UV filter / Protective filter
UV-filter / Beschermfilter · Lens & optics
A clear filter originally designed to block UV light; today mostly used to protect the front lens element.
Digital sensors already have a UV coating, so optically a UV filter has little effect. It is used as sacrificial glass against scratches, sand, salt water and fingerprints. Cheap filters cause flares and reduce sharpness; invest in multi-coated glass (B+W, Hoya HD, NiSi) if you use one. Many photographers skip filters entirely and rely on a lens hood and cloth instead.
Tools that help with this: