The importance and usefulness of a skylight filter cannot be overestimated. Everyone should have at least one. Preferably, they should have one for every lens they own. These should be permanently fixed to the filter thread.

Skylight filters are cheap, freely available and come in various filter thread sizes. This means that everyone can afford one and there will be a filter to fit every lens (or almost).

The primary function of a skylight filter is to cut down excessive UV rays which, in turn, make scenes in the distance appear to have a blue haze. The filter effectively reduces the haze and blue colour cast. Pictures of hills and mountains in the distance look clearer.

The secondary function, and why you should have one on each lens, is that of protection. Like a lens cap, the skylight filter fits over the front of the lens and helps to prevent the ingress of dirt and dust. It also protects the lens from the effects of oily and greasy fingers, stick hands and from accidental knocks. It is much cheaper to replace a skylight than it is to replace a damaged lens.

The filter is very light and has no effect on the exposure value needed for your photography (they do not stop the normal amounts of light getting in).

Eric Hartwell runs the photography resource site http://www.theshutter.co.uk and the associated discussion forums as well as the regular weblog at http://thephotographysite.blogspot.com

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