The ZEISS Lens Shade for 25mm f/2.8 / 28mm f/2.0 is a lens hood for lenses that fit these dimensions. Examples are the Distagon T 25 mm f/ 2.8 and 28 mm f/2.0 lenses, and the Biogon T 25 mm f/2.8 ZM lens. Lens shades are primarily designed to prevent unwanted stray light from entering the lens, by extending and shading the end of it. In addition, since the end of the lens is extended, you also get the added benefit of some extra protection from accidental impact.