Lens hood

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 in order to prevent glare and lens flare.
A selection of lens hoods

In photography, a lens hood or lens shade is a device used on the end of a lens to block the Sun or other light source in order to prevent glare and lens flare.[1] Lens hoods may also be used to protect the lens from scratches and the elements without having to put on a lens cover.

Cause of lens flare

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[2] The light source itself may be in the lens' angle of view, but it does not have to be to cause a lens flare. It is only necessary that stray light from the bright light source enters the lens.

Types

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.[3] 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 (reduction of light around the periphery) 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.[4]

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.[5] 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.

Petal-shaped lens hood
Cylindrical lens hood
No lens hood
EF 70-300mm f/4-5.6 IS USM lens

Storage

Lens hoods that are supplied by the manufacturer of the lens are often designed to fit onto the matching lens facing either forward, for normal use, or backwards, so that the hood may be stored with the lens without occupying much additional space. Rubber lens hoods are flexible and generally collapse for storage. However other lens hoods must be removed if these features are not available and length extension of the lens is not preferred during storage.

In addition, lens hoods can offer some degree of physical protection for the lens due to the hood extending farther than the lens itself.[6]

See also

References

  1. ↑ van Walree, Paul. "Lens hoods". Toothwalker.org.
  2. ↑ Altez, Anna. "Lens Flare: How to Reduce or Avoid it?". PhotoPoly. Retrieved November 14, 2011.
  3. ↑ "UNDERSTANDING CAMERA LENS FLARE". Cambridge in Colour.
  4. ↑ Landreth, Katherine. "The Purpose of a Lens Hood". Pare and Focus. Retrieved 19 August 2012.
  5. ↑ van Walree, Paul. "Lens hoods". Toothwalker.org.
  6. ↑ Harmer, Jim, "What does a lens hood do?", Improve Photography, 15 Jun 2011

External links

Wikimedia Commons has media related to Lens hoods.