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01-04-2008, 10:01 AM   #1
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basic filter affecting AF function?

upon LBA meandering around, i stumbled on an article comparing (sic) canon 70-200 and sigma 70-200. (Sigma 70-200mm f/2.8 EX Vs. Canon 70-200mm f/2.8 IS L 1st im - Canon Digital Photography Forums) Of interest:

"A few months back I reported some odd AF accuracy problems with the Sigma that seemed to crop up out of no where. Upon further investigation,. I realized that all issues disappeared when the UV filter I was using was removed. Since then no problems at all. For this reason both lenses I am using in the field with no filter"

I wonder if anyone noticed this problem before. As i've just jumped from analog camera (complete manual) to digital system with increasingly complicated and sophisticated focusing and lightning algorithm, one could not help wonder if a simple thing such as protective filter could throw a perfectly sharp lens out of focus.

Let me know your thought. Cheers.

01-04-2008, 10:45 AM   #2
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If you look or try hard enough--you can find problems with just about anything: breathing, walking, using a camera. With just a tiny bit more effort you could test the situation yourself. Take the filter off and snap a few shots.

Filters have been blamed for a host of problems and for some very spectacular results--go figure.

QuoteOriginally posted by truonganh Quote
upon LBA meandering around, i stumbled on an article comparing (sic) canon 70-200 and sigma 70-200. (Sigma 70-200mm f/2.8 EX Vs. Canon 70-200mm f/2.8 IS L 1st im - Canon Digital Photography Forums) Of interest:

"A few months back I reported some odd AF accuracy problems with the Sigma that seemed to crop up out of no where. Upon further investigation,. I realized that all issues disappeared when the UV filter I was using was removed. Since then no problems at all. For this reason both lenses I am using in the field with no filter"

I wonder if anyone noticed this problem before. As i've just jumped from analog camera (complete manual) to digital system with increasingly complicated and sophisticated focusing and lightning algorithm, one could not help wonder if a simple thing such as protective filter could throw a perfectly sharp lens out of focus.

Let me know your thought. Cheers.
01-04-2008, 11:16 AM   #3
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QuoteOriginally posted by jfdavis58 Quote
Take the filter off and snap a few shots.
Sadly, 'snapping a few shots' is all too common a practice here, and on other photographic forums, when it comes to testing things.

Designing an experiment that isolates the issue being tested from all the other possible causes of variation (including the imagination of the tester) is often not at all easy, and people in general do not seem to be good at it.
01-04-2008, 12:08 PM   #4
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And so his eyes will deceive him?

What some folks are about here is 'fickle': get technical and they want something more intuitive, go the route of intuitive and they want technical.

I doubt the OP has access to a complete optics lab--he'll just need to 'wing-it', just try it with and without and pick the one he likes.

Or he could do as what seems sadly all too inevitable: stir the forum pot until he brews up a tempest.


QuoteOriginally posted by ChrisA Quote
Sadly, 'snapping a few shots' is all too common a practice here, and on other photographic forums, when it comes to testing things.

Designing an experiment that isolates the issue being tested from all the other possible causes of variation (including the imagination of the tester) is often not at all easy, and people in general do not seem to be good at it.


01-04-2008, 12:22 PM   #5
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QuoteOriginally posted by jfdavis58 Quote
And so his eyes will deceive him?
Not his eyes.

On another forum I read someone was posting examples taken with a lens that was supposedly 'soft' and therefore needed to be returned for a refund.

The test shots were taken in virtual darkness, with an exposure of 1/15s, of a moving target. They were then reduced to fit a web page.

The pictures posted were indeed slightly soft, but were they evidence of anything at all?

Of course not. The lens might have been defective, or it might not. The softness might have been caused by focus error, lack of depth of field, subject movement, camera shake, maybe even the reduction process - but from the 'test' it was impossible to tell.

My point stands. People are notoriously bad at eliminating from their 'tests' other things that could cause the issue under investigation, and are sadly prone to jumping to all sorts of premature conclusions.
01-04-2008, 01:34 PM   #6
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+1

....................

QuoteOriginally posted by ChrisA Quote
...notoriously bad at eliminating from their 'tests' other things that could cause the issue under investigation, and are sadly prone to jumping to all sorts of premature conclusions.
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