Polarizer Question

badrano

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After many photos taken at airshows using my Tamron 70-300 with a circular polarizer, I've always wondered why my pic never really came out all that crisp.

The other day(more like a month ago), I was outside playing around with the lens and polarizer only to realize that I may have been setting it incorrectly. The other issue may be due to not have a 100% clean lens/filter (at least from the airshow back in June). I never realized that the whole lens barrel rotates on focusing there for causing the polarizer to rotate. I would set the polarizer looking at something fairly close (within 70mm). Most of my shots were between 200-300mm which I guess may have an effect on the focusing.

Are there any techniques to help out this problem? The other thought is to upgrade to a different 70-300mm lens.

Thanks.
 

KmH

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Good CPL filters are not inexpensive. If yours is not multi-coated or otherwise a quality made filter it's likely the filter itself is a large part of any sharpness issues you may be having.

Also, a CPL filter gives an uneven effect when the Sun is more than 30° above the horizon and at more or less than an angle of 90° to the lens long axis.

A CPL filter also reduces the amount of light entering the lens (at least 1 stop) and many DSLR cameras cannot AF effectively if the light is f/5.6 or less of effective lens aperture.
With any lens zoomed to 300 mm that has a maximum lens aperture of f/5.6 the CPL filter adds a stop making the effective lens aperture f/8.
 

SCraig

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There are no techniques to eliminate the problem other than another lens that does not rotate the filter when the focus changes. About the only other thing is to focus manually and make sure the subject remains within your depth of field. That's what I did when shooting airshows with a manual-focus lens.
 
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badrano

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Thanks for the info guys.

My Tamron lens is probably close to the bottom of the barrel model, so I should start saving to get a better one. Maybe even get a lens that can go out to 400mm, granted that will be more expensive to get one at 2.8 or larger.

Using the CPL when I shouldn't probably be using one is probably another problem I may have. I have done some reading on when/when not to use a CPL and I think, at least in the most recent air show, used the CPL when I shouldn't have.
 

bratkinson

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Lens barrel rotation while zooming and focusing is fairly common in many zoom lenses. It's rarely, if ever, indicated in the 'factory specs' for the lenses. Instead, check out the reviews of the lens(es) you are considering. They will often indicate the barrel turns while zooming/focusing.

Like the OP, it took me a while shooting with my CPL mounted on a Canon EF-S 18-135 to figure out that was the cause of my oddball coloration problems when shooting through a window. All a part of the learning process, I guess. As a work-around, I simply adjusted the CPL again after I zoomed and focused, when time permitted. Ultimately, I upgraded my lenses to Ls...problem solved.
 

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