A lot of people will tell you, with so many powerful image-editing software packages available, that filters are unnecessary. (Just to be clear, I'm talking about the kind of filters you attach to your lens, not the kind you find in Photoshop.) That’s true… to an extent. You can pretty well replicate the effect of many popular filters with a click or two in your favorite image editor. Split neutral density, graduated neutral density, and color correction filters, as well as many “special effects” filters have awfully good software equivalents, but there are a few filters you can’t replace easily (or even at all). One of these is the polarizing filter. Keep reading, and I’ll tell you why you want to keep one of these in your bag and how to make the best use of it.
Polarization is a physical property of light. The easiest way to explain it is to think of wiggling a rope to make waves travel along it; if you move the end of the rope up and down, the waves will be vertical:
If you move the rope side to side, the waves will be horizontal. Un-polarized light has both kinds of waves, while “perfectly” polarized light has waves that oscillate in only one direction. A polarizing filter does to light kind of like what a picket fence would do to your wiggling rope. If the rope runs through a picket fence, only waves in one direction will pass through and waves in the other direction will be cancelled out. The polarizing filter is like that picket fence for the light going into your lens; light oscillating in one plane passes through and light oscillating in the other plane is blocked. Put two polarizing filters together (two picket fences), with their “slats” turned ninety degrees to each other, and ALL the light is blocked and NO light gets through.
Now here’s the photography part: light can get polarized in different ways. Light
reflected from a non-metal surface (think glass or water) gets polarized; light passing
through the glass, or the surface of the water, remains non-polarized. Put a polarizing filter on your lens and rotate it to the right angle, and you block the light reflected from the water or the glass, i.e. you block the contrast- and saturation-robbing glare you get from those reflections. Ever seen pictures of the ocean where you can see right through the water’s surface to the fish swimming beneath? Probably shot with a polarizer. Likewise photos of shop windows where you can hardly see the glass. Need to kill reflections? Throw on a polarizing filter and fire away. How do you know what the “right angle” is, you ask? Easy! Compose your shot and then rotate the polarizer until the reflections are minimized.
Are there other situations where a polarizer can come in handy? Absolutely! Have you ever seen a landscape with a beautiful, dark blue saturated sky? Chances are there was a polarizer involved there, too. As sunlight is scattered by the atmosphere, it also gets partially polarized. By using a polarizing filter, you can block some of the polarized light, which will darken and saturate that blue sky. There’s a catch, though; the whole sky isn’t polarized equally. Some areas of the sky are more polarized, and therefore more susceptible to the effect of a polarizing filter, than others. You’ll get the maximum darkening effect if you point your lens ninety degrees to the path of the sun through the sky. Put more simply, keep the sun off one of your shoulders for the maximum effect. If the sun is directly behind or directly in front of you, you won’t get nearly as much out of your filter. Also, be careful using a polarizer with a wide-angle lens. The large angle of view you get means the sky can end up lighter at one side of the frame and darker at the other side, which can look pretty funny. The bright side is that you see the effect of the polarizer through the viewfinder, so you can catch this gotcha before you trip the shutter if you’re paying attention.
How much would you pay for this magical filter that can cut glare and give you gorgeous saturated blue skies? Don’t answer yet! There’s more! Many kinds of leaves have a waxy coating that polarizes reflected light. By cutting that reflected glare with a polarizing filter, you can nicely saturate the greens of summer foliage or the beautiful golds and reds of fall leaves.
There are a couple of things you’ll want to keep in mind about polarizing filters, though. First of all, there are two types available, linear and circular. Which do you want to buy? If you have an autofocus camera, get a circular polarizer. Autofocus cameras may be unable to focus with a linear polarizer mounted. I have also heard that evaluative metering systems in newer cameras may not work properly with a linear polarizer. Another caveat: regardless of what kind of polarizer you use, you’ll lose about one stop of light; for example, say you shoot a scene without a filter and your aperture and shutter speed are f/4 and 1/1000 of a second for the desired exposure. Add a polarizer and you’ll need to use either (about) f/2.8 and 1/1000 or (about) f/4 and 1/500 to achieve the same exposure. Not necessarily good or bad, just something to be aware of.
So do you need a polarizer? Like so many other questions in photography, the answer is a definite “it depends.” It depends on the kind of shooting you do and what effects you want to achieve. I will tell you this, however: a polarizer can do things for you that are very difficult, if not impossible, to do in software-based post-processing.
