Pretty much anyone looking at this knows that a UV Filter blocks ultra violet rays from your photos. This article will attempt to explain what ultra violet rays are and why you would want to block them. We will also help you determine what size filter you will need for your lens.
Visible light is measured in nanometers (nm) and ranges from red to violet with red having the longest wavelength and violet the shortest. Light with a wavelength longer then red is called infrared and light with a shorter wavelength then violet is called ultra violet.
So why do we want to block ultra violet rays? Color film has three layers of sensitivity, one to red light, one to green light, and one to blue light. If there are a lot of ultra violet rays around where you are shooting the blue layer of sensitivity in your film will get extra exposure and your photograph can take on a blue color as in the photograph below.
Without UV Filter
With UV Filter
One of the biggest factors affecting the amount of UV rays around is altitude. At sea level there are not a large amount of UV rays because they are scattered by the atmosphere. The higher up you go, the thinner the atmosphere, the more UV rays are present. That is why this photo of Cook Summit has quite a bit of blue tint without the UV Filter attached.
Digital sensors are not as sensitive as film to ultra violet light, but they can be affected especially in the winter when the snow is reflecting UV rays. Digital sensors are sensitive to infrared light, but most digital cameras have infrared blockers built into them.
The other main use for UV Filters is as a protective shield for your expensive lens. If the glass on your lens is scratched or damaged you cannot replace just the one scratched piece. The entire lens is ruined and needs to be replaced. Today's high quality lenses can cost hundreds, even thousands of dollars versus less then $15 for a UV Filter at Plumtree Photo.
What is the difference between a UV and a Skylight Filter? Both will block ultra violet light but a UV Filter is clear glass while a Skylight Filter is pinkish and may produce a warming effect in your photographs.