In the comments to Scott Hanselman’s blog post about our podcast, someone brought up the issue of UV filters, and suggested that one should put a filter on a lens and never take it off. This is an area of debate amongst photographers. On one hand, it sounds like a great idea. Put a piece of glass in front of your expensive lens, and let that glass get any scratches, fingerprints, dust, or other debris which may come into contact with the lens.
There’s a downside, and it’s one that puts me into the “don’t leave a filter on your lens all the time” category. Digital camera sensors are susceptible to lens flare, which is caused by light reflecting back and forth through the various layers of glass in a lens. The result is that bright lights in a photo will often show up mirrored on the “opposite” side of the photo. The problem becomes evident when shooting bright lights, especially lights directed toward the camera. The other downside to having a filter on is that it’s just one more layer of glass for the light to pass though, which will lessen the quality of the image. There are a variety of filters out there, and there are some very good ones that will make image degradation almost unnoticeable, but it’s still there.
Lenses are made from hard glass. About the only time I’d say a filter is good protection is if you’re in a situation such as blowing sand. You’re not going to permanently damage the lens from a light touch. A better protection against hard impacts is the use of a lens hood, which is a hard plastic attachment that goes on the end of the lens and extends outward. Optically, the purpose of a lens hood is to help prevent lens flare from sunlight striking the lens from outward angles, but most lens hoods (for anything but wide angle lenses) will extend at least a couple inches forward, absorbing any impact if you accidentally bump the lens into a wall or god forbid drop it and it lands lens-down.
When I’ve spent $1200 on a high-quality “L” series lens, I’m not going to compromise the engineering by adding another layer of glass.