For as many who feel as Joseph does about *always* using a UV or Skylight filter, there are at least as many who would tell you to *never* *ever*, under any circumstances, use a Skylight or UV.

Me?
I *sometimes* use a UV for several reasons, but it's *not* to protect the lens, and it's only in broad daylight or with a flash. Any time any filter is placed on the front of a lens it adds yet another glass element and group to the lens. Only the more expensive filters are have anti-reflective optical coatings. The inexpensive ones do not; they degrade contrast and can easily be a source of severe flare. In addition, inexpensive filters will also introduce aberrations because their front and back surfaces are neither absolutely dead flat nor parallel to each other. As you might guess, I'm very finicky about filters and consider the cheap ones little better than shooting through a common window pane.

I absolutely, positively, will not use any filter, for any reason, when shooting existing light photographs at night. The front and back filter surfaces are a source of obvious light reflections that end up in the photograph, ruining it, especially from point sources of light. Even filters with optical AR multi-coatings will do this.

Use a filter if you wish to, but recognize any time you do it's a compromise to your optics, and under some conditions even the finest filters can be a source of severe problems that ruin the photograph.

Lens Cleaning 101:
After the blower or brush, there may still be some crudmium on the lens. If there is, the next step is lens cleaning fluid and a soft, *clean* cloth. I've found the better ones to be a relatively modern invention made from "microfiber." Don't use common glass cleaner, such as Windex! It's too harsh chemically for some lenses. Use a lens cleaning fluid. Put it on the cloth, *not* the lens. If you put it directly on the lens it can migrate to the other side of the front lens element/group and leave a spot there after it dries that cannot be removed unless the lens is disassembled. The object of the lens cleaning fluid is to loosen anything stuck to the lens surface that the blower or brush could not remove. Wipe gently, but don't rub hard. Scrubbing will grind things into the lens surface. Start at the center of the lens and work toward the edges; some recommend slow circular motions. Use more fluid on the cloth if you need to and be patient if there's stubborn crud on the lens. I've yet to encounter anything that would not eventually come off. Last is continuing the wiping using dry portions of the cloth until the last of the fluid has evaporated and the lens is dry. If you find crudmium on the the cloth used for lens cleaning, wash it in the laundry (but don't use fabric softener on it; that leaves chemicals behind in the fabric).

A last remark about fingerprints:
Clean them off immediately! The oils and acids in fingerprints can etch lens coatings if left on a lens for a long time.