How a Camera Works
The term photography (from the Greek, phos, "light," and graphein, "to draw") means the production of a negative or positive black-and-white or colored record initiated by the action of radiant energy--usually in the form of light--upon a sensitive surface.
The fundamental physical principle of photography is that light falling briefly on the grains of certain insoluble silver salts (silver chloride, bromide, or iodide) produces small, invisible changes in the grains. When placed in certain chemical solutions known as developers, the affected grains are converted into a black form of silver.
When a photograph is taken with a camera, light reflected from the object passes through the shutter, diaphragm, and lens to form a real inverted image. For the brief period during which the shutter is open, this image falls on the surface of a film or plate sensitized by silver salts and causes an invisible latent image to be recorded on it.
With an instant camera, development and printing are carried out while the film is still in the camera. In most cameras, however, pictures are taken until all the film has been exposed; then the still-undeveloped film must be removed in darkness or in greatly subdued light and placed in a developing solution. This solution darkens the affected grains of silver salt and converts the latent image into a negative image, in which dark and light areas in the object are recorded as light and dark areas, respectively, on the negative.
The negative is then placed in a fixing solution, which dissolves the unaffected grains of silver salt and prevents any further action by light on the image.
To produce a positive contact print, light is passed through the negative so that it falls on a piece of printing paper held in close contact with the negative. Once again a latent image is produced; it is then developed into a positive image and fixed. Just as with the formation of the negative, this process reverses the dark and light areas and reproduces the original tones of the subject in a positive print.
Negatives today are usually too small to make a useful contact print. An optical ENLARGER is therefore used to throw a magnified image of the negative onto the printing paper, which is then developed and fixed to yield a large positive print called an enlargement.
A camera consists essentially of a box carrying a lens, diaphragm, and shutter that are arranged to throw an image of the scene to be recorded onto a sensitive film or plate.
The LENS is usually made up of several components. It forms a real, inverted image of the object. In the popular 35-mm cameras the focal length is typically 50 mm (2 in), but it can be shorter or longer according to the size of the camera.
In the focusing mechanism provision is made for moving the lens backward or forward to focus the image on the film. Three main methods are used to determine the position of the lens for correct focus: focusing scale, range finder, and reflex finder.
Two types of shutters are commonly used. The between-the-lens shutter is mounted between the components of the lens. The focal-plane shutter consists of a roller blind containing a slit that moves rapidly across the plane in front of the film. In popular cameras the shutter provides a range of exposures from about 1 second to 1/1,000 of a second.
The diaphragm may also be placed between the components of the lens. It provides a circular hole of variable size that regulates the amount of light that reaches the film.
If the light is weak, or if a short exposure is required, the diaphragm is opened wide to admit sufficient light. Under good lighting conditions with moderate exposures the diaphragm is set to a smaller APERTURE, thus reducing the amount of light reaching the film. The smaller aperture can also reduce effects of aberrations and of any error in focusing, thus producing a sharper picture.
Of the various kinds of viewfinders, the simplest consists of a small hole, which serves to position the eye, and a wire frame a few inches in front of the hole, which delimits the field of view that corresponds to the image on the film. Most cameras today use optical or through-the-lens viewfinders.
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