Douglas Herr , Apr 19, 2002; 10:46 a.m.
I've heard many arguments on this matter. I won't argue but I'll state my opinion. Whether your lens needs protection or not depends in part on the particular enviornment, the age of the lens, and the lens' manufacturer. In a sandstorm or in salt spray some protection may be a good idea, but if you leave a UV or Skylight filter on at all times you may notice increased flare or loss of contrast due to the added reflections caused by 2 additional air/glass surfaces.
At least one high-end lens maker has been using extremely tough coatings for many years, and has also been using UV-absorbing cement between the glass elements. For these lenses only the harshest enviornmental conditions are a problem. I haven't used any protective or UV filters in over 20 years (in arctic, sub-tropical, alpine and desert conditions) and the glass of my lenses is still in perfect condition.
If you do feel the need to use a UV or Skylight filter, use a good multi-coated one, not the cheap junk often pushed as part of a camera outfit. Funny, isn't it? An expensive lens that you'd be afraid of damaging doesn't need protection and filters the UV itself, while a cheap lens that you're less likely to worry about is more likely to need an expensive filter to protect it from damage.