All about UV Filters

On a digital SLR camera, the primary purpose of the UV Filter is lens protection for expensive lenses (such as the Carl Zeiss Lens ). I regard the B+W MRC UV filter as the best UV filter available but also the most expensive ones, so I choose a little lesser cost but effective UV filter... HOYA Super HMC UV Filter

There is an ongoing debate over the use of UV filters for lens protection. Neither side is wrong - they simply have differing opinions - and the freedom of choice.

The non-UV filter users say that their lens hoods provide all of the protection they need. They say UV filters degrade their pictures. Their contingency is that they can have a damaged lens repaired under their warranty coverage.

I am a member of the UV filter users. A high quality UV filter such as the HOYA Pro 1 Digital UV Filter will not degrade the final picture in most instances. A very bright light in the background of your picture *may* cause the filter to generate a small amount of flare, but I rarely detect any other filter-caused image defects. If I scratch the filter, I can simply remove it and keep shooting. I feel much more comfortable cleaning dirt off of an $28 filter than I do from a very expensive lens. I feel a little less protective of my lenses when they are protected with UV filters as well. Note: A UV filter is not going to save the lens from a significant impact. You will still want warranty coverage if the lens is valuable.

Even non-regular filter users will want a filter when shooting in very dusty or harsh conditions (think salt water spray). Some lenses require UV filters to complete their weather sealing.

UV filters are supposed to block UV light. So, for the newcomers to photography let's first look at what UV light is and why you would want to block it.

The "traditional" visible spectrum runs from red to violet. Red light has the longest wavelength and violet the shortest. Light which has a longer wavelength than red is called infrared, and light which has a shorter wavelength than violet is called ultra violet or UV. The wavelength of light is measure in units of nanometers (abbreviated as nm), and 1nm is a billionth of a meter (that's a US billion or 1000 million, not a UK billion which is a million million!). Light shorter in wavelength than about 400nm is called ultra violet, light longer in wavelength than 700nm is called infrared.

So now we know what UV light is, why would be want to block it? Well the answer lies in the way that color film works. There are basically three color sensitive layers, one sensitive to red light, one to green light and one to blue light. The blue layer not only responds to blue light, but also to UV light, so if there is a lot of UV around the blue sensitive layer gets extra exposure and the final image takes on a blue color. Since film isn't normally sensitive to infrared, you don't need an infrared blocking filter. Interestingly though, digital sensors are infrared sensitive and most digital cameras have an infrared blocking filter built in.
Now there isn't usually a huge amount of UV around at sea level. There is some (that's what gives you a suntan or a sunburn) but most of it is scattered by the atmosphere. However as you gain altitude, for example by going up a mountain, the amount of UV increases. Under these conditions a UV filter can prevent a blue cast in photographs.
Since UV filters look clear and neutral to the naked eye, some people also use them as a protective filter which they leave on their lens at all times. Some people think this is a good idea, other question the wisdom placing a $20 filter in front of a $1000 lens and potentially affecting image quality. Both schools of thought have some valid points. It's your choice.

B+W and Heliopan make excellent filters. Although many brands of UV filters exist, the quality and performance of B+W MRC and Heliopan SH-PMC filters place them among the world's finest. Black anodized brass rings reduce binding or cross threading and to ensure optimal alignment. The MRC and SH-PMC multicoating prevents light from being reflected off the surface of the filter - you want the light to go into the lens. High end Hoya filters are also good, but can be a pain to clean (they tend to smear).