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Reason for so many focus errors we see today?

 
 
RichA
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      06-23-2009
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.
 
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Pete D
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      06-23-2009

"RichA" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> 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.


You mean like with Canon D1's and L glass, cos they all have this problem?

Sometimes Rich even I think you are an idiot.


 
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Doug Jewell
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      06-23-2009
RichA wrote:
> Plastic? Thermal expansion of plastic is much greater than metal and

Bzzt, wrong.
Coefficients of Linear thermal expansion (10^-6 m/m/C):
Aluminium 23.1
Magnesium 26
Brass 19
Stainless Steel 17
Steel 11-13
Glass reinforced Polycarbonate 22
So there's not a lot in it.

> 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

To put expansion in perspective, a 30 degree C change in
temp on a 300mm lens made of Magnesium would be a mere
0.2mm. Every lens I've ever seen has that much play in it's
movement if not more.

> don't remember all metal AF SLRs needing focus fine-tuning (or having
> that facility) in the film days.

Because of the lower resolution of the sensor it wasn't as
critical. Consider that a typical 35mm frame of film can
resolve the equivalent of maybe 12-15MP. Modern DSLR's will
cram 12-15MP on a sensor that has half the surface area, or
are putting 24MP onto the 35mm frame. Because of this higher
resolution they require more critical focus from the lens,
and so errors that have always existed are now noticed.




--
Don't blame me - I didn't vote for Kevin Rudd or Anna Bligh!
 
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Don Stauffer
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      06-23-2009
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.
 
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Nobody
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      06-23-2009

"Doug Jewell" <(E-Mail Removed)> wrote in message
news:4a40c593$0$2602$(E-Mail Removed)...
> > don't remember all metal AF SLRs needing focus fine-tuning (or having
>> that facility) in the film days.

> Because of the lower resolution of the sensor it wasn't as critical.
> Consider that a typical 35mm frame of film can resolve the equivalent of
> maybe 12-15MP. Modern DSLR's will cram 12-15MP on a sensor that has half
> the surface area, or are putting 24MP onto the 35mm frame. Because of this
> higher resolution they require more critical focus from the lens, and so
> errors that have always existed are now noticed.
>

How thick is a film emulsion, versus the sensor plane of a chip. I would
hazard to suggest a CCD/CMOS is more critical than film.


 
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Charles
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      06-23-2009

"John O'Flaherty" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer
> <(E-Mail Removed)> 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.


 
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Chris Malcolm
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      06-23-2009
In rec.photo.digital.slr-systems John O'Flaherty <(E-Mail Removed)> wrote:
> On Tue, 23 Jun 2009 08:56:14 -0500, Don Stauffer
> <(E-Mail Removed)> 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
 
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David J Taylor
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      06-24-2009
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

 
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Rich
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      06-24-2009
On Jun 23, 8:07*am, Doug Jewell <(E-Mail Removed)> wrote:
> RichA wrote:
> > Plastic? *Thermal expansion of plastic is much greater than metal and

>
> Bzzt, wrong.
> Coefficients of Linear thermal expansion (10^-6 m/m/C):
> Aluminium 23.1
> Magnesium 26
> Brass 19
> Stainless Steel 17
> Steel 11-13
> Glass reinforced Polycarbonate 22
> So there's not a lot in it.


That is for glass-filled. Non-glass filled is three times that. Not
all polycarbs are glass-filled. It is
also the differential of expansion between two different materials
that is the problem. All aluminum and maybe all plasic on their own
might be ok, but combined it causes problems, like the binding of lens
elements when temperatures drop.
 
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Chris Malcolm
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      06-24-2009
In rec.photo.digital.slr-systems John Navas <(E-Mail Removed)> wrote:
> On 23 Jun 2009 20:38:58 GMT, Chris Malcolm <(E-Mail Removed)> wrote
> in <(E-Mail Removed)>:


>>In rec.photo.digital.slr-systems John O'Flaherty <(E-Mail Removed)> 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
 
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