Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
OP Mishkin™ Contributing Member • Posts: 917
I agree

Non-loop design easily produces great results when both camera and lenses are properly calibrated, and can track moving subjects without posing any extra strain on motors.

Some of the AF problems can be thus attributed to miscalibrated bodies and lenses, while some (misfocused pictures at f

Jason Hutchinson wrote:
Loop stability is the most likely one, but what about the response
time of the older motors? They're clearly not designed as well as
modern USM motors for the small repetitive motions that'd be
required for that. Who knows.


Mishkin2 wrote:
You're right, direct link between measured lens position and a
signal from AF sensor allows for lens focusing on moving subject. I
guess, the loop system can do this, too, but it will require
extremely fast convergence of the loop and running the loop many
times per second, which is probably beyond capabilities of AF
systems. This is also mentioned in that Honeywell patent.

Other than that, I don't see WHY AF is not implemented as a closed
feedback loop.

hyslopc wrote:

Unless I've misunderstood, you couldn't make the AF a closed
feedback loop and still support moving targets. Since many targets
move, the AF would constantly be re-adjusting itself and would
never "lock" - sound familiar? That's right, I've just described
Canon's AI-Servo mode, which constantly and continuously
re-focusses for as long as you half-press.

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