Most photographers, particularly landscape photographers know the single most useful filter is a Circular Polarizer or CPL. It can remove glare, saturate colors, especially on wet fall foliage, double as a 1.5 stop neutral density filter, and of course is best known use, darkening skies when your lens is aimed about 90 degrees from the sun to help balance large tonal differences and add contrast against clouds. However, there is sometimes confusion regarding its use with wide-angle lenses. The difference is simply this, Partial Polarization instead of even polarization.
Partial polarization simply means the effect is not spread out evenly across a composition with a wide FOV (field of view). (Note: on non-wide compositions partial polarization simply means the polarizer is not turned all the way to produce its full effect.) This is because a polarizer works most efficiently at 90 degrees to the light source. Wide angle zooms can have a FOV of view of 90 degrees or more which means the effect will range from 0 to 100% and result in horrible, unusable skies. As such its use to ‘darken skies’ should be limited to compositions captured around 35mm on full frame and about 24mm on Dx format. At the wide end the best tool to darken the sky in camera is a grad-ND filter, or by using HDR or Bracket/Blend methods in the digital darkroom.
Personally, I frequently use a CPL for wide-angle landscape work when a clear sky is absent in the composition and when there is either foliage or water. This filter will selectively remove glare from shiny leaves to increase saturation, help to tame hot spots, remove surface glare from still (see top image) or moving water and wet rocks, and slow down the shutter speed to get a silky look for many waterfall captures. I simply turn until I get something desirable in viewfinder. My Waterfalls of Ricketts Glen gallery, most of which are shot at wide angles to create dramatic compositions, was shot almost exclusively with just a CPL filter. Bottom line, when in doubt, try it out!
Lastly, a few tips for selecting the right CPL filter. First, make sure you invest in a multi-coated, name brand product like Nikon, B+W or Hoya Pro1. Second, depending on the wide-angle lens you use it might be important to get a thin mount version. I cannot comment on the requirement of all lenses but I can on the four that I have owned or tested. The Nikon 12-24 and Sigma 10-20 f/5.6 can use regular size CPL’s without vignetting problems. The Tokina 12-24 will need a thin mount for the widest end. The new Nikon 10-24 also needs a thin mount since a regular size will show a small amount of hard vignetting at 10mm. If in doubt just get a thin version to be safe. Currently I am using the Hoya Pro1 CPL which works perfectly. Fisheye lenses do not work with any filters and lenses like the Nikon 14-24 and Sigma 12-24 do not have filter threads. If you can only afford one good filter this is it. Don’t be afraid to experiment since sometimes the magic works and sometimes it doesn’t.
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