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Some time ago I bought a polarizing filter for my GoPro and I was pretty amazed by how it can change the resulting image when I turn the filter on top of the lense. Then I bought a DSLR and also wanted this filter for it. So I bought one from Amazon basics and comparing it with the first one, it does nothing. Now I don't know what to buy to not end up like I did with the Amazon one.

Is there some unit in which effectiveness of these filters is measured?

I already asked at a local camera shop, but the lady was also not sure how to distinguish them other than by vendor and price and she did not allow me to try the ones she has on the shelf.

  • Did you try rotating the amazon basics filter on the front of your lens to get the maximum effect? Were you shooting a scene that had strong off-axis light? – Michael C Aug 21 '16 at 23:16
  • "[I]t does nothing." Really? – osullic Aug 21 '16 at 23:36
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    Possible duplicate of How do I choose a polarizer? – mattdm Aug 22 '16 at 8:25
  • Check here for comparison of performances, in case you want to buy another one: lenstip.com/index.php?art=139 Expensive ones are not always better. – FarO Aug 22 '16 at 10:20
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    And that said, even in the context of the whole sentence, it's unclear what you mean exactly. Does it literally do nothing, or does it do something, but not very much compared to the first, such that you are using hyperbole and saying "it does nothing"? – mattdm Aug 22 '16 at 18:01
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The polarizing filter (should be called a polarizing screen) is likely the most valuable optical filter you can possess. The polarizing screen mitigates reflections. It does not work on all surfaces but it works on most. You can use it to diminish annoying reflections on glass and on water. This filter darkens a blue sky making clouds stand out without changing the other colors of the vista. The polarizing screen also cuts haze like a UV filter. The problem is, you can’t just mount this filter and expect topnotch results. You mount and then, peering through the viewfinder, you rotate the filter for effect. You see, the filter screws into the rim of the lens barrel, and its mount allows you to manually rotate the filter for effect. You will find that the maximum effect will be realized when you are taking a picture at a right angle (90⁰) to the sun.

Most likely the only difference between your first polarizing filter experience and this one is beginners luck. You should try again, and this time rotate the filter as you compose for best effect.

On the technical side: The polarizing screen acts as if it has microscopic lines ruled all parallel to each other. This arrangement acts like a picket fence and only allows light waves that are orientated with the pickets to transverse the filter. This allows the filter to pass some light rays and reject others. That’s why we must rotate the filter for effect. There are actually two types of polarizing screens. You likely have a circular polarizing filter. This is the approved design for the digital camera. This is actually two filters sandwiched together. The first one is a standard linear polarizing screen. It is the first one that does the deed. The second is called a retarder -- it de-polarizes. This sandwich design is necessary as most modern cameras will have their auto-focusing and auto-exposure determination disturbed by the polarizing screen. The circular polarizer is a work-around.

It’s time to re-try your polarizing filter; there is likely nothing wrong with it.

  • I did rotate the free part agains the part screwed into lens, but it just darkens small bit of the picture and leave the reflections there. – Mailo Světel Aug 22 '16 at 16:23
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    What are the reflections from? Metal surfaces and mirrors (which are metal coated) reflect all polarisations equally so the filter won't help. – Chris H Aug 22 '16 at 16:30
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    Light from the sun arrives vibrating in all planes, it is not polarized. When light hits some light rays are absorbed and some reflected. The reflected rays may or may not be polarized. This phenomenon is based on the material. The polarizer works to mitigate reflections from surfaces that are non-conductors of electricity. Note: early scientist working with materials that polarized light speculated that light had both a positive and negative component. The reasoned that polarization was likened to the north and south pole of a magnet hence the origin of the name. – Alan Marcus Aug 22 '16 at 16:46
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So I was in different shop:

  • Answer to my question: No, there is no unit to measure effectiveness of polarising filter.
  • When filter does not work at all, check whether light source is not coming directly from behind of the camera. Polarising filter will not dim reflections which are about 180°

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