The Ultraviolet Filter

An Ultraviolet filter or a Skylight filter provides one major function over and above their filtration characteristics - they protect the front element of your camera lens. Better to have one of these cheap filters get scratched rather than the glass on your expensive lens. Not only do they protect your lens from scratches, they also protect the threads on your lens when you attach other filters. You should should have one of these filters for every lens you own.

An Ultraviolet filter (UV) absorbs the ultraviolet light which often makes outdoor photographs hazy and indistinct. They're multi-purpose, fine-weather filters for color as well as black and white films.

Ultraviolet filter nomenclature is a bit confusing. The term "UV filter" by itself usually refers to a neutral (untinted) filter blocking the shorter wavelength UV-B (320-280 nm) and UV-C (10-280 nm) bands while letting a good bit of the UV-A (320-400 nm) through. [nm = nano-metres]. I you're looking for an explanation of light, try Color and Vision. You could also take a look at How Light Works.

UVC and part of UVB are absorbed high in the atmosphere. UVB is very damaging to the skin and eyes, causes sunburn and is implicated in skin cancer induction. UVA is less damaging than UVB but lately there has been concern over long term hazards of exposure

Using an Ultraviolet filter generally requires no exposure compensation. Since uniform coverage of the entire field of view is usually what you want, round UV filters make perfect sense.

Why Worry About Ultraviolet?

By definition, your eyes can't see Ultraviolet light (bees and some birds can, though), but ordinary film can, and many film sensors can as well, although to a much lesser extent than film. In theory, there's something to be gained by filtering UV out of the picture. Due to the very short wavelengths involved (below 400 nm), the UV band carries more than its fair share of atmospheric scatter. Scattered Ultraviolet light can cloud distant backgrounds and impart an unwelcome bluish cast in film images. Ultraviolet contamination tends to be most problematic at very high altitudes (well over 10,000 feet) and over long stretches of water.

So much for theory. In practice, these theoretical Ultraviolet filter benefits simply don't show up on the digital side. Under nearly all conditions and for nearly all cameras, Ultraviolet filters are a waste of money and a potential source of flare for digital photographers.

Purple Fringing Artifact

This image shows a typical scene that results in purple fringing

The only practical optical role for Ultraviolet filters in digital photography seems to relate not to scatter, but to an artifact known as Chromatic Aberration or "purple fringing"). This is most commonly seen in digital images but also occurs in film images. It typically appears as a multipixel band of bright purple surrounding the peripheral edges of dark objects cast against a bright background. Central edges are spared. Backlit leaves set against a bright sky are a common purple fringing scenario.

In certain exceptionally Ultraviolet-sensitive digital cameras, there may also be a contribution from sensor blooming due to Ultraviolet contamination of pixels already near saturation.

How Sensitive are Digital Cameras to to UltraViolet?

Ultraviolet sensitivity seems to vary from one digital camera to the next, but most digital cameras seem to be substantially less Ultraviolet-sensitive than film. In fact, few digital cameras are Ultraviolet-sensitive enough to reap a noticeable optical benefit from Ultraviolet filtration, even in the most extreme Ultraviolet conditions — at very high altitude (well over 10,000 feet) or in very long shots over water. A good quality multicoated neutral Ultraviolet filter is as good a choice as any for a lens protector if you feel compelled to use one, but don't expect to see much of a benefit in your digital images.

Removing Purple Fringing

If you own a copy of Adobe PhotoShop, removal of purple fringing from images is a fairly painless affair: Remove Purple Fringing