The Nikon HB-31 lens hood is compatible with the Nikon 17-55mm f/2.8G lens. It is used to prevent stray light from entering the front lens surface causing flare. By reducing the amount of non-essential light falling onto the front lens element, lenses can better achieve their optimum optical performance, producing more saturated images. Use of specifically engineered Nikon lens hoods is highly recommended. In less-than-ideal lighting conditions, the performance difference can be substantial.