How does light travel through a camera?

Answer

Picture this... you're home for thanksgiving, its you and grandma for one big photo oportunity, big smile. A flash lets out from the camera. The light from the camera is reflected off your face and bounced straight back into the film. Leaving a permanent image on film. The film is exposed for only a fraction of a second. The amount of time the film is exposed should be a setting be the camera operator.

Answer

THE PARTS OF THE CAMERA The LENS is the image-forming device on a camera. It may be composed of from one to as many as 10 or 12 elements. The first cameras were fitted with a single-element meniscus lens (a lens with one concave and one convex surface). In addition to its very low speed, this type of lens suffered from a number of inherent optical defects and it was soon replaced with greatly improved, more complicated designs. The single-element lens remained in use on inexpensive cameras, however, and within limits was capable of producing very acceptable results.

The three basic types of lenses are normal, wide angle, and telephoto. The lens's focal length--the point at which light rays converge, or focus, through the lens--determines the size of the image that will be produced on the film.

With a normal lens, the viewing field is approximately 50 degrees. The objects photographed appear normal in size and shape, relative to the picture's background. A camera that uses a 35-mm film will usually have a 50-mm lens for normal coverage; on a medium-format 6 x 6-cm camera, the same coverage is obtained with an 80-mm lens.

In a wide-angle lens, the field of view is much wider: about 90 degrees. These lenses are used where the distance between camera and subject is limited, as in interior photography. The wide-angle lens is also to make smaller objects look larger (to give a spacious impression of a small room, for instance), or to photograph large objects from close up.

Telephoto, or long-focus lenses, have a smaller field of view than a normal lens, and show an enlarged detail of the image over the same film area. Interchangeable-lens cameras offer the photographer the opportunity to select a focal length that is optimum for any given situation. In recent years, variable-focal-length, or "zoom," lenses, have become very popular. A single lens of this type can replace many individual lenses, and offers a great convenience to the photographer.

The speed, or light-gathering power, of a lens is indicated by the f number, called the aperture. The lower the f number, the faster the lens--that is, the more light it lets through. A fast lens has an aperture of at least f/2.0. As the speed increases, the cost of the lens tends to increase, since it is more costly to maintain high standards of optical correction at very high apertures.

Diaphragm - Aperture One of the two factors that determines correct film exposure is the amount of light allowed to pass through the lens. The aperture is the camera adjustment that controls how much light enters the camera. Mechanically reducing the aperture improves optical performance, particularly toward the edge of the picture, and increases the DEPTH OF FIELD, which is the zone of good focus. Most cameras use an iris-type diaphragm, which consists of a number of very thin metal blades. They are so mounted that by rotating a ring or moving a lever, the size of the lens opening can be varied.

On automatic cameras the diaphragm is adjusted by a built-in mechanism to produce the optimum exposure over a wide range of lighting conditions. The various openings of the diaphragm--called f-stops--are stamped on the lens mounting. Each change of diaphragm opening changes the amount of light passing through the lens by a factor of 2. For example, the amount of light allowed through the lens at a setting of 2 is twice the amount allowed through the lens at a setting of 2.8.

The standard diaphragm settings found on most lenses are 2, 2.8, 4, 5.6, 8, 11, 16, 22, and so on. The smallest lens opening on a lens whose f-stops end in 22 is, in fact, 22.

Shutter Shutter is the second exposure control factor is the shutter, a mechanical device that acts as a gate, controlling the duration of time that light is allowed to pass through the lens and fall on the film. Two types of shutters are in general use. The leaf type, like the diaphragm, is made up of a number of thin metal blades that are opened and closed either by a spring-driven clockwork mechanism, or--in many recent models--by an electromechanical device. Shutters of this type usually have a maximum speed of 1/500th of a second. The focal-plane shutter in modern cameras usually consists of two pieces of rubberized fabric that move across the focal plane. The spacing between the fabric edges and the speed of transit determine the effective shutter speed. Some recent models use ultrathin pieces of titanium instead of fabric. Shutters of this type are capable of very high speeds, in some cases 1/4,000th of a second. The entire shutter mechanism is independent of the optical system, and it is therefore ideal for cameras with interchangeable lenses.

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