Circular and Linear Polarizers

• Linear polarizers may cause SLR auto-focus and auto-exposure to malfunction. They can only be used on rangefinder cameras, such as Leicas, or older completely manual film SLRs. Circular polarizers are used for all digital SLRs and film SLRs which rely on auto-focus or auto-exposure.
  
  

Polarizers and Lenses

• Circular polarizers work well on normal and telephoto lenses but can cause problems on extreme wide angle (28mm equivalent and wider) lenses. Because circular polarizers actually consist of two separate pieces of glass which rotate independently, they are much thicker than other types of filter, and will actually show up on the edges of photographs taken with wide-angle lenses, an effect known as "vignetting." Select an ultra-thin polarizer to avoid this problem. The second issue is that polarizers darken the sky far away from the sun more than they darken the sky close to the sun, an effect that is magnified with a wide angle lens. To darken the sky with a wide-angle lens, try an 81A or other moderate warming filter instead.
  
  

When to Use a Polarizer

• Polarizers can improve your photographs in several situations. They can reduce haze, darken very light blue skies and increase overall color saturation. They also reduce glare and reflections, something especially important if you are shooting highly reflective surfaces such as glass, metal or water. Polarizers will help you shoot through glass or water as well.
  
  

When Not to Use a Polarizer

• If you are shooting a nonreflective surface on an overcast day, a polarizer adds another layer of glass between your camera and your subject, reducing the amount of light coming into your lens and making your image slightly less sharp. For reflective surfaces, use a polarizer to minimize reflections, but remember not to use it if the reflections themselves are worth including. A tree or moon reflected from a still lake may be more interesting than a muddy lake bed. Darkening the sky with a polarizer can look good or cliched, depending on the composition; it might be better to adjust your shooting angle or change lenses to include less sky or more interesting areas of sky. Let the needs of the subject guide you, rather than letting a filter become a crutch to compensate for bad composition.