When to use a circular polarizer

[bigone5500]

When does one know when to use a circular polarizer filter?

[OsmosisStudios]

A circular polarizer is used to reduce glare from non-metallic objects or darken blue skies. Depending on which effect you want, throw it on or keep it off.

[kirbinster]

Use it when you want to cut refections off of water or to get that rich blue in the sky. Need to be about ninety dgrees from the light source to get max impact.

[bigone5500]

Cool, thanks!

[dcclark]

In a pinch, a polarizer can also act as a quick-and-dirty neutral density filter too, too. Most absorb between 1 and 2 stops of light, in addition to the reflection-removing properties (which, if you're taking waterfall photos, can be a helpful combo).

[cbrf4ivy]

Great question and answers!
I was going to ask a similar question after searching for a polarizing filter for my camera and ran across someone that said not to use a cirular polarizer for manual focus ... I think that after reading these responses, I have concluded that isn't necessarily the case ...

[kirbinster]

It has no direct impact on manual focus. It does cut down on light by one to two stops, so it could make it harder to determine if you are in focus - but that impacts autofocus also.