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Polarizer impacting image quality on telephoto lenses?
  
 
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p.1 #1 · p.1 #1 · Polarizer impacting image quality on telephoto lenses?


I've always been a fan of polarizers on all my lenses. Recently I've been testing the Sony fe 100-400mm GM and notice that at the long end, polarizers seem to be impacting sharpness and focus consistency. Is this typical for polarizers on longer telephoto lenses? The impact seems to be insignificant at the 100-300mm range. I'm wondering if it effects the ability to focus or if it just degrades the image sharpness at longer focal lengths, or maybe it's just user error? Anyone had similar experiences with long lenses?


Aug 02, 2017 at 09:37 PM
hmzimelka
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p.1 #2 · p.1 #2 · Polarizer impacting image quality on telephoto lenses?


Not all polarisers are made equal, and like UV or protective filters, the quality of the glass' flatness will affect the quality of the image to varying degrees. The longer the focal length, the worse a filter can affect the image quality.

I once helped a chap complaining that his AFS 300/4 was rubbish and not sharp, and wanted to sell the lens. Out of habit I removed the filter, and turned out it was excellent. Before, it kind of gave the impression of not being sharp, slightly directionally smeared in some areas, and especially the blurry areas looked like they were affected by atmospheric distortion. So I guess the image effects can range from imperceptible to quite drastic.



Aug 03, 2017 at 10:33 AM
Mikehit
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p.1 #3 · p.1 #3 · Polarizer impacting image quality on telephoto lenses?


A polarising filter generally has a 2-stop penalty on light reaching the sensor, and if you reduce the light you reduce the amount of information that the AF sensors have to work with and they become less reliable. At 100 mm the aperture will be at f4.5, at 400mm will be f5.6 so the effect will be dependent on focal length.

You then have the issue of the quality of the filter itself, as hmzimelka pointed out and blurring of detail will be more apparent at longer focal lengths.



Aug 03, 2017 at 12:46 PM
 

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p.1 #4 · p.1 #4 · Polarizer impacting image quality on telephoto lenses?


Thanks guys. Appreciate the feedback. I have a decent filter. It's a B+W MRC polarizer. Do filters deteriorate over time? I've had the filter for about 10 years I'd guess


Aug 04, 2017 at 04:46 AM
hmzimelka
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p.1 #5 · p.1 #5 · Polarizer impacting image quality on telephoto lenses?


B+W should be good, although it's a often counterfeited brand too.
It's really hard to say, as I'm no expert on the foils used in polarisers, but I know they can deteriorate with moisture, heat and impact.

Could you try another polariser just to compare?



Aug 08, 2017 at 09:57 AM
elkhornsun
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p.1 #6 · p.1 #6 · Polarizer impacting image quality on telephoto lenses?


The mirrorless cameras use a different autofocus method than a DSLR so a polarizing filter might affect focus accuracy. It would not hurt to have your camera and this lens on a tripod and then manually focus on a subject with and without the filter and do the same with the camera autofocusing on the subject and compare the results.

I would want to know that the camera and lens were working as expected without the polarizing filter (and assume that you are using a circular polarizing filter).

Not obvious but AF sensors need light to detect contrast and determine when a lens is properly focusing the image. A filter that cuts the light in half makes this processing more difficult for the camera to achieve. My 2x teleconverter also cuts light by 2 full f-stops and in many situations I need to focus manually when using it.



Aug 11, 2017 at 08:57 PM







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