Reason for so many focus errors we see today?

RichA wrote:
Plastic? Thermal expansion of plastic is much greater than metal and
it could very well be why we are seeing focus issues that need "lens
re-calibration" at service depots or that we see the need for in-
camera focus fine-tuning. Even cameras and lenses that appear to be
metal today may have plastic cells holding lenses, components in
cameras. The cameras are produced in a control temp environment but
that isn't real life use where temps can vary by 10's of degrees. I
don't remember all metal AF SLRs needing focus fine-tuning (or having
that facility) in the film days.


I can see that in open loop focusing, where you estimate the distance
and dial that distance on lens. However, in any closed loop operation
that source of error would not lead to a focus error. Also, there are a
some plastics that have a thermal expansion less than many metals. So
one cannot use generalities on this.

On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer
wrote:

RichA wrote:
Plastic? Thermal expansion of plastic is much greater than metal and
it could very well be why we are seeing focus issues that need "lens
re-calibration" at service depots or that we see the need for in-
camera focus fine-tuning. Even cameras and lenses that appear to be
metal today may have plastic cells holding lenses, components in
cameras. The cameras are produced in a control temp environment but
that isn't real life use where temps can vary by 10's of degrees. I
don't remember all metal AF SLRs needing focus fine-tuning (or having
that facility) in the film days.


I can see that in open loop focusing, where you estimate the distance
and dial that distance on lens. However, in any closed loop operation
that source of error would not lead to a focus error. Also, there are a
some plastics that have a thermal expansion less than many metals. So
one cannot use generalities on this.

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.
--
John

John O'Flaherty wrote:

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.

(jargon as per
http://en.wikipedia.org/wiki/Autofocus
)

Interesting - since contrast measurement can be done in software,
a camera with phase detection (which can, as you say, get out of
calibration) could use contrast measurement to self-calibrate.

BugBear

On Tue, 23 Jun 2009 16:20:03 +0100, bugbear
wrote:

John O'Flaherty wrote:

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.

(jargon as per
http://en.wikipedia.org/wiki/Autofocus
)

Interesting - since contrast measurement can be done in software,
a camera with phase detection (which can, as you say, get out of
calibration) could use contrast measurement to self-calibrate.

That sounds like a good idea - you'd just have to take a special shot
of something with detail after each lens change.

Also, I wonder if it would be possible to make a main sensor chip with
special data paths to some small subset of the pixels, so they could
be digitized really fast, doing phase contrast right there. But maybe
that would be too hard to do on a uniform sensor with maximum pixel
density.
--
John

"John O'Flaherty" wrote in message
...
On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer
wrote:


Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.

Agreed about the separate sensor array. However, the phase system (at least
on some cameras) seems to be a hybrid servo (open loop for fast response,
then switches to closed loop for the final tweak ... that's where the
hunting comes in). There was a protracted debate about this on one Canon
forum and it never was resolved, since some of the information is
proprietary.

In rec.photo.digital.slr-systems John O'Flaherty wrote:
On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer
wrote:

RichA wrote:
Plastic? Thermal expansion of plastic is much greater than metal and
it could very well be why we are seeing focus issues that need "lens
re-calibration" at service depots or that we see the need for in-
camera focus fine-tuning. Even cameras and lenses that appear to be
metal today may have plastic cells holding lenses, components in
cameras. The cameras are produced in a control temp environment but
that isn't real life use where temps can vary by 10's of degrees. I
don't remember all metal AF SLRs needing focus fine-tuning (or having
that facility) in the film days.


I can see that in open loop focusing, where you estimate the distance
and dial that distance on lens. However, in any closed loop operation
that source of error would not lead to a focus error. Also, there are a
some plastics that have a thermal expansion less than many metals. So
one cannot use generalities on this.

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.

True, but thermal expansion in the lens *is* within the AF closed
loop, so that particular problem won't give rise to focus errors in an
AF system.

--
Chris Malcolm

John Navas wrote:
[]
It's the dark side (secret) of phase detection. Fast focusing
inevitably involves some focus error, since it's predictive, and
affected by lens errors. To fix that requires fine tuning, which can
slow down focusing considerably. The reason more people don't notice
is that it's lens sensitive, so focusing with a good lens, as in the
case of most reviews, may still be fast. The issue doesn't exist for
contrast detection, which is now fast enough for speed not to be an
issue.

But, contrast detection relies on detecting a maximum, with no information
about what direction the focus system should be driven. With phase
detection you are seeking a zero, and the sensor output tell you which way
to drive the focus, making a very fast, one-shot, open-loop movement
possible. Iterate further /i/f there is any need. On the other hand,
detecting a maximum can be quite a lot slower as you need to seek on
either side of the maximum and make a best guess as to the peak. Swings
and roundabouts for both systems.

David

John O'Flaherty wrote:
On Tue, 23 Jun 2009 16:20:03 +0100, bugbear
wrote:

John O'Flaherty wrote:

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.
(jargon as per
http://en.wikipedia.org/wiki/Autofocus
)

Interesting - since contrast measurement can be done in software,
a camera with phase detection (which can, as you say, get out of
calibration) could use contrast measurement to self-calibrate.

That sounds like a good idea - you'd just have to take a special shot
of something with detail after each lens change.

Also, I wonder if it would be possible to make a main sensor chip with
special data paths to some small subset of the pixels, so they could
be digitized really fast, doing phase contrast

The article speaks of "phase detection" and "contrast measurement"; what do you mean
by "phase contrast", which sounds like a mixture?


BugBear

In rec.photo.digital.slr-systems John Navas wrote:
On 23 Jun 2009 20:38:58 GMT, Chris Malcolm wrote
in :

In rec.photo.digital.slr-systems John O'Flaherty wrote:

Focusing with a phase contrast system isn't closed loop with regard
to the picture sensor, since it uses a separate, simple sensor array
for focusing. Thus the focus system can get out of calibration.

True, but thermal expansion in the lens *is* within the AF closed
loop, so that particular problem won't give rise to focus errors in an
AF system.

What closed loop? Many (most?) phase detection autofocus is open loop.

Most is open loop with respect to focus on the image sensor, and
closed loop for at least the final approach on the AF sensor. There's
a lot of misunderstanding and misinformation on this topic. Try the
experiment of locking focus with a half press on something near when a
big slow focusing lens is at infinity. Then complete the press, and
after the lens has started to move, but before it has stopped, remove
the thing it's focussed on from the view of the AF sensor. If the lens
runs past that point without stopping then it's a closed loop system
whose terminating condition has not been found. If it's a predictive
system it will stop focused on the distance the object was earlier
found to be at, even though it's no longer there.

--
Chris Malcolm

John Navas wrote:

Fast focusing
inevitably involves some focus error, since it's predictive, and
affected by lens errors.

Please explain how lens errors are affecting focus detection in
DSLRs, but not when using contrast detection.

To fix that requires fine tuning, which can
slow down focusing considerably. The reason more people don't notice is
that it's lens sensitive, so focusing with a good lens, as in the case
of most reviews, may still be fast. The issue doesn't exist for
contrast detection, which is now fast enough for speed not to be an
issue.

Sure, and P&S cameras focus near instantly, too. Pull the
other one.

-Wolfgang