A CPL filter cuts reflections from water or glass which gives a subtle look and objects do not appear very shiny.
It also enhances color saturation (gives the sky a royal blue hue) and reduces haze (increases clarity).
Let you see under the surface of lakes and streams.
A CPL filter works best when you’re pointed 90 degrees from the sun. It is doing nothing shooting directly into the sun, or directly away from the sun.
Use a wide angle CPL filter for wide angle lenses otherwise you might see uneven colour tone in the sky because the filter will create a stronger or weaker polarization effect in different parts of the frame.
You can adjust how strong you want the polarization to be by turning the filter.
Polarizing filters are very useful when taking photographs indoors. You may have experienced that when you take a picture of a person wearing glasses their eyes don’t show since the reflection from their glasses obscures them, a polarizing filter will cut out the reflection and the persons’ eyes won’t be obscured any more.