A lens filter is a photographic tool that is placed over (either screwed or snapped onto) the camera’s lens to alter the wavelengths of the light that enter the camera. These flat round disks can come in a variety of colors, ranging from yellow to red to green to blue. Depending on the lens filter a photographer chooses, he can mildly or dramatically alter the resulting photos.
While lens filters are more often tools of professional photographers, they add an interesting flair to amateur shots. Because they are relatively inexpensive camera accessories, lens filters are easy tools for amateurs to start experimenting with.
However, photographers should keep in mind that filters tend to decrease the amount of light that enters the camera’s lens, thereby affecting the final exposure. Similarly, scratched or unclean lens filters may seriously blur or ruin photographs. Consequently, photographers should always examine their filters before placing them on their camera’s lens to take pictures.
Lens filters are categorized by a numerical system known as “Wratten numbers,” named for the British inventor Frederick Wratten. The Wratten number system works by assigning lens filters a number and possibly a letter by which the photographer can gauge what light tint and the degree of warmth or coolness in the resulting photos.