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How do I understand focus calibration ?

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Tessarboy

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I suddenly realise that there is much about this subject that was never discussed in this forum.

There is a lack of understanding of what parts of the camera can be miscalibrated and which are the available adjustment options to the Canon repair center ?

I see 2 main possible fault areas : (excluding the typical handling errors by photographers)

a) AF

Most threads complain that the Auto-Focus doesn’t work as expected.
This involves miscalibrated lenses, miscalibrated bodies or both combined.

I.o.w. most of these threads assume that the problem is only a question of lens vs body calibration, be it mechanical or electronic.

b) MF

The widespread idea is that MF is always correct and should be used to improve on AF and test AF.

Almost everybody on this forum takes for granted that best manual focus in the viewfinder results in optimal sharpness on the sensor. Is that really always the case ??
(assuming perfect dioptric adjustment to the eye of the individual photographer)

I question this idea, as there are 2 independent optical paths involved : one from the lens to the sensor and one from the lens over mirror-prism-viewfinder lenses to the eye.

I.o.w. Am I wrong to assume that a particular dslr body could have a misalignment of components as such that when I MANUALLY focus for best sharpness in the viewfinder this results in front focus or back focus of the image ?

Is there someone on this forum who has the insider knowledge to enlighten us Canon users ?

Can you tell us whether the lens-sensor distance of 5D bodies (+ on other Eos dslr ) is adjustable ? Is it mechanical adjustment on the body and electronical on the EF lenses ?

Is it a myth that the AF can be adjusted with a small Allen key in a slot on the inside of the mirror-box ?
Are the mirror-prism-viewfinder components adjustable to conform to the sensor ?

(B.t.w. what was the follow-up to the thread of someone who showed that the focus (AF!) of the 350D body could be adjusted with a small Allen key? )

--
Tessarboy
'photography is about the quality of light'
 
Tessarboy said:
I suddenly realise that there is much about this subject that was
never discussed in this forum.
There is a lack of understanding of what parts of the camera can be
miscalibrated and which are the available adjustment options to the
Canon repair center ?

I see 2 main possible fault areas : (excluding the typical
handling errors by photographers)

a) AF

Most threads complain that the Auto-Focus doesn’t work as expected.
This involves miscalibrated lenses, miscalibrated bodies or both
combined.
I.o.w. most of these threads assume that the problem is only a
question of lens vs body calibration, be it mechanical or
electronic.

b) MF

The widespread idea is that MF is always correct and should be used
to improve on AF and test AF.
Almost everybody on this forum takes for granted that best manual
focus in the viewfinder results in optimal sharpness on the sensor.
Is that really always the case ??
(assuming perfect dioptric adjustment to the eye of the individual
photographer)

I question this idea, as there are 2 independent optical paths
involved : one from the lens to the sensor and one from the lens
over mirror-prism-viewfinder lenses to the eye.

I.o.w. Am I wrong to assume that a particular dslr body could have
a misalignment of components as such that when I MANUALLY focus for
best sharpness in the viewfinder this results in front focus or
back focus of the image ?

Is there someone on this forum who has the insider knowledge to
enlighten us Canon users ?
Can you tell us whether the lens-sensor distance of 5D bodies (+ on
other Eos dslr ) is adjustable ? Is it mechanical adjustment on
the body and electronical on the EF lenses ?
Is it a myth that the AF can be adjusted with a small Allen key in
a slot on the inside of the mirror-box ?
Are the mirror-prism-viewfinder components adjustable to conform to
the sensor ?

(B.t.w. what was the follow-up to the thread of someone who showed
that the focus (AF!) of the 350D body could be adjusted with a
small Allen key? )

--
Tessarboy
'photography is about the quality of light'
Click to expand...
Good post, but also realize that the AF is a different path from the image sensor. And quite a bit of it is in common with the MF path.
 
I am aware of this theoretical stuff but that doesn't answer my D5 specific calibration questions.
Thanks anyway
--
Tessarboy
'photography is about the quality of light'
 
Hi,
You are correct that there are multiple opportunities for focus misalignment.

For MF , front mirror and focus screen position must accurately match the
sensor distance.

For AF the focus screen position is not relevent , howevever the AF sensor in the base of the camera uses light transmitted THROUGH the front mirror and reflected down into the camera AF sensor by a secondary mirror mounted behind the front mirror.
This path must also match the sensor distance.

Rgds Phil
 
A related article (in French) is at http://www.astrosurf.com/buil/autofocus/adjust.htm

One can notice a focus-adjustment hexagonal screw in the chamber of a 350D. HAven't checked for it in the 5D, but it may be there too.

Alex
Tessarboy said:
I suddenly realise that there is much about this subject that was
never discussed in this forum.
There is a lack of understanding of what parts of the camera can be
miscalibrated and which are the available adjustment options to the
Canon repair center ?

I see 2 main possible fault areas : (excluding the typical
handling errors by photographers)

a) AF

Most threads complain that the Auto-Focus doesn’t work as expected.
This involves miscalibrated lenses, miscalibrated bodies or both
combined.
I.o.w. most of these threads assume that the problem is only a
question of lens vs body calibration, be it mechanical or
electronic.

b) MF

The widespread idea is that MF is always correct and should be used
to improve on AF and test AF.
Almost everybody on this forum takes for granted that best manual
focus in the viewfinder results in optimal sharpness on the sensor.
Is that really always the case ??
(assuming perfect dioptric adjustment to the eye of the individual
photographer)

I question this idea, as there are 2 independent optical paths
involved : one from the lens to the sensor and one from the lens
over mirror-prism-viewfinder lenses to the eye.

I.o.w. Am I wrong to assume that a particular dslr body could have
a misalignment of components as such that when I MANUALLY focus for
best sharpness in the viewfinder this results in front focus or
back focus of the image ?

Is there someone on this forum who has the insider knowledge to
enlighten us Canon users ?
Can you tell us whether the lens-sensor distance of 5D bodies (+ on
other Eos dslr ) is adjustable ? Is it mechanical adjustment on
the body and electronical on the EF lenses ?
Is it a myth that the AF can be adjusted with a small Allen key in
a slot on the inside of the mirror-box ?
Are the mirror-prism-viewfinder components adjustable to conform to
the sensor ?

(B.t.w. what was the follow-up to the thread of someone who showed
that the focus (AF!) of the 350D body could be adjusted with a
small Allen key? )

--
Tessarboy
'photography is about the quality of light'
Click to expand...
 
I had a look in sensor cleaning mode and found similar lateral lugs in the 5D mirror box.

When you look for yourself it becomes obvious that these are the 4 resting points for the mirror.
I don't have a small mirror to verify if they contain hex screws.

I can't see if these lugs are centric or oval. If not, they could be used to adjust the mirror position, but here i am speculating.

Was this article real or a practical joke ? In another linked article he after all completely dismantled a D350 to remove the IR filter and shows all the components. He should have identified an adjusting mechanism !

I'll try to contact the french OP of the article.

Nobody here with insider info ?

--
Tessarboy
'photography is about the quality of light'
 
Tessarboy said:
I had a look in sensor cleaning mode and found similar lateral lugs
in the 5D mirror box.
When you look for yourself it becomes obvious that these are the 4
resting points for the mirror.
I don't have a small mirror to verify if they contain hex screws.
I can't see if these lugs are centric or oval. If not, they could
be used to adjust the mirror position, but here i am speculating.
Was this article real or a practical joke ? In another linked
article he after all completely dismantled a D350 to remove the IR
filter and shows all the components. He should have identified an
adjusting mechanism !
Click to expand...
I'm fluent in French and browsed that guy's bio at some time. He seems to be a geek of astrophotography, doing digital imaging for astro photos since tens of years, with equipment self-manufactured, long before DSLRs existed. I believe that article is serious.
Tessarboy said:
I'll try to contact the french OP of the article.
Click to expand...
Yes, please do.
Tessarboy said:
Nobody here with insider info ?
Click to expand...
Nah... the usual advice here is to send the equipment for calibrating to Canon. Which is very sain advice. The skill required is indeed very precise. The only DIY advice is suggested by some stories about sending equipment to calibration and the returned camera is less calibrated.

On peer-to-peer networks one can find a document about EOS 1v parts, with a detailed "explosion" graph with what's inside it. I have or had it at some point in time but can't seem to find it anylonger.

Alex
 
Thanks !
I am not poking for methods to adjust the 5D myself.

I'd just like to have a better technical insight on the AF-MF relation and first of all an answer to the nagging question : does perfectly sharp in the viewfinder ALWAYS means perfectly sharp on the sensor ?

--
Tessarboy
'photography is about the quality of light'
 
I suspect almost all of my images are out of focus, but I have no way of confirming this without taking a poll. People here are always saying an image of mine is front focused, or soft. I never know this until somebody tells me. I can tell the real bad ones myself, but when they are nearly correct, thats another story.

In some cases it is a post processing problem, or lack of contrast or some other factor. I also suspect some of it is game playing, but again, how would I know? But there are too many cases to doubt that many of my images that I think are sharp are not.

For me this is the starting point.

--
http://www.pbase.com/roserus

Ben
 
Absolutely no insider knowledge on this forum ?
I am disapointed !
--
Tessarboy
'photography is about the quality of light'
 
Phil Humphreys answered your question 6 days ago.
Bottom line, there are 3 "focus points"
1) The Sensor
2) Focussing screen on the bottom of the prism
3) Focus sensor in the mirror box.

If the world were perfect, all would be at the exact same distance from the lens back element.
We all know how perfect the world is, right ??
Chris.
 
Thanks for summarising it so clearly.

I guess I had to many questions in one thread.
The unanswered question in short : which of these 3 can Canon adjust ?
(and perhaps how ?)

--
Tessarboy
'photography is about the quality of light'
 
Canon can adjust all of them, and can also adjust the mirror alignment. They can also repair parts that may be broken that relate to focusing. I had a camera a while back where a spring on the back side of the mirror that controls the little AF mirror had broken. The camera was in perfect focus on the first shot in drive mode, but it had trouble maintaining focus during a sequence of shots. Turned out the spring that makes sure the little mirror in the back returns to the correct position (against it's stops) had broken.
Tessarboy said:
Thanks for summarising it so clearly.

I guess I had to many questions in one thread.
The unanswered question in short : which of these 3 can Canon adjust ?
(and perhaps how ?)

--
Tessarboy
'photography is about the quality of light'
Click to expand...
 
EOS3User said:
Phil Humphreys answered your question 6 days ago.
Bottom line, there are 3 "focus points"
1) The Sensor
Click to expand...
I don't think they can change the sensor alignment. Just a supposition. It is enough to change the two others to bring it at same distance as it (no?).
EOS3User said:
2) Focussing screen on the bottom of the prism
Click to expand...
I think this is adjustable by adding/removing (shiny, metal) shims between the focus screen and the prism. My 5D has 2 and I saw a 300D having one.
EOS3User said:
3) Focus sensor in the mirror box.
Click to expand...
I think this is changeable by turning the hex screw on the right lateral wall of the mirror box.
EOS3User said:
If the world were perfect, all would be at the exact same distance
from the lens back element.
Click to expand...
I didn't know that, thanks for pointing it out.

Alex
 
There are a set of shims/spacers between the sensor housing and the shutter box assembly.

These are approximately 0.2mm in thickness. And on the 300D, there are 3 of them, two on the bottom, 1 on the top.

I just did an IR conversion, which required me to take apart the camera. I removed the shims, but unfortunately, I'm still front focusing.

Hopefully I'll be able to adjust the focus screw to fix it.

Does anybody have any other information besides that french site?
 
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