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Author Topic: Autofocus accuracy  (Read 2929 times)

Mark F

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Autofocus accuracy
« on: December 25, 2008, 07:17:11 pm »

I've been using my 400mm 5.6L with the 1.4 extender attached for years and have always found it to be very sharp.  Last week I used the same combination but at the close end of the focus range. Focused on the bird's eye as usual, same tripod and ballhead setup but the images are too soft to use.

So I tested the lens / extender combination on close and more distant subjects and found that the close images are definitely soft.  Doesn't seem to me to be an allignment issue since distance shots are fine.  Has anyone has this problem?
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Mark F

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Autofocus accuracy
« Reply #1 on: December 28, 2008, 02:38:18 pm »

Well, since no one else has had this problem, let me ask a slightly different question.  In your experience does auto focus with long lenses work just as well at close range as for distant?
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Morgan_Moore

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Autofocus accuracy
« Reply #2 on: December 28, 2008, 02:43:07 pm »

Quote from: Mark F
I've been using my 400mm 5.6L with the 1.4 extender attached for years and have always found it to be very sharp.  Last week I used the same combination but at the close end of the focus range. Focused on the bird's eye as usual, same tripod and ballhead setup but the images are too soft to use.

So I tested the lens / extender combination on close and more distant subjects and found that the close images are definitely soft.  Doesn't seem to me to be an allignment issue since distance shots are fine.  Has anyone has this problem?

Lets say you were on manual focus

Lets say your lens was out by 1/16th of a turn of the focus barrell

1/16th of a turn is 3ft-5ft or 100ft to 120ft

focus at 120ft and on a  100ft object that object looks quite sharp

forcus at 5 foot and the 3 foot object is muzz

You need to do some tests on boring things like brick wall preferably one you can paint some marks on dot it on a windless  but bright day with your tripod sandbagged down and 1000th shutter

then you will know your problem

You may then be able to calibrate if you have a camera that allows else its repair shop time

my guess is at near infinity you are not noticing the inaccuracy

400 5.6 and a converter is always going to be a challenge to use - it is surely offering such a small aperture that challenges both AF and judgement on the screen in manual mode - try losing the converter ?

s
« Last Edit: December 28, 2008, 02:45:26 pm by Morgan_Moore »
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Mark F

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Autofocus accuracy
« Reply #3 on: December 29, 2008, 11:18:18 pm »

I can't lose the extender until I can afford a real 600mm and the Sherpa to carry it  

But I do not think it is the extender as I have closely looked at distant shots at large magnification and they are sharp.  The problem is with subjects about 15 feet distant.
 


Quote from: Morgan_Moore
Lets say you were on manual focus

Lets say your lens was out by 1/16th of a turn of the focus barrell

1/16th of a turn is 3ft-5ft or 100ft to 120ft

focus at 120ft and on a  100ft object that object looks quite sharp

forcus at 5 foot and the 3 foot object is muzz

You need to do some tests on boring things like brick wall preferably one you can paint some marks on dot it on a windless  but bright day with your tripod sandbagged down and 1000th shutter

then you will know your problem

You may then be able to calibrate if you have a camera that allows else its repair shop time

my guess is at near infinity you are not noticing the inaccuracy

400 5.6 and a converter is always going to be a challenge to use - it is surely offering such a small aperture that challenges both AF and judgement on the screen in manual mode - try losing the converter ?

s
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Mark

james_elliot

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Autofocus accuracy
« Reply #4 on: December 30, 2008, 04:51:27 am »

Quote from: Mark F
I can't lose the extender until I can afford a real 600mm and the Sherpa to carry it  

But I do not think it is the extender as I have closely looked at distant shots at large magnification and they are sharp.  The problem is with subjects about 15 feet distant.
Are you certain that shots at 100ft are that sharp?
As stated by  a previous poster, you might not notice the difference at 100ft because depth of field is much greater.

The general formula for DOF is (when the distance to the subject is much larger than the focal length):
DOF = 2 * c * a * (d*d) / (f*f)
Where c is the confusion circle, a is the aperture number, d is the distance to the subject and f the focal length.
If we take f=0.560m, d=30m, a=8 and c=5E-6m we have a DOF of 23cms
If we take d=5m, we have a DOF of 0.6cm (36 times less)

Thus something which is in focus at 100ft might be out of focus at 15ft, and unpleasant things can happen if you lens is misaligned or not properly calibrated.

(If you want more details on calculating DOF, look here for example).
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Mark F

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Autofocus accuracy
« Reply #5 on: December 30, 2008, 10:10:10 pm »

Well, I can only say that at 100% in Lightroom the distant shots (probably about 60 feet away) appear sharp, even though I am shooting wide open (f8 effective with the extender) at 400 or 800 ISO.  Still, I was thinking of using the micro-adjustment on my camera and this would be a good excuse.


Quote from: james_elliot
Are you certain that shots at 100ft are that sharp?
As stated by  a previous poster, you might not notice the difference at 100ft because depth of field is much greater.

The general formula for DOF is (when the distance to the subject is much larger than the focal length):
DOF = 2 * c * a * (d*d) / (f*f)
Where c is the confusion circle, a is the aperture number, d is the distance to the subject and f the focal length.
If we take f=0.560m, d=30m, a=8 and c=5E-6m we have a DOF of 23cms
If we take d=5m, we have a DOF of 0.6cm (36 times less)

Thus something which is in focus at 100ft might be out of focus at 15ft, and unpleasant things can happen if you lens is misaligned or not properly calibrated.

(If you want more details on calculating DOF, look here for example).
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Mark

rod edwards

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Autofocus accuracy
« Reply #6 on: January 09, 2009, 07:58:39 pm »

Hi !

Have just read your post about autofocus problems / microfocus adjustment on long lenses ...

You may find my new post of interest to get the answers (and questions!) flowing !

[a href=\'index.php?act=findpost&pid=251036\']AF Microfocus Adjustment with Canon 1Ds Mark 3[/a]

Regards,

Rod  

Quote from: Mark F
Well, I can only say that at 100% in Lightroom the distant shots (probably about 60 feet away) appear sharp, even though I am shooting wide open (f8 effective with the extender) at 400 or 800 ISO.  Still, I was thinking of using the micro-adjustment on my camera and this would be a good excuse.
« Last Edit: January 10, 2009, 05:30:10 am by rod edwards »
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stever

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Autofocus accuracy
« Reply #7 on: January 09, 2009, 11:13:02 pm »

rod, the link doesn't work for me
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rod edwards

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« Reply #8 on: January 10, 2009, 05:31:47 am »

Just fixed the above link but here it is again
[a href=\'index.php?act=findpost&pid=251036\']AF Microfocus Adjustment with Canon 1Ds Mark 3[/a]
Rod

Quote from: stever
rod, the link doesn't work for me
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