A lens filter is attached to the end of a camera lens to add special effects that improve a camera's ability to capture certain types of images. Like camera lenses, filters are tied to certain types of pictures and will not be useful for every shot. The most common types of filters are UV and haze filters, neutral density filters, graduated neutral density filters, temperature filters and polarizing filters.
UV and Haze Filters
These filters limit the amount of UV light that gets to the film or image sensor. These are more important for film cameras because UV light does not damage digital sensors like it does film. These filters also provide protection for the lens.
Neutral Density Filters
Neutral density filters come in varying strengths and limit the amount of light that gets through the lens to the film or image sensor, allowing for longer shutter speeds. This is particularly useful when taking pictures of rushing water to allow the water to blur softly.
Graduated Neutral Density Filters
Like neutral density filters, graduated neutral density filters also limit light. However, the limits change as you move across the lens. For example, the top may severely limit light, but as you move down the lens, the lens may allow more and more light to pass through. These filters are useful for landscapes with a bright sky and darker ground.
Temperature Filters
"Temperature" refers to color temperature. These filters can be either warming or cooling, and are used for altering the white balance of a scene. These are not as necessary with digital cameras because they can automatically adjust the white balance.
Polarizing Filters
Polarizing filters saturate colors and decrease reflections of non-metallic surfaces. For example, if you were shooting down at a pool of water, it would be transparent with a polarizing filter attached rather than showing a reflection.
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