Polarizing Filter

Up to now I've mostly been concerned with interchangeable lenses for my digital SLR (Canon 20D), for different amounts of zoom and wide-angle. The only filters I have are UV filters that I leave on the lenses all the time, more for scratch protection than anything to do with the pictures themselves.

I've been spending a lot of time cleaning up landscape shots by darkening the sky and/or lightening the ground though. Sometimes mashing together 2 or 3 auto-bracketed shots to get decent contrast in bright outdoor situations. I finally got a polarizing filter, and I can see that the editing time will be much reduced!

Despite what you might think from the name, a polarizing filter has nothing to do with Joe Lieberman. It acts like sunglasses for your camera -- reducing glare, increasing colour saturation, reducing blown-out highlights. It reduces the overall amount of light that comes in, but a camera like a DSLR that meters "through the lens" (TTL) can automatically compensate. Camera mavens consider a polarizing filter the #1 most important filter for photographic effects. (They'll advise you not to skimp based on price. In this case at least, I'll concur with that advice.)

There are 2 types of polarizing filters, circular and linear. DSLRs require a circular polarizer, whose effect increases and then decreases again as you spin its front element around. This fact triggers several interesting consequences. A circular polarizer would be useless if zooming or auto-focusing actually caused the lens element itself to rotate, because you could never rely on the polarizer to stay at the rotation where you left it. Cameras that do rotate their front lens elements must therefore need a linear polarizer, which I won't mention again. Because the circular polarizer has two pieces, the back that stays steady and the front that you spin around, it has the potential to be relatively thick. You can pay extra for a "slim profile" filter. (I did.) Now the DSLR owner has three things to spin around on their lens: the zoom ring, manual focus ring if you want, and the polarizer. Luckily, once you get into a bright situation with refections and glare, you can spin the polarizer to the setting you want and then leave it there the rest of the time.

Although a DSLR like the Canon 20D does compensate for the decreased light coming in, that's at the expense of shutter speed. Be prepared to boost your ISO value by a couple of steps -- 400 where you might usually use 100, 800 the moment the daylight is anything less than bright.

The polarizing effect doesn't apply equally in all directions, and its amount depends on the angle you are shooting vis-a-vis the angle of the sun overhead. You can predict where the effect will be strongest by pointing at the sun and tracing an arc directly perpendicular to it; the usual advice is to make a "gun" shape out of your index finger and thumb, aim your index finger at the sun, and rotate your wrist (or imagine doing so) and your thumb traces out a path that points at all the places where the filter will have the most effect. But I only find that useful for figuring out where to look as I turned the ring to gauge the effect. Looking towards reflective water, I could actually see texture appear and glare fade as I adjusted the filter.

I've always been annoyed that Canon DSLRs by default produce photos with low saturation. I've actually got a Photoshop action set up to boost saturation, that I run practically all shots with. (In pictures with bright reds, sometimes the right adjustment is boosting saturation for blue and green, but leaving it unchanged for red.) However, now with the polarizing filter, I can see that this dull-looking default leaves room for the natural increase in saturation you get with a polarizer.

Net result: My pictures taken against the sky or ocean came out much better than usual -- no burn out, good cloud definition, even correct exposure for an eagle in flight instead of the usual black silhouette. I had to boost exposure time and ISO maybe more than I'd like for a set of fireworks pictures, but those pictures had good colour that required less adjustment and saturation-boosting than normal.

One other tip for shooting with a polarizer. Because the darkening effect on the sky varies depending on the direction, you might want to leave the filter off when shooting multiple shots for a wide panorama, to avoid uneven brightness in the sky.