that the situation is as complex as you are suggesting.
Feel free to correct me if I am wrong but I as far as I understand it, the autofocus system works in a different way than you seem to think.
The system is more like:
(AF system) Hey lens, you're out of whack, according to my calculation, you should move forward. And you're way off so make it quick
(Lens) Roger that, forward full speed.
(AF) Slow down you're getting there.
(Lens) slowing down.
(AF) OK STOP!!!
(Lens) Done! According to what I can measure, I think I am currently at about 4.6m from the target, just so you know. Take your shot.
And not like this:
(AF) Lens, I command you to turn 135.456392 degree in clockwise rotation in order to achieve focus, this is 165 1/3 turns of the focus motor.
(Lens) Copy that, I'll let you know when I get there.
(Lens) I'm there, and write down in your report that the object in question is located exactly 4.653344 meters from us. Fire at will!
I don't think anything in the system is aware of the number of degree the focus ring moved in order to achieve focus. There is no way for the focus motor to know how many turns are required to get from close to far focus (screw drive doesn't know for sure, SDM/DC only maybe, but why bother adding this since it's not required). PDAF knows when stuff is in focus or not and by how much to a certain extent but it won't know how long it will need to ask the lens to turn in order to maintain a proper focus for stacking. A 90 degree focus throw lens might require the screwdrive to turn 100 times where a 230 degree one will require 600 turns... and another 230 degree one only requires 60 turns of the screwdrive... and then there is the gearing inside the lens, and quickshift and lens variation and temperature variation. And what if your lens had to be adjusted?
Simple in theory, not so simple in practice.