Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
Ben Strasser Senior Member • Posts: 1,763
Re: yes, but...

It is a closed loop system that shuts down on focus lock to conserve battery power and allow USM lenses to be manually tweaked for focus. Servo mode is left in closed loop mode, continually sampling the AF sensor output. AI Focus provides a slightly wider window prior to shutting down the closed loop. When AI Focus has decided the target is stationary and locks in One Shot, movement by the target will not reactivate and switch to Servo mode. Lens servo motors are stepper type motors that give feedback on how many steps the move. Accuracy is dependant on the distance between steps. No half step instructions or feedback, only whole steps. If one lens has, lets say 200 steps from near to far range and another has 100 steps, you can see how there would be a difference in focus positioning. The feedback assembly would have to be coupled to the actual lens focusing element. The calibration would be to allign the feedback pulses to the lens element. If the AF sensor says move 13 steps and the lens indicates it moved 13 steps, the AF system checks the target again or even many times and determines 12 steps is not enough and 14 steps is too much and locks. The error is between the steps and where fine calibration takes place. Coarse calibration would be to define how many steps it would take to focus from infinity to lets say 10 feet but I don't think the system works that way. I think the AF sensor is continually being looked at while the focus motor is driving and reaches a null point where movement in either direction would cause mis-focus. The only difference is where the feedback is coming from, motor or AF sensor.

Mishkin2 wrote:
it's still subject to body or lens miscalibration. The AF sensor
perhaps rectifies the position several times, but it still suffers
from miscalibration.

DavidP wrote:
Doesn't this, in effect, make it a semi-closed loop?

I simply can't imagine the lens being told "move 13 steps" and it
then moves 13 steps, without a final "OK, now how far should I
move?" step in there.

Mishkin2 wrote:

This talk may happen several times during focusing procedure,
though - in case, for example, camera moved since the focusing
began but before it locked. This also helps AF sensor to gather
more data and tell the lens: "Did I say -12.5? I meant -12.54" at
the last moment before it shouts "Stop!".

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