Films and CCDs behind camera lens are sensitive to UV rays. UV rays are invisible to our eyes but when they are recorded on the CCD, the image might look bluish and color casts might thus be distorted.

Excessive bluishness frequently occur in outdoor photography especially in open shade under a clear, blue sky and near water front while haze is caused mostly by dust particles in the air. When daylight shines through the sky, the particles reflect the short wavelength of the light more than the long one. This is UV filter effectively helps elminating the lack of sharpness, absorbing UV that cause blue cast, as well as reducing distant haze that caused by UV radiation. It also protects your camera lens from dust and scratch.