Multi-coated surface to help improve contrast and reduce reflections. For best results, the quality of the filter should be on a par with the quality of the glass in your lens.
Appropriate thickness for the type of lens you're mounting the filter on. On my 16-35mm wide-angle zoom, I have a filter with a thin mount so it doesn't cause vignetting when the lens is set to its widest field of view. For my long zoom lens, such as a 70-200mm, it doesn't make any difference how thick the filter mount is.
Brass or aluminum for the mount. The theory is that brass mounts tend to be easier to unmount from the lens than their aluminum counterparts. Some photographers believe that they don't "freeze up" as often, when they then require a filter wrench to remove. I don't have any conclusive data on the superiority of a brass mount, but I will say that I like the feel of brass better.
The two main benefits of a polarizing filter are to reduce or eliminate reflections on some surfaces and to darken a blue sky. The most widely used type, the circular polarizer, has two glass elements. Depending on the angle of the light, you can increase or decrease the polarizing effect by rotating the front element.
Polarizers are effective for both color and black and white photography. They can add drama to a sky, clarity to an object in water, and saturation to foliage in a landscape. As with protection filters, look for multi-coated polarizers with high-quality mounts. Keep in mind that polarizers are dense and will usually absorb two stops of exposure.
Since polarizers tend to be relatively expensive--up to $250 for a 72mm mount--you may want to buy the size for your largest diameter lens, then purchase cheaper step-down rings to mount the filter on your other lenses. But I like to have a polarizer for each of my major zooms.
Neutral Density filters
For photographers who like to shoot at wide apertures or use slow shutter speeds, even in normal daylight, neutral density filters are a valuable asset. They come in two basic types: single density and faders.
Single-density ND filters are commonly available in the following options:
ND.3 = 1 stop exposure adjustment
ND.6 = 2 stops exposure adjustment
ND.9 = 3 stops exposure adjustment
ND1.2 = 4 stops exposure adjustment
You can buy them individually or in a kit. Kits typically run $100 to $200.
The second type, variable neutral density filter, sometimes referred to as "fader ND filters," are often seen in ranges of 2 to 8 stops of exposure adjustment. You choose the density by rotating the outer ring of the filter. I've seen faders as cheap as $35 and as expensive as $350.
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