Registered: January 2006
Location: Boston, USA
I'm probably the penultimate person on dphoto to acquire a polarizer - so I thought I'd give a primer on what it can (and cannot) do... I had last used a polarizer some 18 years ago, and the first outing showed up a lot of differences between polarizer and non-polarizer photos apart from the proverbial bluer sky that most everyone knows about. I also knew I needed a circular polarizer for my camera, but why on earth I had no clue. Time to hunker down and do some googling.
Nota bene: This is not a technical or necessarily overly physically correct treatise, so physicists will cringe and should abstain from reading this whithout proper cushioned seating arrangements.
First off, a polarizer works by -you guessed it- polarizing the light entering the camera. Sunlight and a lot of other light sources emit non-polarized light (the light beams/rays/waves/particles have random orientations). The polarizer only allows those light waves to pass that are in a specific orientation. So no matter what orientation your polarizing filter is in, it will block a vast portion of light particles from entering your lens. This explains one fundamental aspect of polarizers - using them will reduce the amount of light hitting your sensor by as much as 2 f-stops. That's why it's not a good idea to use a polarizer as a UV- or scratch-blocker on your objective, but it does open up the possibility of using it as a type of 'neutral density' filter. (It can saturate colors and increase contrast, though, so results are slightly different from using true neutral density filters.)
Most light entering your camera will be reflected light, though, from trees, grass, faces, water... Some materials like glass, water, clouds and leaves are prone to reflecting light that is now partially or totally polarized, and this is where the polarizer on your camera strikes. It still only lets through a specific orientation or plane of light rays, and if you turn your polarizer so that it is at a 90-degree angle to the already polarized reflected light, it won't let that polarized portion through. Basically, you subtract already polarized light from your photo.
Finding the right angle is pretty easy, of course, just observe the scene through your viewfinder or on your LCD screen and try to get the most dramatic effect. In-between effects are achieved by turning the polarizer to less than 90-degree angles.
So what does it all do? It reduces 'atmospheric scatter' (think haze control, bluer skies, higher contrast overall) and specular reflections (notably on glass, water and leaves, think reduced specular highlights, see-through water, clear windows - and often greener, more detailed foliage). What it cannot do is conjure a blue sky on a totally overcast day, fix a bad composition or deal well with metallic highlights, which are mostly non-polarized light.
Still not yawning? Let me up the ante - not all polarizers are the same. There's the good old linear polarizers, but now there's also the circular kind. What's that - circularily polarized light? Actually no, it's basically a linear polarizer with a little backing that is variously called 'birefringent material', 'quarter-wave plate' or 'wave retardant' (gotcha! your eyes just glazed over!). Something worth noting in purely practical terms is that a second material is introduced in a circular filter, which could potentially lead to (more) image degradation. I think it is therefore even more critical to use a high-quality filter!
I can't say I have a firm idea of what this second sheet of material does in purely physical terms, but de facto it 'depolarizes' the light a little bit. Wait a minute - we just went to all that effort to polarize it, and now we're undoing it all? So is this some weaker polarizer?
Actually no, because the polarizing front element has already filtered out the unwanted light or reflections, so a circular polarizer is just as good as its linear counterpart. But the polarized-then depolarized light is needed for SLR-type cameras (and also some non-SLR models), because some of their features won't work well with polarized light. The foremost culprit seems to be the beam-splitting meter that helps you get a proper exposure, but AF might work sluggishly with linear polarizers as well.
And last but not least, a helpful hint in deciding whether you do indeed hold a circular polarizer in your hands: look through the polarizer into a mirror, with the threads on the polarizer pointing towards you, then flip it to have the threads pointing at the mirror. The image of the circular polarizer in the mirror will turn black, while a linear one will stay translucent.
As you can see above, using the polarizer didn't fix the composition , but quite a few differences can be seen between the two sample photos.
The photos are original jpegs, identical manual exposure, both with the polarizer on camera, then downsampled and sharpened lightly identically, no other photoshop work has been done. In the top photo, the polarizer is in the 'off' position (acting basically only as a neutral density filter, image close to actual perceptions on scene). There is very little difference to what a photo taken without the filter would look like. The bottom photo shows the polarizer at the position of maximum effect.