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I am testing out a B+W 49mm Circular Polarizer filter, but there is no marking, or indication where the starting position supposed to be.

If the filter is supposed to be turned to turn polarization on/off, how do you tell which is which?

I must be missing something obvious, but i've googled around and checked the manufacturer page and there is no mention of this.

I am only guessing that some of the markings on the filter should be used to figure this out.. Also, should these effects be visible through the viewfinder (provided it's looking through the lens)?

  • The reason there is no marking on the filter for the starting point is that it all depends on the angle of the light sources in relation to the the angle that light is reflected off the items in the scene in your viewfinder. – Michael C Sep 8 '14 at 3:30
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That's because there is no set "starting position". You simply rotate it until it is blocking whatever angle the glare is coming in at. Adjust by how it looks. You will see glare either increase or reduce as you turn it, when you get to the angle with the least glare (or whatever visual look you are looking for), you have it set correctly. That's all there is to it.

There is no On or Off. A polarizing filter is always only letting light with a particular polarization angle through and the light coming through it is always polarized. The rotation just adjusts which angles of polarized light are allowed to pass. (Note, that's a rough approximation, circular polarization is a little more complicated than linear polarization, but the basic principal remains the same, we use CPLs because they allow phase detect auto-focus to work.)

  • ahh.. that makes perfect sense, thanks! i tried previewing this in viewfinder but it didn't seem to make a difference, perhaps because i was testing indoors – Sonic Soul Sep 7 '14 at 15:56
  • @SonicSoul - yeah, if there isn't any light that is polarized in your seen, then it will be roughly even in every direction of polarization. In that case, you would see no change other than the filter making things darker in general. It is still blocking out particular angles of light, but there is the same amount of light at every angle. – AJ Henderson Sep 7 '14 at 16:00
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    For practice, I'd recommend pointing the camera at something where the effects are really obvious, such as the reflection off a pond or a plate-glass window. – Mark Sep 7 '14 at 19:18
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    @SonicSoul You won't see much difference through a polarizer indoors, unless you try viewing an LCD screen through it: for the most dramatic demonstration, look at one of the old calculator-style LCDs through it! Other places you can see the effect are a blue sky, when you're looking at 90deg to the sun (e.g., looking east or west if the sun's to the south) or reflections from non-metallic objects such as water or glass. – David Richerby Sep 7 '14 at 19:57
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    @AJHenderson Digital is not the reason we need circular polarizers instead of the older linear polarizers. Auto Focus, even in film cameras, is why we need them. – Michael C Sep 8 '14 at 3:26
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There's no "on" and "off". The following explanation isn't completely accurate but I think it's a good enough approximation to understand what's going on. In particular, I'm really talking about what's called "linear polarization", which, as you can guess, isn't quite what a circular polarizer deals with.

Polarization refers, roughly speaking, to the direction in which the light waves are wiggling. Consider a rope that has one end tied to the wall. If you wiggle the free end up and down, you'll create a wave in the rope, with everything in it wiggling up and down. But you could also wiggle the end from side to side, or at any other angle you choose.

A polarizing filter only lets light through that's wiggling in a particular direction (or, rather, in a particular fairly narrow range of directions). To continue the rope analogy, consider passing the rope through a board that has a slot cut in it. If you make a wave in the rope that's moving in the same direction as the slot, the wave will pass through the slot; but if you make a wave in a different direction, the wave will just happen between the slot and your hand. Of course, in the polarizing filter, it's the equivalent of the slot that's rotated, rather than the light waves.

Light can be either completely polarized (all the waves wiggling in the same direction) or completely unpolarized (all wiggling in random directions) or somewhere in between (all directions present but some directions more common). Whenever you put light through a polarizing filter, it blocks out most of the light that isn't wiggling in the direction the filter allows. Most ordinary light is unpolarized: if you put that through a polarizing filter, it just blocks out some fraction of the light coming in. That's why you didn't see anything in your tests.

However, the light from the blue part of the sky away from the sun is quite strongly polarized. Face at 90° from the sun (e.g., face west or east with the sun in the south) and look at the sky through the polarizer. Rotate the polarizer to the angle that makes everything as bright as possible. Your polarizer is now aligned with the light from the sky and it's letting nearly all of it through. However, as you rotate the polarizer, you'll see that the sky gets much darker, while the land stays at about the same level of brightness. That's because you're blocking out a lot of the polarized light from the sky but not the unpolarized light bouncing off the ground. Turning the polarizer a quarter turn should take it from brightest to darkest.

The other main source of polarized light is reflections from non-metallic objects. If you look at reflections off glass or water through the polarizer, you'll see the same effect: turn the polarizer so the reflections are at their brighest and then, as you turn it through a quarter turn, the reflections will fade away. Another example is reflected glare from foliage, as demonstrated in this blog post.

Perhaps the most dramatic demonstration is to look at an old-style calculator LCD display through the filter. At one angle, the display will look completely normal; a quarter-turn later, it'll be completely black.

Finally, yes, the effect will be visible through the viewfinder, as long as that's showing an image through the lens. It's a "pure" optical effect and doesn't rely on the sensor at all. In fact, all the demonstrations I suggested above are easiest if you just hold the filter in your hand and look through it without using a camera at all! :-)


Thanks to MichaelT for reminding me about foliage.

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