Circular Polarizer (CLP) can boost sky colors and contrast but could be very tricky to use. Common issues include vignetting on your photo corners and uneven light bands in the sky. The following five circumstances are what typically caused the problems.
1. Shooting in the wrong angle
The polarizing effect is directly related to the angle from the sun. They work best at 90° to the sun. If you take a picture with the sun behind you or in front of you, you won’t get polarization. If you don’t manage the angles right, sometimes the sky in your image will show darker blue either to the left or the right, which apparently doesn’t look good. See following bad example (photographed in Snæfellsnes National Park in Iceland). ISO320, 16mm, f/9 ,1/640sec
2. Shooting with a wide angle lens on a full frame, at the widest angle of your zoom
Wide angle lenses can cover an extraordinarily wide field of view. Therefore, if a large part of your image is blue sky, there’s a good chance you’ll end up with vignetting on the corner or bands of lighter and darker shades across the sky. Sometimes you won’t notice this on your camera’s LCD screen and by the time you see it on your computer screen, it might be too late to fix – particularly for the light bands.
You can easily solve this problem by zoom in your lens, perhaps just a few mm. See following bad example. You can see the dark vignetting in the top and bottom right corner of my image. I was using a full frame camera (Canon 5D Mark III) plus a wide angle lens 16-35mm at its widest angle. Camera setting was ISO100, 16mm, f/22, 1/30. Of course the issue in this image was relatively minor and I could simply cropped the vignetting corner out.