A lens hood is a shield fitted to prevent rays of light from outside the picture area from falling on the lens. Although rays outside the angle of view of the lens do not affect the sensitive surface directly, they set up internal reflections in the lens and may cause ghost images and a general degradation of the contrast of the negative.
There is no need to use a lens hood with the sort of box cameras which have the lens set well back into the body (often behind the diaphragm) but a hood is almost a necessity on the more expensive cameras with wide aperture lenses which are usually without any protection from stray light.
The simplest type of lens hood is no more than a short piece of tube, blackened on the inside to reduce surface reflections. It is made to slip directly on to the rim of the lens or filter mount. Some cameras have a screw fitting.
Most lens hoods are made funnel-shaped with the object of providing a relatively deep hood without cutting off the corners. The angle of the funnel conforms more or less to the angle of view of a lens of normal focal length. With this shape of hood there is always a risk of unwanted reflections from the inside surface if it is at all smooth or bright. The cheaper hoods of this sort are painted with dead black paint; in the more expensive types there are fine ridges machined on the inside surface to reduce reflections still further.
The most efficient hoods are those which conform to the shape of a square or round box with an aperture cut on the side opposite the lens. With this sort of shape there is no risk of internal reflection as none of the surfaces is so inclined that it can reflect incoming light rays on to the front of the lens. The aperture of the hood should, ideally, be of the same proportions as the negative; it should also be as small as possible without actually obstructing the angle of view of the lens (the distance between the lens and the hood aperture will govern this). Unfortunately, such hoods cannot be used with lenses which have front cell focusing unless the hood is put on after focusing.
Most lens hoods for hand and miniature cameras are of rigid metal construction; a few are collapsible. Larger hoods for studio and field cameras are often in the form of a rectangular bellows which can be extended to the limit where it starts to cut off the corners of the image on the focusing screen.
When choosing a lens hood for a camera with a rising front, it is important to check it for cut-off with the front raised to the limit. A hood which just fails to cut the corners of the image with the lens central will cut off the bottom corners when the front is raised.
The shorter the focal length of the lens, the shallower the hood that can be used with it. Telephoto lenses, because of their narrow angle of view, can be fitted with very deep and efficient hoods which are a great help in producing a brilliant, fog-free image.