Lens Hood - Kinds of Lens Hoods

Kinds of Lens Hoods

EF 70-300mm f/4-5.6 IS USM lens with (top to bottom) petal-shaped lens hood, cylindrical lens hood and no lens hood

The geometry of the 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 higher portions of the lens hood, while allowing more light into the corners of the image through the lowered portions of the hood, thereby reducing the amount of vignetting in the final image.

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.

Maximum aperture also affects the shape of a lens hood. As the aperture gets larger the amount of light and consequentially the amount of the frame the sensor "sees" increases. This can be seen when comparing two lens hoods of the same focal length but with differing apertures - compare the lens hood of a telephoto f/4 lens with that of the same lens but with a maximum aperture of f/2.8.

Square or rectangular lens hoods are generally more efficient because their shape closely resembles the shape of the photograph, and can be longer than circular lens hoods without blocking the angle of view or causing vignetting. However, square or rectangular 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 the angle of view. For these types of lenses, only circular hoods will work effectively.

Lens hoods with an extending bellows design (much like the bellows of a medium or large format camera) can be adjusted for depth. This means that the depth can be increased when used on longer focal length lenses, and reduced as necessary for shorter focal length lenses.