The 86mm Long Metal Lens Hood from Heliopan is screwed to the front of the telephoto lens to block stray light and prevents flare on the front element of the lens. The aluminum hood also protects the lens against sudden or accidental impact. The front diameter of the lens hood is the same as it is in the back, which means a lens cap can clip to the front of the hood without having to remove the hood.
Primarily, longer lens hoods are used for telephoto lenses since the field of view is narrower. It is not recommended to place a long lens hood on a wide-angle lens, which could cause vignetting in your image.