Penflex UV Filter

Penflex UV filters are uncoated glass filters, with a metal ring which screws into the front of the lens. The ring also has a female thread on the front of the lens to accept threaded lens hoods or additional filters. 

UV filters are designed to prevent ultraviolet (UV) light from getting through. UV filters are used in photography to reduce the level of ultraviolet light that strikes the recording medium.

Historically, photographic films were mostly sensitive to UV light, which caused haziness or fogginess, and in color films a bluish hue. Therefore, as a standard, a UV (blocking) filter was used, transparent to visible light while filtering out shorter ultraviolet wavelengths. 

Many photographers and cinematographers still use UV filters as a protection for their lenses' glass and coating, due to their low cost and lack of effect on the exposure of the shot.