For FE 100-400mm f/4.5-5.6 GM OSS Lens, Side Window for Filter Adjustment, Blocks Stray Light from Entering Lens, Protects Lens from Impact, Spare or Replacement Part. The ALC-SH151 Lens Hood from Sony is specifically made for the FE 100-400mm f/4.5-5.6 GM OSS lens. The hood is made to block stray and extraneous light from entering the lens and causing lens flare. A window is located on the side of the lens hood for filter adjustment such as a circular polarizer or an ND filter. Also the lens hood sustains accidental or sudden impact instead of the front element of the lens which would normally cause damage. Additionally it can be used as a spare or as a replacement to the original lens hood.