Filters are used to improve image quality by altering the properties of light that enters the camera lens. They can be used individually or as combinations to affect contrast, sharpness, highlights, reflections, colors, color balance, and light intensity.
Coatings applied to filters (and lenses) enhance light transmission and reduce reflections that can lead to undesirable aberrations in an image. In general, light transmission improves as more layers of coating are applied. A multicoated filter is more effective than a single coated filter, which is far more effective than a filter with no coating at all. Filters are especially beneficial for older cameras with lenses that weren’t coated.
A filter such as a skylight or UV filter was commonly used primarily to protect the lens barrel, the front lens element, and its coating from dust, scratches, and other damage that could lead to costly repairs. A filter is much easier and cheaper to replace than a front-lens element, and vintage cameras found with a filter attached generally have well-preserved lenses. However, a cheap filter may have a negative effect on the optical performance of a finely tuned coated lens, in which case a suitable lens hood is a better option to protect the front of the lens from accidental impact.
Filters generally screw into the front of the lens barrel and come in various sizes to match the thread of your lens. The size is indicated by the symbol ø and specified in millimeters.
Exposure Compensation
Although all filters alter qualities of light, some have no effect on the amount of light that reaches the film. Others reduce the amount of light that reaches the film, in which case you must consider exposure compensation.
If using a light meter away from the lens, or calculating light readings manually, exposure should be increased according to the number of stops by which the filter reduces light, increasing either the aperture size or the exposure time. For example, if a filter reduces light by -2 stops, exposure should be increased +2 stops, back up to 0. If you are using a light meter that sits behind the filter, there is no need for compensation.
