Keeping a filter on every lens protects your expensive optics. We highly recommend the brass-threaded B+W pro-line of UV MRC filters. While blocking invisible Ultra Violet light (which can cause blur or blue-casting), these broadband anti-reflection coated filters prevent internal ghosting and allow 99.8% light transmission with no vignetting. More than once, these filters have saved several thousand dollars in lenses by taking the scratches and cracks from rocks and branches.