No matter whether you shoot with a dSLR, compact digicam, film SLR or rangefinder -- your photography can benefit from a polarizing filter. It modifies one of the basic properties of the light falling in your camera and its effects cannot be digitally duplicated, which makes it the No. #1 filter in photography.
Effects range from unwanted reflection removal to enhancing the pureness of colors and much more. Read on for a more in-depth look at polarizers, the way they work and the way you can benefit.


Introduction

First I need to dispel some misconceptions -- for example, a widely spread myth is that polarizing filter is used only for getting bluer sky in you shots. This is wrong, or at least imprecise for more than one reason, but to really understand why, I'll first have to explain one specific property of light...


Behind the scenes

(skip this section if you don't want the physical background)

Polarization is property related to wave-part of light's wave-particle duality. In simple terms, polarization (measured in degrees) is direction of oscillation of the wave of light (measured from vertical axis).
For example, waves on a sea always oscillate up and down. If you imagine a wave of light coming towards you doing the same, it would be polarized at 0 degrees. Light can deviate from this in the range from 0 to 180 degrees. (then angles are repeated, for e.g. 270 degrees is identical to 90 degrees and so on.)
Light around us is mostly unpolarized, which is equivalent to saying "polarized in all directions". It means roughly equal number of waves oscillate in all directions.

Now, there are ways of polarizing light, and the ones we are concerned about occur in nature, as they are the only ones polarizer will have effect on. When light reflects off water, metal or glass surfaces, for example, it will become partially or completely polarized.

So what does a polarizing filter do? You can imagine it as a narrow slit that will only let through light waves oscillating in one selected direction, direction of the slit, while rejecting others.
This means that you can choose which light waves you will admit to your film/sensor and which to reject. This influences the final image.


Effects

Now let's get into the effects. First off, screw on polarizing filter on your lens, and you will lose about 2 stops of light (give or take half a stop). Short explanation is: you're rejecting significant amount of light, letting through just part polarized in desired direction. No matter which direction you choose to admit, you're still rejecting a lot of differently polarized light. Using just a portion of available light means less light left for your camera.

As I said, light reflecting off surface of water, glass or metal will be polarized. However, even light reflecting off water molecules dispersed in the atmosphere (vapor) will be polarized, albeit to a lesser extent. That's why on certain parts of the sky (not on whole of the sky), polarizer will have effect of darkening and lightening, depending how you turn it, making the clouds stand out in the process. (It won't make the sky look bluer however, only darker.)
*EXAMPLE 1* *EXAMPLE 2*
(you can also note how sea surface darkened as well)

When shooting waterscapes, polarizer is useful to select only light reflecting off the bottom, and reject light off the surface of the water. What that does is remove nasty reflections off the surface like sun, surrounding trees, clouds etc. It is also immensely useful for glare reduction. That's one of the reasons why eyeglasses with polarized lens are so popular when going out on the sea.
*EXAMPLE 1* *EXAMPLE 2*
(second example illustrates how polarizer can make a difference between tropical looking sea, and muddy harbor water looking sea (both shot on same location))

When shooting through a glass window or similar, polarizer will be able to remove reflections of surrounding objects from the glass and make it completely transparent, to the point you wouldn't notice it was shot through a glass window. This can be immensely useful.
Same goes for some metal surfaces.
*EXAMPLE*

However, there is one more use for polarizing filter that most people overlook or don't even know about, and it's got to do with colors on a sunny day.
Light coming directly from the sun renders warm and saturated colors. However, part of the light finishes up in the atmosphere where due to Rayleigh scattering mostly blue light continues towards Earth (that's why sky is blue). This bluish wavelengths combined with warm direct sunlight will decrease pureness of colors (ie. their saturation). You can notice this during beautiful clear days when all colors look so bright and bold and the sky is dark blue, compared to hazy day when sky is grayish-blue and all colors around you tend to be muted.

As light coming from the sky is polarized, and sunlight isn't, you can use your polarizer to remove bluish light and get purer, warmer, more saturated colors by letting only direct sunlight reflected off your surroundings on your film/sensor.
(Of course, you can do the inverse of that, but you won't like the results).

This is a trick I often use to get naturally saturated colors without post-processing, and it works especially well with contrasty and saturated slide film or negative film designed for same purpose (Kodak Portra VC, Fuji Pro C etc.). For example:

(These non-photoshopped examples don't have 'before' and 'after', but colors speak quite enough I think. Read descriptions for exposure information.)


Shooting in practice

Take any polarizing filter in your hand and you will notice it was designed so you can easily rotate it on your lens. Rotation is means of selecting desired polarization you want to let through. As you rotate it, you should see different effects in your viewfinder. After it was turned 180 degrees (half-circle), effects will repeat.
When shooting with it, it's just a matter of turning it until you like what you see, and taking a shot. Simple as that.

Now let's get into the "but" part.

It all sounds well and good, but for light to be completely polarized by reflection, there are some conditions to be fulfilled, namely one: incident angle. Incident angle is the angle at which ray of light falls into a surface measured from the plane perpendicular at the surface. Given that the light is diffused, it's falling onto the surface at many angles, so the imperative becomes the reflection angle, or the angle at which you're looking/shooting at the surface.
Or, in English: depending on your perspective on the object you shoot, polarizer may have full effect, no effect or somewhere in between.

Let's illustrate. Imagine you're shooting through a horizontal glass surface (e.g. floor) you want to eliminate reflection from. If you take your camera and almost lie down on it (reflection angle of nearly 90 degrees), your polarizer will have no effect.
As you gradually stand up, it will have more and more effect and will remove more reflection. It will reach maximum somewhere around 55 degrees, and then again fall as you near 0 degrees (perpendicular on the surface).
This goes for water, metal and glass, although they each have their unique angle of maximum polarization (all fall between 53 and 56 degrees, though).
To illustrate, take another look at this *example*, but this time pay attention to side window of the car. There is almost no reflection without polarizer, and a pronounced reflection with polarizer, due to different angle of reflection when comparing to windshield.

That means if you want to shoot through a surface of the lake or glass, you need to adjust your perspective for maximum effect.
Same would be valid for shooting the sky if there weren't for a million molecules reflecting at million different angles. That's why when shooting sky you don't get black sky (when polarizer is at maximum effect) but just slightly darker sky.


One final note on conditions when polarizer is useful. You usually won't use it unless it's sunny outside to influence luminosity of the sky or color saturation of the scene. When removing reflections off the water, lighting source doesn't matter, it can be cloudy or artificially lit.
Shooting under artificial lighting doesn't make sense as polarizer won't have any effect except reducing amount of light reaching your camera, which usually means exposure problems. There is one exception: when removing reflections off glass, metal and water, lighting source doesn't matter, it will always work. You may need a tripod if there's not enough light, though.


Filter types

There are two types of polarizing filters, linear polarizers and circular polarizers (CPL). Without going into details, let's just say linear polarizers were used with old cameras, and tend to confuse auto-focus and light-meters of modern cameras. Circular polarizers are designed to work around this, while maintaining the same effects, and are also more expensive. Most (if not all) of filters you'll see in shops are circular type, but still -- make sure before purchasing.


Digital reproduction

There's no way to replicate the effect of a polarizing filter digitally. Neither digital sensors nor film record any information on the polarization of light which falls on them. Once you shoot a lake with bunch of reflections, you can't remove it the way polarizer would and see through the surface. Same goes for everything else: you can try to clone, saturate, darken or whatever, but it will never be as if you used an actual filter. Look at examples again if you need more convincing.
There are plug-ins for various image editing applications which suggest in their name that they convert your shot to a polarized one, but it's misleading. All they do is darken the blues, corresponding to polarizer's effect on the sky, which is what most amateurs use them for, and sometimes boost other colors. Nothing that couldn't be done with curves, if you're that desperate.


Getting the filter

Finally, how do you choose which polarizer to buy? It's simple. If your lens have expensive high-end optics, get an expensive multicoated filter that won't degrade the picture. If they're cheap kit lens, get whichever filter you can afford.




I hope you found this article useful.

If you want to learn more about ND filters, ND gradients and diffusers, read more *here*, as I don't plan on posting it as news because of (probably) relatively small amount of people who would actually benefit from it and own these types of filters.