A CPL filter, or circular polarizer filter, is a glass attachment that can reduce or eliminate glare and reflections from surfaces in your photographs. It consists of two slim metallic frames that are very close to each other, with each metallic frame holding one piece of glass similar to an ND glass. Polarized light comes from light that has been reflected off of something, and the CPL filter filters out this polarized light, leaving you with an image with enhanced colors and a boost in the overall contrast of the image. The most common use of a CPL filter is in landscape and travel photography, as it can enhance the image drastically on the fly. It is also useful when photographing through glass. The CPL filter effect still remains something that even the most advanced software cannot reproduce accurately.