Shocking insight: AF is not a closed feedback loop!

Started Jul 18, 2003 | Discussions thread
OP Mishkin™ Contributing Member • Posts: 917
Re: Aha! and why???

The arrow from electronic mount to camera microprocessor should be bi-directional. It is an error in diagram.

Bob Watt wrote:

What in the diagram leads you to the
conclusion they used feed forward error
correction. Without a line from the body to
the lens telling it to move, which seems lacking in
the diagram, this is a "do not move" control
system. Clearly the diagram must be wrong.

All outputs and no inputs from the lens simply
can not work.

Nothing in the diagram or text at Canon suggest
feed forward or feedback. The subject is completely

Bob Watt

MarkG wrote:
You are right, this does explain everything. In control theory this
system is called "feed forward error correction". The advantage is
that there are no stability problems, the disadvantage is that the
error (misfocus amount) is not reducable as much as with negative
feedback system. My guess is that due to variation in motor
parameters of different lenses the decision has been made early on
not to worry about stability problems, and rather calibrate lenses
to work with feed forward system. Oh, well... It is worse than I
thought. Thank you for digging this out. MarkG

Mishkin2 wrote:

Yes, you're right, the triangle is USM motor. The comparator is
within lens microprocessor (the white box with "Data communication
control" label).

This doesn't change a thing in my conjecture.

A feedback loop doesn't need any data from the lens. The lens just
needs to respond to the signal "move closer/move farther" from the
body in this case. And this would achieve perfect focus anytime,
with any lens (assuming body is perfectly calibrated).

Hunting is simply searching for better data when AF sensor doesn't
have enough light or contrast.

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