What is a Camera?
The basic function of a camera is to record a permanent image on a piece of film. When light enters a camera, it passes through a lens and converges on the film. It forms a latent image on the film by chemically altering the silver halides contained in the film emulsion. When the film is developed, the image becomes visible in the form of a negative. From the negative a positive image, or print, can be made. Since the time of the invention of the first camera, all cameras have operated on the same fundamental principles. As photographic technology developed, however, various camera functions underwent improvement. Thus, while the basic concept of the camera remains the same today, a wide range of accessories have been created to cope with special situations. In addition, special-purpose cameras have been developed that meet a variety of needs.
The basic parts of a camera are a lighttight body, or box, and a lens. In addition to the fundamental lens and body, a camera has a shutter, a film-holding and transport system, focusing and viewfinding systems, and sometimes a system for determining length of exposure.
The simplest camera body is the one designed for the snapshot camera. It holds the lens in a fixed position at one end and the film, under lightproof conditions, at the other. In simple cameras the distance between lens and film remains fixed, but in more advanced cameras it is possible to vary the distance between the lens and film plane for precise focusing. The camera body can range in size from the minute subminiature, the inside diameter of which may measure less than 1 inch (2.5 cm), to extremely large special-purpose machines used for cartography and engraving.
The function of the lens is to gather light and focus it on the film. A lens provides an angle of view that depends on its focal length (the distance between lens and film plane when the camera is focused on a distant object) and on the film size. On a 35mm camera, for example, a 50mm lens will provide an angle of view of 45° on the horizontal. A 100mm lens has an angle of view of 22°, while a 500mm lens covers a scant 5°. Conversely a 35mm lens covers an area of about 62° and a 28mm lens encompasses 74°. One of the most fascinating is the 8mm fisheye with a coverage of 180°.
The normal lens for a 35mm camera (a camera that uses 35mm film and produces a 24 x 36mm (1x1% inch) negative) has a 50mm focal length. The normal lens for a camera using 4x5 inch (100 x 125mm) film is 135mm. Since the angle of view of the two lenses is approximately the same, they provide the same area coverage of the image on their respective film sizes. The image on the 4x5 inch film will, of course, be larger than on the 35mm. But when a 135mm lens is used in both 35mm and 4x5 inch cameras, and the subject is the same distance away in each case, the image of the subject will be the same size on the films in both cameras. The 135mm lens will, however, provide a much wider angle of view on the 4x5 camera than on the 35mm camera.
Wide-Angle and Telephoto Lenses
Any lens with a focal length that is shorter than is normal for a given film size is considered a wide-angle lens. Thus, a 35mm lens for a 35mm camera would be a wide-angle, as would a 90mm lens for a 4 x 5 inch camera. A wide-angle lens produces a smaller image of a subject at a given distance than does a normal lens, but it provides a wider angle of view. A telephoto lens, on the other hand, is one of greater than normal focal length; it produces a larger image of a subject at a given distance than does a normal lens, but its angle of view is narrower. A 135mm lens, for example, would be considered a telephoto for a 35mm camera, while a 250mm lens would be considered a telephoto for a 4 x 5 inch camera.
A zoom lens provides a wide range of focal lengths within a single lens. A control on the outside of the lens allows the photographer to choose a particular focal length by changing the relationship of the lens elements. Zoom lenses usually offer a range of focal lengths of about two or three to one. Typical ranges are 36 to 85mm (1% to 3% inches).
Practically all modern lenses are equipped with a variable diaphragm that controls the amount of light admitted to the film. The opening, or aperture, of the diaphragm is expressed as a fraction of the focal length of the lens. The resulting fractions, or relative apertures, are marked in a series of numbers, such as f/1.9, f/2.8, f/4, f/5.6, f/8, f/11, and f/16, around the rim of the lens mount. All lenses set at the same aperture provide the same image brightness at the film. At a diaphragm setting of f/5.6, for example, a 135mm lens permits the same amount of light to reach the film as a 50mm lens set at the same opening designation. The sequence of numbers is devised so that the next highest number (and therefore smaller lens opening) allows only half as much light to pass as the previous opening does. Thus the light admitted at f/5.6 is half of that admitted at f/4, and at f/8 the amount of light admitted is only half of that admitted at f/5.6.
Depth of Field
When a camera is focused on a particular object, other objects in the image area that are nearer or farther away from the camera may not be as sharply defined.
This change in sharpness is gradual rather than abrupt; there is a zone in front of and behind the exact point of focus where the actual difference in sharpness is too small for the human eye to see. The extent of this zone of sharp focus is called the depth of field. It depends on the lens aperture, the focal length of the lens, and the distance between camera and subject. At a given distance from camera to subject, the smaller the aperture and the smaller the focal length, the greater will be the depth of field. For example, with a 135mm lens set at f/11 and focused on a subject 30 feet (10 meters) away, the depth of field is approximately 26.5 to 34.7 feet (7.9 to 10.4 meters). With a 50mm lens at the same focus and aperture setting, the depth of field is 15 feet (4.5 meters) to infinity. At larger apertures the depth of field for both the 135mm and the 50mm lenses would be shallower, but it would still be relatively larger for the 50mm lens than for the 135mm lens. In addition, at a given aperture for a given lens, the depth of field narrows as the point of focus moves nearer to the lens.
The shutter is the other light-regulating mechanism on a camera. It controls the amount of light that reaches the film by regulating the actual exposure time. Two major types of shutter - leaf and focal-plane - are in use in most cameras. In addition, there are other shutter types for motion-picture cameras and special types of still cameras.
This type of shutter usually consists of either three or five interleaved blades, each pivoted on the outer end. The blades form a diaphragm controlled by a ring. When the shutter release of the camera is depressed, a spring moves the ring, which in turn opens the blades. On completion of exposure a second ring closes the blades. The duration of the opening can be set by adjusting the spring's tension. On most advanced cameras that use such a shutter, the speeds ivith which the shutter opens and closes range from 1 second to ^oo of a second. In such shutters the slow speeds are controlled by geared timing mechanisms. The leaf shutter can be mounted in the camera behind or in front of the lens or between its elements.
The focal-plane shutter is actually a blind with a slit in it. The shutter travels across the film plane, either up and down or from side to side. In focal-plane shutters used on press cameras, both the size of the slit and the tension on the blind can be regulated to control the duration of exposure. However, the modern focal-plane shutter found in 35mm cameras is under constant tension. Exposure time is adjusted only by adjusting slit size. In actual practice the time required for the slit to travel across the film may be much longer than the time a particular section of the film is exposed by the light passing through the slit. These shutters are usually made of rubberized cloth or thin metal.
Among other types of shutters is the electronic shutter, essentially a leaf shutter that is electronically controlled for accuracy of exposure timing. The rotary shutter is a semicircular disk with a wedge-shaped opening. Primarily used in motion-picture cameras, it has also been applied to still cameras, particularly to machines designed to shoot fast bursts of exposures at the rate of several a second. The prism shutter is employed primarily for highspeed camera work, where hundreds or even thousands of images must be recorded in a second. As the prism rotates it synchronizes with the travel of the film across the film plane and forms an image on each frame.
One of the most important attributes of the modern shutter is its ability to synchronize with the flash attachments. Of the two main types of shutters, the leaf-type works best with both electronic flash and flashbulbs, which can be used at all speeds with these shutters. When a flash is synchronized with a leaf shutter, the flash reaches its peak of light output at precisely the same time that the shutter reaches its fullest opening. A flash contact inside the camera closes the electrical circuit at the correct instant.
There are three types of synchronization: M, F, and X. The M-synchronization setting on the shutter is used with medium-peak bulbs; the firing circuit closes before the blades of the shutter begin to move and about %o of a second before the shutter opens fully. The F setting is designed for fast-peak bulbs; the circuit fires the bulb when the shutter is about half open. The X-synchronization setting fires the flash the instant the shutter is fully open. It is intended primarily for use with electronic flash with zero delay. X-synchronized shutter speeds are restricted to 1/125 of a second.
When a camera has a focal-plane shutter, the flash must be synchronized to coordinate the bulb's peak of light output with the instant the slit starts to move across the film. The flash must last until the entire film area has been exposed. There are FP (focal-plane) bulbs for just this purpose; they work best at higher shutter speeds. Focal-plane shutters also have X synchronization for electronic flash.
Another feature of many focal-plane and leaf shutters is the self-timer, a device for automatically tripping the shutter after it has been cocked. With leaf shutters the self-timer is usually located on the shutter assembly itself. On focal-plane-shutter cameras it is most often found on the camera body. The self-timer allows a delay of about 10 to 15 seconds after it is tripped before the shutter opens and closes. In most cases the self-timer is used to permit the photographer to get into the picture himself after focusing and setting the exposure. It is also used occasionally in the absence of a cable release when extreme closeups are being made with a tripod-mounted camera, in order to prevent a jarring effect when the finger depresses the shutter release.
The viewfinder provides the photographer with an image of the subject that is identical (or nearly so) with the image that the camera lens will project upon the film. It makes it possible for him to "frame" the photographic subject within a desired background by changing camera angle or position, and it gives a preview of the photograph he is about to take. Viewfinders are standard parts of nearly all cameras except view cameras, which have a ground-glass back on which the image formed by the camera lens can be viewed before the film holder is inserted.
In all viewing systems except the ground-glass back and the single-lens reflex viewfinder, inaccuracies in viewing and framing may be caused by a phenomenon known as parallax. Parallax results from a difference between the viewpoint of the camera lens and that of the viewfinder, and it is most noticeable in the case of nearby subjects. Many viewfinders are designed to compensate in part for parallax.
The bright-field finder is somewhat more advanced than the sports finder. It is usually found on less expensive cameras, such as box and inexpensive twin-lens cameras. The finder consists of a simple lens that directs an image to a mirror set at a 45° angle inside the camera. The mirror in turn directs the image to another lens where it is brought to focus, producing a very bright image. Bright-field images are rather difficult to view, and usually the camera must be held at waist level. The image is seen reversed from right to left.
The optical viewfinder is based on the Galilean telescope and provides a less distorted, unreversed image, and one that is easier to see. It consists of two lenses, one concave and the other convex. The separation between the two lenses determines the size of the subject area shown. The optical finder is most often mounted inside and near the top of the camera, for eye-level use. A masking system makes it possible to use the optical finder with a variety of focal lengths, providing different angles of view to match different lenses. Some optical finders incorporate a bright frame, a reflection of a white rectangular line drawn on the sight. In this case, the finder covers a greater area than the lens does, for easier viewing, while the frame provides the exact field of the lens.
The twin-lens reflex camera employs two lenses, one for taking the picture and the other for viewing the subject. While similar in principle to the bright-field finder, the typical twin-lens reflex incorporates a ground glass and field lens combination that greatly improves ease and accuracy of viewing.
Another reflex viewing system, the single-lens reflex, does not have a separate lens for viewing. Instead, the light enters the camera lens and strikes a mirror, which directs it to a ground glass. The image formed on the ground glass is viewed by means of a pentagonal prism, or pentaprism, and an eyepiece lens. The penta-prism returns the reversed image to the correct right-to-left orientation and directs it through the eyepiece to the viewing eye. During exposure the mirror moves up and out of the way, permitting light to pass to the film and momentarily blacking out the finder.
In addition to being framed correctly by the viewfinder, the image should be properly focused on the film plane. A range-finder or other focusing system is considered essential for serious photography, therefore, although some cameras do dispense with such systems. The systems fall into two general categories: reflex focusing aids, in which focusing is done by means of the ground glass viewing screen, and coupled range finders.
The two types of coupled range finders in general use are the coincident and the split-image. Both are based on the fact that when an object is viewed from two different positions, the lines of sight converge on the object. The convergence angle increases as the object grows nearer and decreases as the object moves farther away. In the coincident range finder the two viewing points are provided by separate windows, one of which is the viewfinder window. A second image is introduced through another window by means of mirrors and prisms. The images are superimposed, and a double image is seen as long as the lens is out of focus. In the split-image range finder, the unfocused image appears unaligned, or split. When focused, the image is aligned, and the split disappears. The range finders are coupled to the focusing ring of the lens; hence the name.
While split-image range finders are often used in single-lens reflex cameras, the ground glass is most often used for focusing in both single-lens and twin-lens reflex cameras - except the relatively few that do not have ground-glass screens. The ground glass is the same distance from the lens as the film plane is; when the lens is not focused, the image appears fuzzy.