Back in the days of shooting film, color was physically split into red, blue and green layers. Oddly enough, the blue layer was found to be particularly sensitive to ultraviolet rays. In certain situations where there was excessive UV light — for example, a bright sunny day on the beach, cameras would produce pictures with a blue-tinted haze and greatly reduced constrast. Thus the the invention of the UV filter. UV filters are specifically designed to absorb ultraviolet rays, thus eliminating the undesirable blue haze from our pictures.
Todays digital cameras have coatings and filters built-in to stop everything from Infrared to UV light from interfering with your pictures. With todays cameras not really needing that protection, a UV filter doesn't make a whole lot of sense — at least, not for the purpose it was originally conceived.
So why use UV filters? In one word, PROTECTION.
Let’s face it, lenses are expensive. UV Filter protect the front lens element against scratches, dust, flying sand, sea water spray and the like, plus it can be kept on the lens at all times. Many photographers use UV filters as a added layer of protection for their lenses' glass and special lens coatings. Remember, it’s much cheaper to replace a scratched or broken filter than replace a $1000 lens.
A few words on choosing a UV filter: This is not the time to cheap-out. Always go for the high-end glass filters! It makes no sense to spend tens of thousands of dollars on cameras and lenses, only to save a few bucks by buying sub-par plastic filters. The filter is the first piece of glass through which light passes, and its quality must match that of your camera and lens. Cheap filters will introduce lens flare and have an negative impact on contrast and sharpness.
Here are the main points to look out for when choosing a quality UV filter.
Ensure that it’s crafted from high quality, optical glass. Also ensure that the filter is multi-coated for reducing flare, ghosting, and reflections on the filter surface.
I highly recommend B+W filters, these are considered to one of the finest glass filters on the market today. I use B+W on all my lens.