A lens filter is a piece of optical glass that is placed in front of the camera lens. It may modify the image subtly or dramatically, or it may provide protection for the lens and/or the image. A lens filter might be a piece of glass that fits into a holder in front of the lens, or it might be specifically designed for attachment to the exact size lens on a camera, in which case, it would have a specific size such 77mm, which would screw on to the end of a lens with a diagonal measurement of 77mm.
Protective Lens Filters
The most common type of protective filter is the UV (ultraviolet) lens filter. This filter protects your lens from ultraviolet rays by absorbing them. It is generally more effective for that purpose in outdoor photography. However, a uv filter is universally recommended as a constant attachment to all lenses just for protecting a lens from physical damage such as scratches. Whenever you purchase a lens for your digital SLR camera, it should also include the purchase of the correct size UV filter.
Neutral Density Lens Filters
Neutral density filters come in several types. There are solid color neutral density filters that have a gray coating. The purpose of the gray lens is to reduce the amount of light in the image. Because they are “neutral gray,” they do not change the color values, or color balance, of the image. There are two common reasons for ND filters. The first reason is to be able to use slower shutter speeds. On a bright day, this helps to take pictures of moving water or clouds. By slowing the shutter speed, the photo has a silky effect where the water is moving.
The second reason for using a ND filter is to reduce the depth of field by allowing the use of a wider aperture. This essentially gives the photographer more control to be able to separate the main subject from the blurred background.Another kind of filter is the Graduated Neutral Density filter.
The glass of a graduated neutral density filter has a dark half and a clear half. These devices are used when there is a big difference between the dark area and light area of a picture, like a snow-capped mountain where it is difficult to get a good picture because the difference in light intensity is too hard for the camera to handle. The idea is to position the filter in such a way as to have the darker area cover the light area of the photograph, thus fooling the camera’s image sensor into balancing the two areas of light.
Polarizing Lens Filters
Polarizing filters come in two varieties – circular and linear. The circular filters are for auto focus cameras and the linear filters are for lenses without auto focus. Use of a polarizing lens filter does about the same thing a pair of polarizing sunglasses does for ones eyes. It removes reflections of unwanted bright areas while is deepens the blues of the sky or water, making your photo have a much more appealing color saturation.
Specialty Lens Filters
More commonly used in years past, specialty lens filters include warming and cooling filters. They also include other color-casting filters which can produce sepia, soft focus, infrared, and a number of other possibilities. Photographers are not using these specialty filters as much any more due to the inclusion of many more effects in digital camera software that can duplicate the outcome of lens filters.