In order to talk about a circular polarizer, you have to understand its purpose, what it works with.
Lets start with light. Light has many sources, could be the sun, a light bulb, or maybe even a reflection. Light travels out or emanates from its source. It radiates or travels out in a wave; it flows out randomly and in all directions. Light can be reflected, like off of a wall, glass, metal or even off of water. Imagine light traveling horizontally and vertically also in circular patterns clockwise and counterclockwise. By this I mean the light is almost on several different planes.
In photography, trying to capture a picture or image light is always present, in some form. You may or may not be happy with how the light is falling on your vantage point. It may be necessary to filter out the light to intensify work, it maybe necessary for clarity of an object. Your goal may be to enhance the color. It may be a goal to project more depth in an image. To do this, you would have to be able to control the pattern of light. Well thats exactly what a circular polarizer does. The word filter comes to mind. Allowing certain things in or keeping certain things out. In our case, it is light. With the circular polarizer, I am able to capture the light traveling in a certain pattern or it may even be thought of as having the abillity to block scattered light or atleast to be able to adjust it like a beam to make my enhancements. With the circular polarizer, I am in control of the tool in my arsenal to get the results that I need, feeling confident that I have achieved my goal in creating my perfect shot.
There are many available filters out there to help improve your landscape photography, but there is one filter that every photographer should have in their camera bag, and that is a polarizing filter. A polarizing filter can be used in both color and black and white photography and it will help you produce beautiful outdoor photos.
The main effect of a polarizing filter is that it eliminates the reflection from non-metal surfaces. It will enhance the colors in your photographs, making them rich and deep in color. No other filter is able to do that. When taking pictures of trees, leaves, foliage, etc., the polarizing filter will help them to appear less shiny.
A polarizing filter is most effective with skies, color enhancement, water and reflective surfaces, and light absorption. By eliminating the reflection of light, the polarizer produces saturated and slightly darker skies. The filter tends to enhance colors by making shadow areas darker. By eliminating reflections, the filter tends to make water and various other reflective surfaces more transparent. A side effect of the polarizing filter is that it absorbs 1.5 stop of light, so in low lighted situations, some type of camera support will be needed to develop the most effective photos.
Polarizing filters typically come in two types- screw-in filters or fitting one into a square filter. Screw-in filters are the right way to go if you typically use one lens, or if all your lenses have the same filter thread. When buying a polarizer that fits into one of the square filter systems, the Cokin A or P ranges are ideal for the small diameter lenses. The Lee filter system is very adaptable so it will fit almost any lens, including large diameter angle lenses and medium format cameras.