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Do it yourself autofocus adjustment for Canon 350D

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Anonymous
August 17, 2005 8:41:27 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

eawckyegcy@yahoo.com 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
Anonymous
August 18, 2005 4:14:33 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

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.
Related resources
Anonymous
August 18, 2005 7:06:09 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<eawckyegcy@yahoo.com> wrote in message
news:1124316520.890996.197230@g14g2000cwa.googlegroups.com...
> 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
Anonymous
August 18, 2005 8:45:32 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

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
Anonymous
August 19, 2005 3:12:27 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<eawckyegcy@yahoo.com> wrote in message
news:1124392473.143243.105630@f14g2000cwb.googlegroups.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&gt; 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
Anonymous
August 19, 2005 5:00:41 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<eawckyegcy@yahoo.com> wrote in message
news:1124401853.223201.199110@f14g2000cwb.googlegroups.com...
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
Anonymous
August 19, 2005 6:31:36 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<eawckyegcy@yahoo.com> wrote in message
news:1124408732.846073.152540@g49g2000cwa.googlegroups.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&m...;
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
Anonymous
August 19, 2005 2:57:29 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

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.
Anonymous
August 20, 2005 2:32:24 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

In article <1124408732.846073.152540@g49g2000cwa.googlegroups.com>,
eawckyegcy@yahoo.com says...
> 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
>
>
AF on Canon (and probably other) DSLRs is open loop - it is definitely
NOT closed loop. The camera says to the lens "focus by amount X" and
trusts the lens to do what it is told. There is no check that it has
actually done so. Presumably this is because a closed loop AF system
would be much slower.
Anonymous
August 20, 2005 6:08:29 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

"Graeme Cogger" <gcogger@bigSPAMfoot.com> wrote in message
news:RPOdnW1DHtaYzZveRVnyuA@pipex.net...
> In article <1124408732.846073.152540@g49g2000cwa.googlegroups.com>,
> eawckyegcy@yahoo.com says...
SNIP
>> 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
>>
>>
> AF on Canon (and probably other) DSLRs is open loop - it is
> definitely NOT closed loop.

From what I've read, it apparently seems to be open-loop indeed
(http://en.wikipedia.org/wiki/Open_loop).

> The camera says to the lens "focus by amount X" and trusts
> the lens to do what it is told. There is no check that it has
> actually done so. Presumably this is because a closed loop
> AF system would be much slower.

Yes and, because of the relatively wide EF mount, the phase detection
may provide enough information to accurately determine the direction
and amount of movement to succeed in one go. It would partly explain
the focus speed.

Bart
Anonymous
August 20, 2005 6:54:56 AM

Archived from groups: rec.photo.digital.slr-systems (More info?)

<eawckyegcy@yahoo.com> wrote in message
news:1124474249.486021.276810@z14g2000cwz.googlegroups.com...
> 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.

Failure to achieve focus, and trying/failing again, is IMHO not the
same as a closed-loop system with adequate light levels. It also
doesn't explain the difference between Pro and non-Pro body focus
accuracy (caused by smaller/more accurate phase detection elements).

> 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.

Canon's historical backwards-compatibility break going from the FD to
EF mount was not only about the introduction of Electronic Focus. It
was also about mount size, thus allowing more accurate Phase detection
for closed-loop AF. The compatibility break was inevitable and, in
Europe anyway, it started a landslide type of brand switch from Nikon
to Canon for sports and journalism.

http://www.pat2pdf.org/patents/pat6603929.pdf describes "a" method of
closed-loop AF, which requires multiple measurements (of phase *and*
contrast) to achieve both speed *and* accuracy, but there is no
evidence that this (Aug-2003) patent is used in EF-mount designs
(which pre-date the patent). It instead seem likely that this patent
is a new method, not (yet) implemented in the older EF-mount/Camera
intelligence. It might even just apply to P/S cameras which have
life-sensor-review feedback.

Bart
Anonymous
August 21, 2005 8:55:27 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

eawckyegcy@yahoo.com wrote:
>>You are assuming it's a Closed Loop system, any evidence?
> Well, you can engage the "AI Servo" mode if you want ;-)

That's constant refocusing.

> 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.

Sometimes it doesn't work that well. Ever run into a lens with focusing
problems? Typical problems like constant slight misfocusing should not
ever be an issue with a properly designed closed-loop focusing.
--
harri
Anonymous
August 22, 2005 1:41:51 PM

Archived from groups: rec.photo.digital.slr-systems (More info?)

Bart van der Wolf wrote:

> Failure to achieve focus, and trying/failing again, is IMHO not the
> same as a closed-loop system with adequate light levels.

Any system that examines what it is doing in order to behave properly
and/or better is, by any reasonable definition of the phrase, "closed
loop". If you wish to argue otherwise, you'll have to do it on your
own.

> It also
> doesn't explain the difference between Pro and non-Pro body focus
> accuracy (caused by smaller/more accurate phase detection elements).

One presumes that the "one shot" mode stops as soon as the focus
estimate is supposedly optimal. If the variance in the estimates is
larger for non-Pro bodies, we can expect the non-Pro bodies to have
inferior AF properties. And indeed, this is what we _do_ observe.
Also note that this would be observed whether or not the AF system was
open or closed.

With evidence in hand, let's speculate:

The Canon AF system is closed, and works something like this:

1. user engages AF function
2. camera makes an AF measurement
3. if in focus (by whatever measure)
and we are in "one shot" mode, then we are done.
3. camera sends lens a command
4. camera waits for lens to say "done"
5. goto 2

Minor and major details omitted -- e.g., it's clear that when step (2)
fails, there is a sub-mode that commences a focus search by racking the
lens in and out, and probably the AF sensor is tickled continuously
while this happens, looking for any hook to lock onto, and so on.

But the above algorithm explains _all_ the behaviour I have observed
with my old Canon EOS 5, a Canon 10D, and a Canon 1DMkII. In
particular, it is completely consistent with the observations I posted
back on Friday. Has anyone repeated my experiment?
!