Luminous Landscape Forum
Equipment & Techniques => Cameras, Lenses and Shooting gear => Topic started by: Mark F on December 25, 2008, 07:17:11 pm
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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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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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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
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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.
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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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 (http://www.photo-lovers.org/fdof.html.en) for example).
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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.
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 (http://www.photo-lovers.org/fdof.html.en) for example).
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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
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.
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rod, the link doesn't work for me
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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
rod, the link doesn't work for me