Okay...had some folks pm to see some example shots of what a polariser does to enhance certain shooting scenes...so here goes. Just shot this outside my office. Sunny day but unfortunately no blue sky from where I shot the scene so no blue sky shots.
But from this simple examples you will see how useful a polariser really is in cutting reflective glares..etc and let you photograph scene and objects with more "pure" colour coming through to your lens.
Film like Velvia might be able to enhance the colour for your shots but it does not cut glare and reflection which is where the polariser comes into play in tandem to give your shots more details and depth.
Cloudy though abit sunny when light fall on the trees. With the PL tuned, the trees has less reflective shine to the leaves and allowing more green to come through and the flats in the distance has a warmer and better tone to the building's painting. As you can see white over cast sky does not benefit from the PL.
With this carpark scene I shot at about my lens at 250mm, the difference in with and without PL. The roof and tress has a more natural look to it. Btw, I set my D70 to Adobe RGB and auto WB of -2. The scene looks more alive with the greenery when a PL is use to bring out the colours of the plants and also cut reflective glare from the roof.
Here just concentrating on the cars. Properly adjusted, your PL will be able to cut through the reflection on the car's window screen and let you see through to the car's interior.It makes for a less messy distractive shot without the glares. And sometime the colour of the car can also benefit from it if the lighting condition is right.
You may use a better film to capture the best colours or use your PS to enhance the colours of your shots but you can not remove all the reflection off the leaves,as in this case, to give your leaves on the tree a more natural and lively look. With a Polariser, you get better colour and also cut down the glares to get more details into your shots.
Also note that if you are not going to adjust the filter when it is on your lens, best to take it out then leave it there. Not so much because you then have more light going into your lens but also because some PL when not tuned properly will add a slight colour tint to your pictures. That is why some people find this or that PL has a yellow cast or green..etc. If you want to put a PL on your lens, you have to make use of it.
So for those who asked me for some examples...here you go
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