A polarizing filter is one that converts an undefined or mixed-polarization light into a beam of well-defined polarization. In practical terms it can eliminate reflections off a surface (like water) and take away haziness. It also enhances clouds and darkens the blue in skies. The effect is more pronounced when shooting perpendicular to the direction of the light.
A polarizing filter is a useful one to have on your wide angle lens because it will in many situations improve your landscape images. A disadvantage is that it reduces light. This means that your shutter speed will be slower when you have a polarizing filter attached to your lens. So be careful of camera shake.
The most practical application of a polarizing filter is when photographing water. Be careful when using a polarizing filter on an ultra-wide angle lens and photographing skies, as the intensity of blue in the sky will become uneven and unnatural due to the wide angle of view.
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