The many uses of polarizers

Started Sep 27, 2010 | Discussions thread
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Mithandir Senior Member • Posts: 1,766
The many uses of polarizers

I've been off on a holiday for the past two weeks (I trust I didn't miss any important news? :)) and while there I somewhat rediscovered one of the basic tools of photography: the polarizer. Therefore I wrote this little article on it, mostly on my phone during the trip home. You can also find this at my site here:

As always I'm too wordy for a single post and a bit of the article flows over into the first reply

The many uses of a polarizer

There are a handful of photographic filters that image editing program can't readily reproduce. Of these, I find the polarizer the most interesting. Now, some people will say they can mimic a polarizer easily in photoshop, but they will then continue by showing how to merely make your sky bluer. This effect, despite being the best known, is really just the least interesting of the many uses of a polarizer.

A bit of physics

In discussing polarizers, it is helpful to inject a bit of physics to understand what exactly they do. I trust everybody knows light is a wave. To be exact, light is a longitudinal wave. Take a bit of paper and draw a wave on it and draw a straight line through it in the direction it travels. This is your ray of light. Now hold one end of this ray in each hand. You'll notice that you can rotate it around its axis without changing the direction of the beam. This is the angle of polarisation of the ray. In most cases, lightbeams consists of a great many of such rays, polarized randomly in all directions. The net result is an unpolarized lightbeam.

If that was all there was to it, a polarization filter (which filters out all light except that with certain angles of polarization) would be quite useless except as a darkening filter. Luckily there are a couple of things that cause rays of light to alling together and polarize entire lightbeams. The most important of these (for us) is reflection: light that is reflected at a certain angle is polarized at a certain angle. This means that with a polarizer we can selectively enhance or remove reflections (well, you don’t enhance reflections, you darken everything else. A polarizer steals a bit over half your light, but you get to pick which half).

The black sky above

At this point you may be thinking “big deal, I don’t take pictures of mirrors” but reflections are all around us. Take the sky: if you’re luckier than me, you may see some blue. Sky is blue, after all, right? Well no, the sky is transparent. The blue is sunlight that is scattered in the atmosphere: the blue light gets reflected of air molecules and spread all over the sky before they are reflected back at us. Now, using a polarizer we can filter out some of this reflected light, which will turn the sky darker simply because it lets us see the empty space beyond. Cool, huh? Alternatively you can turn the polarizer the other way to enhance the reflections and ... well, clip the highlights in your sky, usually.

The tranquil waters

Now that we’ve seen the obvious one that everybody knows, let’s take a look at some other reflections. You may not often take a picture of a mirror, but mother gaia provides us with some natural mirrors that can be fun to play with: lakes and ponds. Of course, water doesn’t reflect as well as a mirror … except when you use a polarizer. You’ll need a pretty quiet lake for this, but if you come upon one, take out your polarizer and have some fun. You can generally go from full reflection to seeing the bottom.

Somewhere over the rainbow

A bit more physics for you: do you know why a rainbow is always away from the sun? It’s because the light that you see is reflected back at you by the water drops. This means that using a polarizer you can greatly enhance the vibrancy of the rainbow (or make it disappear entirely if, you know, you don’t like pretty things in your pictures).

The glass between us

Alright, I’ll admit that this one sounds better than it is: using a polarizer you can remove reflections from glass when taking pictures through it. However, there’s two problems here: 1) usually when we’re shooting through glass it’s indoors in quite dark surroundings (museums, zoos, …) and we don’t really want to lose a stop and a half of light. 2) because the glass tends to be close to your lens, it doesn’t work as well as we’d hope.

A distant haze

Remember that blue light getting scattered all through the atmosphere? It doesn’t have the decency to stay high up there and out of the way. Take a picture of some distant mountains, and they’ll often colour blueish and get all washed out. Don’t worry, though: grab a polarizer and you can filter out those blues and make those distant mountains a lot crisper (and redder).

Turn that monitor off

LCD screens (flat panel monitors and many TV’s these days) work based on rotating polarizers. This means that you can use a polarizer on your camera to completely block out monitors (and other LCD screens). This can be quite handy when taking pictures in an office or similar (alternatively you can enhance the screen output).

Variable darkness filter

Take two polarizers and put them in front of each-other. Depending on the quality of your polarizers you should now be able to turn them relative to each-other and go from complete blackness to a bit over 1.5 stops of light loss (a bit more than a single polarizer gives).

Mind you, you might experience some colour shifting, but you now have yourself a variable-strength darkening filter.

For best effects you want the back polarizer (the one mounted on the lens) to be a radial one and the front one (the one mounted on the first one) to be a linear polarizer.

Continued in first reply

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Eternal Amateur

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