This Heliopan 39mm Screw-In Metal Lens Hood is designed to prevent unwanted stray light from entering the lens by extending and shading the end of the lens. In addition, since the end of the lens is extended you also get the added benefit of extra protection from accidental impact.
This short metal hood is for medium wide-angle to standard lenses. It is made of black-anodized aluminum, in cylindrical form. Since the hood is threaded all the way from the front to the back, you can build up even longer hoods by combining several together. Other threaded accessories can also be added to the front of the hood. The threaded interior as well as the matte black finish helps to minimize flare and directs stray light away from the lens.