LensRentals has published some particularly useful articles recently on cleaning lenses and choosing telephoto focal lengths.
The first article outlines the procedures that LensRentals uses to clean and check its lenses, but it’s worth bearing in mind that (as they’re rental equipment) those lenses would be seeing far heavier use than any lens owned by an individual photographer. I visually inspect my lenses (particularly the front element) immediately before each shooting session, but this practice is influenced by my habitual shooting style—I usually don’t change lenses in the field, and if I know that I’ll need more than one lens, I’ll normally bring a second camera body.
The second article covers choice of focal length for event photography (in this case, the context is a theatre, but the information is equally valid for other situations needing telephoto lenses). Unless you’ve actually used a lens in person, it can be quite difficult to visualise the photographic results you can expect in terms of framing and the size of the subject in the frame. The sample pictures in the article speak for themselves; take a look, particularly if you haven’t used telephoto lenses before, and are planning on shooting with one.
For the types of low-light action photography that I and my colleagues do, we’ll typically be anywhere from 3 m to 15 m from the subject (or roughly 10′ to 50′ in Imperial measurements). I’ll usually shoot with an 85 mm prime lens, which gets me both light-gathering capability (f/1.4 maximum aperture, though I usually shoot at f/2 or smaller these days) and good subject isolation (i.e., the background is blurred and does not distract attention from the subject in the final image) at the cost of having to do more physical work to position myself at a good shooting distance (as the lens does not zoom in or out). A 70–200 mm zoom lens gives more flexibility in framing, but at the cost of a smaller f/2.8 maximum aperture. Even in modestly low light, the difference in exposure duration between f/2 and f/2.8 is noticeable, particularly if you’re trying to ‘freeze’ action. In a poorly-lit venue, the prime lens is the clear winner, but if the lighting is not too bad, the zoom lens could well be the better tool for the job.
Note that I’m shooting with a ‘full-frame’ (full-sized 35 mm sensor) DSLR body, so an 85 mm prime lens will translate to a 50 mm prime lens (approximately) on a ‘cropped-frame’ (1.5x or 1.6x smaller than a 35 mm sensor) DSLR body. Both 50 mm and 85 mm f/1.8 lenses are supposed to be some of the easiest lenses for manufacturers to make, so if you’re looking at one of these options, you’ll probably be able to get good optical quality at a reasonable price with any DSLR system.