Lens hood

In photography, a lens hood or lens shade is a device used on the front end of a lens to block the Sun or other light source(s) to prevent glare and lens flare. Lens hoods may also be used to protect the lens from scratches and the elements without having to put on a lens cover.
The geometry of a lens hood is dependent on three things: the focal length of the lens, the size of the front lens element and the dimensions of the image sensor or film in the camera.

Flare occurs when stray light strikes the front element of a lens and then bounces around within the lens. This stray light often comes from very bright light sources, such as the Sun, bright studio lights, or a bright white background. If a light source is in the lens' angle of view, a lens hood will hardly have any effect, but the light does not have to cause lens flare. It is sufficient that stray light from a bright light source enters the lens. Multi-layer coatings in newer lenses also help to reduce lens flare.

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. These more complex shapes take into account the final image's shape and aspect ratio. 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) in the final image.

The geometry of a lens hood is dependent on three things:

Ideally, lens hoods should increase in length, and therefore in efficiency, as the focal length of the lens increases and the angle of view reduces. Lens hoods are more prominent in long focus lenses because they have a smaller viewing angle than that of wide-angle lenses. 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.

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