Be sure you are using a circular polarizer (C-Pol or CPL). Most sold these days are, so it's probably what you've got.
But there are also linear polarizers (Pol or PL). Those will interfere with autofocus and, in some cases, with metering. Circular polarizers do not interfere with either.
Not all C-Pols are created equal. "Cheap" polarizers can cause problems... soft images, ugly color tints. It's also best if any filter is multi-coated. This prevents problems with flare.
I recommend B+W circular polarizing filters as a good value (B&H Photo and Adorama sell them at very reasonable prices.). All B+W filters use top quality glass and their "F-Pro" and "XS-Pro" are multi-coated. The F-Pro have 8-layer multi-coatings (fine optically), while the XS-Pro have 16-layer "Nano" multi-coating that are a bit more durable, oil and water resistant, and a little easier to clean. While all B+W filters have slim frames (to reduce concerns about vignetting on wide angle lenses), the XS-Pro are slimmer than most. Probably not necessary if the widest lens you use is 18mm.
Both the F-Pro an XS-Pro are now "High Transmissive", too. This means they aren't as dark gray as "standard" C-Pol filters... so you lose less light to the filter. Standard C-Pol will reduce light passing through by approx. 1.25 to 2.25 stops. The High Transmissive type reduce it by approx. .75 to 1.5 stops. This can be important when shooting in low light conditions, so you don't need to use too slow a shutter speed or too high an ISO.
It's not uncommon for comparable quality C-Pol filters from other manufacturers to cost 1.5X to 2X as much as B+W XS-Pro C-Pol.
I agree with a previous response... Polarizers have their strongest effect at 90 degrees from your subject. To visualize this hold your hand like a gun, point your forefinger at your subject, then notice where your thumb is pointing... that's approx. 90 degrees. So when the light source (sun or ?) is anywhere along that arc, the filter will give maximum effect. As that angle increases or decreases, the filter will have less and less effect.. Ultimately it will have no effect when you are pointing directly toward the light source (0 degrees) or when the light source is directly behind you (180 degrees).
Because of this variation in the filter's effect, sometimes you may see a gradation in your images... Especially if shooting with a wider angle lens (shorter focal length, wide angle of view) and when there's a large, plain expanse in the image (such as a clear blue sky). This isn't necessarily a bad thing, but is something to watch for (you can usually see it occurring in your viewfinder).
There are times to NOT use a polarizing filter... such as when a reflection is an important part of the image. Also, if trying to photograph a rainbow, avoid any polarizer. It will make a rainbow disappear! And, because the filter has no effect anyway, I recommend removing the filter when directly shooting a sunrise or sunset, with the sun within the image. Providing no benefit, the added layers of glass in the filter will likely just cause flare problems (although when multi-coated flare may be minimized).
That said, circular polarizers remain one of the most useful filters you can have in your kit. Most other filters are unnecessary with digital photography. C-Pols are the single biggest exception. They can do things to images that can't be accomplished by camera settings or in post-processing software. (Neutral Density filters also remain useful, but are specialized... only needed for certain types of shots. And some people like to put a clear or UV filter on their lens for "protection", though frankly I find it a bit silly to think that a thin piece of glass can do much to prevent damage. Still I do use them shooting at seashore, to keep "salt spray" off my cameras. I'd also use them if I ever shoot out in a sandstorm or at a paintball battle.)
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7DII (x2), 7D(x2) some other cameras, various lenses & accessories
FLICKR & ZENFOLIO
