Do it yourself autofocus adjustment for Canon 350D

See:

http://www.astrosurf.org/buil/autofocus/adjust.htm

wrote:
See:

http://www.astrosurf.org/buil/autofocus/adjust.htm


Mais, c'est tout en francais sauf le suivant:

Description of a method for tuning the EOS camera's autofocus by using
an AF adjustment screw located in the shutter compartment.

But the other pages at the site are bi-lingual.

--
John McWilliams

wrote in message
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See:

http://www.astrosurf.org/buil/autofocus/adjust.htm

Maybe a word of caution (not about the obvious).

While it is possible to do that adjustment for a single lens, it
doesn't mean that the focus accuracy will improve for all lenses.
There is a small variation between lens mounts, so the adjustment
needs to be checked for all lenses in the bag! That way an 'optimal
compromise' can be reached.

One more word of caution, testing with a ruler at a slope is *not* the
correct method. The AF sensors are larger than the indicator on the
focus-screen, so it may validly focus either on the near side or on
the far side. Both extreme focus positions and any in between are
considered to be within the acceptable range (that range differs
between models, narrower range for the top models).

Bart

Bart van der Wolf wrote:

While it is possible to do that adjustment for a single lens, it
doesn't mean that the focus accuracy will improve for all lenses.
There is a small variation between lens mounts, so the adjustment
needs to be checked for all lenses in the bag! That way an 'optimal
compromise' can be reached.

For proper AF function, the AF sensors must be in the same plane as the
imaging sensor. The lens doesn't matter: any variations will be
"driven away" by the focus motors.

wrote in message
oups.com...
Bart van der Wolf wrote:

While it is possible to do that adjustment for a single lens, it
doesn't mean that the focus accuracy will improve for all lenses.
There is a small variation between lens mounts, so the adjustment
needs to be checked for all lenses in the bag! That way an 'optimal
compromise' can be reached.

For proper AF function, the AF sensors must be in the same plane as
the
imaging sensor. The lens doesn't matter: any variations will be
"driven away" by the focus motors.

From the Canon document
http://photoworkshop.com/canon/EOS_Digital.pdf they state after
describing the AF system (camera) calibration:
"Because each autofocus lens contains its own microcomputer and many
other internal devices such as focusing motors and diaphragm
actuators, lenses occasionally require calibration. If a focusing
error is detected, the circuitry of the lens itself can be adjusted to
ensure that it is operating according to design specs. Calibrating a
lens does not compromise its performance with other camera bodies
because the calibration standards for the lens are independent from
the calibration standards for the body. Please keep in mind that we do
not recommend sending equipment to the Factory Service Center unless
you are sure that the source of your image quality problems is not one
of the issues we have already discussed in this document".

If the focus motor would "drive away" variations, the lens calibration
would be unnecessary, wouldn't it?
The point is that the AF phase-detection sensor says how much the
image is OOF, and the camera tells the lens to move the right amount
to correct it. The right amount to move may differ from the intended
factory specs. Thus calibrating the camera (mirror) only adjusts for a
single lens (which may be out of tolerance for all I know), and it is
not necessarily valid for all lenses (which may also be out of
tolerance but in the opposite direction). That's why Canon uses a lens
with known (factory optimal) characteristics to calibrate the camera.

Bart

Bart van der Wolf wrote:

Bart van der Wolf wrote:

While it is possible to do that adjustment for a single lens, it
doesn't mean that the focus accuracy will improve for all lenses.
There is a small variation between lens mounts, so the adjustment
needs to be checked for all lenses in the bag! That way an 'optimal
compromise' can be reached.

For proper AF function, the AF sensors must be in the same plane as
the
imaging sensor. The lens doesn't matter: any variations will be
"driven away" by the focus motors.

From the Canon document
http://photoworkshop.com/canon/EOS_Digital.pdf they state after
describing the AF system (camera) calibration:
"Because each autofocus lens contains its own microcomputer and many
other internal devices such as focusing motors and diaphragm
actuators, lenses occasionally require calibration. If a focusing
error is detected, the circuitry of the lens itself can be adjusted to
ensure that it is operating according to design specs. Calibrating a
lens does not compromise its performance with other camera bodies
because the calibration standards for the lens are independent from
the calibration standards for the body. Please keep in mind that we do
not recommend sending equipment to the Factory Service Center unless
you are sure that the source of your image quality problems is not one
of the issues we have already discussed in this document".

If the focus motor would "drive away" variations, the lens calibration
would be unnecessary, wouldn't it?

Lens calibration serves a different purpose. I'll quote the document
you quote:

Calibrating a lens does not compromise its performance with
other camera bodies because the calibration standards for the
lens are _INDEPENDENT_ from the calibration standards for the body.

Emphasis added. Continuing with your remarks:

The point is that the AF phase-detection sensor says how much the
image is OOF, and the camera tells the lens to move the right amount
to correct it. The right amount to move may differ from the intended
factory specs. Thus calibrating the camera (mirror) only adjusts for a
single lens (which may be out of tolerance for all I know), and it is
not necessarily valid for all lenses (which may also be out of
tolerance but in the opposite direction). That's why Canon uses a lens
with known (factory optimal) characteristics to calibrate the camera.

Crappy ASCII diagram:

A
^
|
|
I----R----L

A is the AF sensor, I is the image sensor, L is the lens.

The body AF calibration is needed to ensure that the distance from R to
I is the same as the distance from R to A. If these distances are not
the same, then when the camera's AF mechanism drives the lens ("L")
into focus at "A" (where the focus sensor is), then the final image at
"I" will be out of focus.

The distance LR is not important, and any variations present (imagine a
macro extender), will, as I said, be "driven out" by the AF mechanism,
given enough back focus the optics is capable of.

What, then, does lens calibration do? I can't say exactly, but likely
it has to do with the dynamics of the close-loop AF system: some gain
parameters or whatever. These would indeed be per-lens, not per-body,
as the document you quote notes. But whatever it is, it is completely
independent of the body-side calibration.

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SNIP
What, then, does lens calibration do? I can't say exactly, but
likely
it has to do with the dynamics of the close-loop AF system: some
gain
parameters or whatever. These would indeed be per-lens, not
per-body,
as the document you quote notes. But whatever it is, it is
completely
independent of the body-side calibration.

Correct, lens calibration is independent of camera (mirror)
calibration, that's why camera calibration alone will only adjust for
a given lens, other lenses may be (relatively) off.

To quote Chuck Westfall (Canon U.S.A.):
"The primary job of the camera's AF sensor is to detect the amount and
direction of defocus at the focal plane regardless of the lens
involved. This procedure provides initial focusing data, which can
then be modified by the camera's CPU based on a number of different
factors including the type of lens that's being used. I'd love to tell
you more, but there's a limit on how much information Canon Inc. is
willing to release, for obvious reasons".

The initial focusing data (based on phase differences) will be
improved by camera calibration. The following info exchange between
camera and lens will determine how much that particular lens will need
to move the optics. However the lens needs to supply the correct info
for this to work, that's where lens calibration kicks in.

Bart

Bart van der Wolf wrote:

The initial focusing data (based on phase differences) will be
improved by camera calibration. The following info exchange between
camera and lens will determine how much that particular lens will need
to move the optics.

And when the lens doesn't move enough, the camera sees this lack of
focus, and asks it to move some more.

http://en.wikipedia.org/wiki/Closed_loop

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oups.com...
Bart van der Wolf wrote:

The initial focusing data (based on phase differences) will
be improved by camera calibration. The following info
exchange between camera and lens will determine how
much that particular lens will need to move the optics.

And when the lens doesn't move enough, the camera sees
this lack of focus, and asks it to move some more.

http://en.wikipedia.org/wiki/Closed_loop

You are assuming it's a Closed Loop system, any evidence? It seems to
be quite an open (pun intended) issue.

For your amusement, try
http://forums.dpreview.com/forums/read.asp?forum=1031&message=13292671
and do follow the links in that thread (if you haven't already) if you
want to read the actual quotes from Chuck Westfall.

Bart

Bart van der Wolf wrote:

You are assuming it's a Closed Loop system, any evidence?

Well, you can engage the "AI Servo" mode if you want ;-)

But even in "one shot": pick up EF 500/4 (or probably any f/4 lens),
and connect to a stacked pair of teleconverters: 1.4x + 2x. Mount the
mess to 1DMkII, point at object, and hit the AF. Watch, listen and
even feel as the focus motors slowly, but surely, bisect onto the
correct focus. Even with "lesser" lenses, you can occasionally feel
the focus motors tweak more than once in "one shot" mode.

I'm fairly amazed it would be an "open issue"; AF wouldn't work well
if it was based on a single observation, single command, and no follow
up.