The Vello 55mm Snap-on Tulip Lens Hood (Version II) is designed to fit lenses with a 55mm filter thread. The hood is designed for use with internal focus type zoom lenses. When used with "full frame" sensor cameras, the starting focal length should be above 20mm. With "APS-C" sensor cameras, the starting focal length should be above 15mm. With wider lenses, especially on full frame cameras, some slight vignetting might occur.
The "petal" shape of this lens hood maximizes the amount of hood coverage with the minimum amount of weight and hood area. In addition, the design of this hood allows it to be reversed and stored on the lens, when not in use. It is also designed to accept lens caps.
Lens hoods are primarily 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 some extra protection from accidental impact.