Lens hood

The shape of a lens hood can vary from a plain cylindrical or conical section (much like a lamp shade) to a more complex shape, sometimes called a petal, tulip, or flower hood.

[3] This allows the lens hood to block stray light with the longer portions of the lens hood, while allowing more light into the corners of the image through the shorter portions of the hood, thereby reducing the amount of mechanical vignetting (reduction of light around the periphery)[4] in the final image.

For wide angle lenses, the length of the hood (away from the end of the lens) cannot be as long as those for telephoto lenses, as a longer hood would enter the wider field of view of the lens.

However, rectangular or square lens hoods should not be used with zoom lenses whose front elements rotate as the focal length is changed, as the hood will rotate as well, blocking parts of the angle of view.

For these types of lenses, only cylindrical or conical lens hoods will work effectively.

A selection of lens hoods: one cylindrical, one conical and three petal ones (one of them is chopped).
Scheme of a lens with lens flare. A lens hood is designed so that it does not block the angle of view of the lens. Lens hoods block the Sun or other light source(s) to prevent glare and lens flare.