Your lens, of course, is exposed to the same conditions as your camera and the same precautions should be taken. Inspect your lens (off the camera) for fogging, rattle or gritty sounds. Use a commercially designed cloth to clean your lens: They also clean better. Tee shirts or the old handkerchief should be used only as a last resort. Too frequent cleanings are unnecessary and can be harmful to the coating on your lens. Buy the best lens you can afford; the really expensive, top of the line lenses are designed to provide maximum sharpness at all stops and they come with a tripod collar and lens hood. Use them! Experiment at different f-stops; start a couple of stops down from fully wide open to see which give you the best results.