Light moves through the air in waves. These waves actually have an orientation in space. They can move horizontally, or vertically or any angle in between. This angle is known as polarization.
For the most part polarization does not matter to a photographer, any polarization will expose the recording medium the same way, but when a polarizing filter is employed it allows one to select which polarization of light is allowed to enter the camera. The practical upshot of this is that a polarizer can:
- Enhance saturation.
- Remove reflections and glare.
- "See through" humid air.
Linear vs CircularEdit
Circular and linear polarizers for imaging applications are virtually the same in how they function in polarization effectiveness and image quality. After all, the circular polarizer is at its heart a linear polarizer to which has been added a clear retarder layer that makes it into a circular polarizer. The retarder does not degrade the image in any noticeable way.
General imaging rule of thumb...a linear polarizer has many applications but may not work best in all...a circular polarizer can be used in any application. When in doubt, go circular.
If your viewfinder has a partially silvered mirror or prism in it, it may be susceptible to being hard to see through with a linear pol. Check your manual for this equipment, as it may be an issue.
The key issue you face is in panning with a polarizer in general, realizing that it doesn't matter whether it's circular or linear. The degree of polarization effect changes as you change your angular direction relative to the sun in the sky. So if you are panning to follow a plane as the angle changes the sky will go from lighter to darker and back again. This may (or may not) be visually objectionable. The way to minimize this is to try to set up your shots, as much as possible, so that the camera's viewing angle stays perpendicular to the line-of-sight of the sun in the sky. This will maximize the sky-darkening effect. If you find that your pan causes too much change in the sky, then turn the polarizer so that it isn't at maximum effectiveness so that the image will be improved but not be so dramatically different from where it isn't affected at all.
Note that either type of polarizer has no effect when aimed directly toward or away from the sun. And that you need a blue sky to start with...and that you want to be sure that the change in sky darkness is not causing your camera to change exposure as you pan...you will want to use the recommended filter factor for exposure compensation with a fixed iris if that is otherwise acceptable for your situation. Typical pols use a 1-2/3 stop increase.