That's a very interesting web site... I enjoyed it. Thanks. It's
great to get a chance to see the inner workings of something like
USM lenses.
However, if you intended it to shed some light on the discussion of
focus problems, unfortunately it doesn't. What the photocoupler
discussion describes is how the USM stepper motor works and how the
onboard lens electronics sense the focus ring movement. Remember,
the focus sensor and the focusing electronics/software are in the
camera, not the lens. The focusing "decisions" are made in the
camera. This was the control system I was describing, not the
mechanical movement system in the lens. The system described in
the web site is simply the electronics which activate the stepper
motor and then sense the focus ring movement... it is not making
focus decisions.
I will say, the one thing this web site does shed some light on is,
as I stated earlier, modern AF lenses are very complicated beasts.
Certainly, there are lots of things to go wrong. But, focusing
decisions are controlled in the camera.
My disagreement with your original post was not that a problem
might exist in the lens, it was with your statement:
If the lens has an internal AF error, the camera AF can be correct,
but the ultrasonic motor "moves" to the wrong position.
This is not consistent with modern feedback control systems, and
certainly not the way the D60 focus system works. Any error
generated because the motor "moves" to the wrong position would
show up on the camera AF sensor and new instructions would be
issued to the lens... this process continues until the lens is in a
focus state that the camera AF decides is "best focus".
I have no disagreement with another of your statements that sending
in both camera and lense might save time by having both checked.
But it seems to me one could do that at home by simply trying a
different lens and determining if the problem is the camera AF out
of calibration or a faulty lens.
Regards.
Check out in particular the description of the photocoupler as
described by Chuck Westfall.
Best wishes, Pete.
Thus if the lens is a little out of whack, an active feedback
control system will null out any mismatch error with the lens... it
will keep pulsing the lens until focus is achieved (minimizing the
error).
If the system was open-loop, as you describe, accurate focus across
many lenses would be hopeless... there is no way to control
manufacturing tolerances to those levels.
It is highly unlikely that this is a lens-camera combination
problem. The only way one can imagine the lens causing this
problem is if it has an electronics problem causing the stepper
motor to oscillate between front and rear focus... thus a lens
failure, not a combination failure.
The way the Canon EOS System functions, at least with the
Ultrasonic lenses, is that the camera body tells the lens AF motor
how may "pulses" and in which direction to move the lens assembly.
If the lens has an internal AF error, the camera AF can be correct,
but the ultrasonic motor "moves" to the wrong position. Many Pro's
have had to send in their fast EF teles for adjustment after heavy
use ( and abuse! ) to correct AF error in the lens itself.
What I'm saying is that maybe the 50 1.4 lens is off, and the D60
camera body is fine. I'd check it with some other fast lenses. It
would also be interesting to check the camera/lens combo at
infinity to see what the lens indication is ( how far off the mark
is the inf. mark on the barrel ). Possibly the CMOS sensor is off,
but Canon's check on the AF sensor distance reads as OK. Pete.
http://www.petestone.com
(And he SHOULD be testing his equipment-he makes money at this!)