A polarizing filter is a great little accessory to have when shooting images in bright sunlight. They are perfect for increasing the saturation of the colors; reducing glares and allowing for diffused light to be controlled making the shots sharper and well balanced. A lot of times specially when using wide angle lenses and shooting landscapes, photographers find the composition lacking in the right color saturation. While one can always do a bit of editing during post production using Photoshop, using a polarizing filter one can eliminate much of it during shooting.
So how does a polarizing filter work?
A polarizing filter will allow polarized light of a particular orientation to pass through. This has the effect of cutting down unwanted polarized lights which can cause glares, reflections and haze that we normally see in a picture shot. Glares are one of the primary reasons why your composition can look soft and hazy. Even in bright sunny days the dirt and dust in the air can diffuse the light affecting the color saturation of your compositions. Polarizing filter can correct that, increasing the color saturation, eliminating unwanted polarized light and sharpening the over all picture.
Types of polarizers
There are two types of polarizing filters, circular and linear. Modern Auto-Focus cameras have a built in split metering system that can often be offset by the use of a linear polarizer. This is because when a surface that reflects linearly polarized light hits the mirror of the DSLR (with AF) with split metering system the intensity of light waves often gets affected by the polarization angle of the reflecting mirror inside the camera. With a linear polarizer this problem is aggravated when the shot is taken; i.e.; when the mirror is out of the path and the sensor is recording the image. As such most manufactures of DSLRs recommend circular polarizers for their cameras which does not affect the metering systems of the AF enabled DSLRs. When purchasing a filter look for the words CIR, CIRCULAR or CPL to correctly identify that this is a circular polarizer.
They are known as linear polarizers because when they are setup at a particular angle they would accept or deny only either vertically or horizontally polarized light. Light when bouncing of a surface can be either vertically, horizontally or circularly polarized. A linear polarizer is designed to pass only a light beam of a linear nature. This can however be adjusted as mentioned above to allow either horizontal or linear polarized light.
Circular polarizer is designed to allow or block circularly polarized light beams. When we speak of reflected light from a surface such as mirror, windscreen of a car or even a shop window the reflected light is linearly polarized. Thus why would we be discussing a circular polarizer when the reflected light is linearly polarized? This is because a circular polarizer will also stop linearly polarized light when dialed at a particular angle. They have apart from the same elements that make up a linear polarizer an additional layer that actually transforms the oncoming light waves to circularly polarized. This is also why they are known as circular polarizers.