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Adjusting AF screw is not enough

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automaniak

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Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF screw. I have found that AF precision is FL dependent in my 300D so when it is spot on on my 50mm then it will front focus on my 70-200, first I though that it is a lens problem but when I put on a 24-85 found that it is spot on till 50mm and front focuses at 85mm, of course if I adjust the AF to be spot on on my 70-200 then it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the problem is. If some of you have some links to the description of how AF works or can explain why this happen I will be thankful.
 
Hi, I did the same and adjusted the screw. Somehow it stuck now and I can't adjust it anymore.

The thing I noticed is that because the mirror isn't exactly at an angle if 45 degrees anymore the different AF points have their own focus point. My Sigma 24-60 EX F2.8 is spot on at the (horizontal) middle row of AF point. My 70-200/F4L is spot on at AF point that's at the bottom. So I can choose the AF point that matches the lens best.

It's not ideal but it works. Try it out and determine what is best in your situation.
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
 
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
Where is the adjustment screw located? Thanks.

Brad
 
its a small screw behind tle mirror ca 6 mm in diameter. flip the mirror up with your fingers and its at the ride side. The screw is "hollow" an it fits a small ca 1/2 mm "sqare ? " screwdriver.

seee here for an example

http://216.239.59.104/search?q=cache:GnSLA_EBlEoJ:www.pbase.com/terrylovejoy/daylight_use+300d+af+adjustment+screw&hl=en

The screw is concentric and controls the angle of the mirror+little AF mirror. By adjusting the screw the distance of the light rays to the AF sensor are adjustable.

Any attempted changes are on your own risk.
Brad Hollister said:
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
Where is the adjustment screw located? Thanks.

Brad
Click to expand...
 
emklap said:
Hi, I did the same and adjusted the screw. Somehow it stuck now and
I can't adjust it anymore.

The thing I noticed is that because the mirror isn't exactly at an
angle if 45 degrees anymore the different AF points have their own
focus point. My Sigma 24-60 EX F2.8 is spot on at the (horizontal)
middle row of AF point. My 70-200/F4L is spot on at AF point
that's at the bottom. So I can choose the AF point that matches the
lens best.

It's not ideal but it works. Try it out and determine what is best
in your situation.
Click to expand...
Yes, I can confirm your findings but have to say that in my experience the center focus point give more consistent results.

My 70-200 is a Sigma f2,8 it have less DOF wide open than your canon f4 is why I need more precision.
 
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
Surely then this means that your lenses are out of calibration? The camera's AF needs to be adjusted using a 'known good' calibrated lens. The lens needs to be calibrated using a 'known good' calibrated body. In your situation, you've managed to adjust it to correct a problem in one lens, but causes the other lens to be out of focus. It is quite possible that you've now made your camera out of focus with good/calibrated lenses - and only be in focus with bad lenses.

David
 
David Hearn said:
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
Surely then this means that your lenses are out of calibration?
The camera's AF needs to be adjusted using a 'known good'
calibrated lens. The lens needs to be calibrated using a 'known
good' calibrated body. In your situation, you've managed to adjust
it to correct a problem in one lens, but causes the other lens to
be out of focus. It is quite possible that you've now made your
camera out of focus with good/calibrated lenses - and only be in
focus with bad lenses.

David
Click to expand...
Hi David!

I'm looking for detailed information How AF works but can not find.

Maybe I'm wrong but is logical for me after my test that the lens have nothing to do with AF precision(maybe only the af-motor precision), is the camera that focuses I have 4 lenses and the front/backfocusing is in my case not lens dependent but FL dependent.

If you have some knowledge about AF systems, please explain me why I'm getting this results?
 
All this demonstrates why most of us would send the camera in to be calibrated. There may be other adjustments that you are not accessing, to correct the problems you are seeing.
As the saying goes - a little knowledge can be a dangerous thing.

--
Come and look at my ego site (I mean website)
http://www.outnumbered.ca
 
automaniak said:
David Hearn said:
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
Surely then this means that your lenses are out of calibration?
The camera's AF needs to be adjusted using a 'known good'
calibrated lens. The lens needs to be calibrated using a 'known
good' calibrated body. In your situation, you've managed to adjust
it to correct a problem in one lens, but causes the other lens to
be out of focus. It is quite possible that you've now made your
camera out of focus with good/calibrated lenses - and only be in
focus with bad lenses.

David
Click to expand...
Hi David!

I'm looking for detailed information How AF works but can not find.

Maybe I'm wrong but is logical for me after my test that the lens
have nothing to do with AF precision(maybe only the af-motor
precision), is the camera that focuses I have 4 lenses and the
front/backfocusing is in my case not lens dependent but FL
dependent.

If you have some knowledge about AF systems, please explain me why
I'm getting this results?
Click to expand...
I don't personally have any knowledge of AF systems - I'm just going by the number of posts on here, and from Canon's own info, that states that people have had lenses recalibrated to correct focus problems, and also bodies recalibrated to correct similar problems.

For example - http://www.rogercavanagh.com/helpinfo/29_calibration.stm

David
 
Hi,

I need to adjust my 300d's AF screw also as it is front focusing on every lens, so looks like AF screw adjustment will fix it.

Can you please tell me which screw driver did u use, size? you have to use the L shape one right?

You reply me at [email protected]

Thanks

Adnan'
automaniak said:
Hi All!

Finally I manage to do some AF adjusting in my 300D by rotating AF
screw. I have found that AF precision is FL dependent in my 300D so
when it is spot on on my 50mm then it will front focus on my
70-200, first I though that it is a lens problem but when I put on
a 24-85 found that it is spot on till 50mm and front focuses at
85mm, of course if I adjust the AF to be spot on on my 70-200 then
it back focuses in the 50mm.

I need some feedback on auto focus system to figure where the
problem is. If some of you have some links to the description of
how AF works or can explain why this happen I will be thankful.
Click to expand...
 
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