Re: Do it yourself autofocus adjustment for Canon 350D

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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