Lens hood

The lens hood ( other common names: Sun visor, hood, Lens Shade) is an accessory in photography and a part of telescopes or mirror telescopes in astronomy.

Function

A lens hood is designed to prevent lateral incident light is reflected by lenses or replaced parts and so reaches the image sensor or film, or in the eyepiece. This can lead to considerable impairment of the image, such as optical overlays in the form of halos in the shape of the lenses, so usually circles or rings - the so-called lens flares. Furthermore, the image of the photographed or observed object is flat and low contrast by such stray light.

In practice, the lens hood also offers a degree of protection of the front lens, such as rain, spray or to touch and light shocks.

When shooting with direct backlighting such as the setting sun, the lens hood has no effect. Also the formation of ghost spots or other reflections caused by light sources within the image field, naturally can not be prevented by a lens hood.

In photography there for nearly all commercially available photographic lenses matching lens hood as a separate accessory in the form of rigid tubes made of metal or plastic or foldable rubber diaphragms. Mounting by screw thread, bayonet, by plugging and lens hoods aftermarket with screw thread on the filter thread. Zoom and telephoto lenses are partly equipped with removable baffles. For lenses with very short focal length, the lens hood is partially integral part of the lens and thus inseparable.

For compact cameras, particularly with recessed at rest zoom lenses, mounting a lens hood is usually not possible only with special accessories or. For cameras with optical viewfinder review, it may happen that the lens hood partially obscured the viewfinder image. Also there may be shadows when using built-in flash devices with cameras.

Interpretation

The optimum geometry of the scattered light depends on the angle of aperture of the lens, the diameter of the front lens and the size of the sensor and the film surface. A lens with a small angle of view requires a longer lens hood as a lens with large picture angle. Lens hoods for zoom lenses are always for the shortest focal length (largest angle ) and designed for the maximum sensor or the usable image circle of the lens. The use of constructed for the small picture format lenses on cameras with smaller-size imagers, such as the APS-H or APS -C format, but generally allows the use of larger lens hoods with better effect than intended by the manufacturer of the lens.

The wrong choice of lens hood either leads to a reduced protective effect, if it is too short or vignetting, if it is too long or has too small a diameter. When using additional filters may cause vignetting at for further Einschraubblenden especially with wide angle lenses.

If not rotate the front panel fixings or the front lens during focusing or zooming, the lens hood can be built a little longer. Since vignetting in the corners occurs first, then cut out ( tulip shape ) are. The advantage of this design is somewhat better protection against stray light at longer focal lengths.

A special form of the lens hood is the compendium. This is an intent, usually in the form of an adjustable bellows, which is common in film or professional cameras. A compendium can be adjusted to optimally lenses with different angles of view. Most have compendiums also supports for photographic filters or other accessories.

A new kind of compendium was invented by Roger Field and manufactured by Chrosziel, with a solid plastic housing and an inner axially adjustable mask for lenses with different focal lengths.

Astronomy

In astronomy, there are three types of lens hoods:

• As a dust cap seated on the lens or in front of the opening of the aperture is referred to telescope. This has on the one hand the same function as the lens hood in photography. But it should also protect against moisture night. Therefore, it is heated at more complex telescopes.
• In the barrel of the lens telescopes are located at certain intervals flat diaphragm whose outer diameter adapts the tube and the inner diameter of just allowing the passage of the beam path.
• Inside of mirror telescopes Blend tubes are used.