UV Filter
A clear filter that blocks ultraviolet light, now used mainly as physical protection for the front element.
In depth
On film, ultraviolet haze genuinely fogged distant landscapes and the filter earned its place. Digital sensors have their own UV-blocking layers, so the optical benefit is negligible and the filter is really an insurance policy. A poor one adds two reflective surfaces at the most flare-critical point in the optical path, so if one is used it should be a good multi-coated example.
Examples in practice
Photographers working in surf, sand or dust keep a UV filter fitted and accept the small flare penalty for the protection.