Most of us know the main benefits of a circular polarising (or polarizing) filter or CP as we refer to. It suppresses surface reflections and intensifies colour saturation. The effect is directly related to the angle of the lens from the light source. Best results on polarising a blue sky can be achieved when the light source (the sun) is at an angle of 90˚ with the lens; basically, right above your head.
You can also use two CP filters screwed together. By rotating the filters on opposite directions, you can have an adjustable neutral density filter of up to ten stops.
However, using a CP filter in conjunction with ultra wide lenses for landscape photography may cause some problems. Ultra wide lenses are considered the ones wider than 24mm on a full frame body. Here are a few things that will help you avoid potential failure.
Parts of a blue sky will turn out darker than others. This is due to extreme wide field of view with these lenses. Unless you love the effect, it is best to avoid using a CP.
Be aware that CP filters darken the exposure by almost 2 stops. Make sure you account for the loss of light when adjusting exposure.
Make sure you remove the CP filter when you are shooting multiple frames for a panoramic picture as the sky will look unnatural.
It is advisable to remove the filter when shooting a scene with the sun in front of the lens. Wide lenses do not handle lens flare very well, the problem is intensified with a CP filter on.
Read this article, and many more, in High Definition, inside Issue 26 of Landscape Photography Magazine.