Cameras and Lenses

When one speaks of the technology of photography one usually refers to the equipment, techniques, and processes that are used in the production of photographs.

The most widely employed photographic process used to be the black-and-white negative–positive system (see the figure below). In the camera the lens projects an image of the scene being photographed onto a film coated with light-sensitive silver salts, such as silver bromide. A shutter built into the lens admits light reflected from the scene for a given time to produce an invisible but developable image in the sensitized layer, thus exposing the film.




This illustration shows the sequence of negative-positive process, from the photographing of the original scene to enlarged print





This illustration shows the sequence of negative-positive process, from the photographing of the original scene to enlarged print.




Pictured in this photograph are a roll of black-and-white film, strips of coloured negative film, and coloured slides in and out of their cases





Pictured in this photograph are a roll of black-and-white film, strips of coloured negative film, and coloured slides in and out of their cases.

Bright subject details record as dark or dense areas in the developed film; dark parts of the subject record as areas of low density; i.e., they have little silver. After development the film is treated with a fixing bath that dissolves away all undeveloped silver salt and so prevents subsequent darkening of such unexposed areas. Finally, a wash removes all soluble salts from the film emulsion, leaving a permanent negative silver image within the gelatin layer.

A positive picture is obtained by repeating this process. The usual procedure is enlargement: the negative is projected onto a sensitive paper carrying a silver halide emulsion similar to that used for the film. Exposure by the enlarger light source again yields a latent image of the negative. After a development and processing sequence the paper then bears a positive silver image. In contact printing the negative film and the paper are placed face to face in intimate contact and exposed by diffused light shining through the negative. The dense (black) portions of the negative image result in little exposure of the paper and, so, yield light image areas; thin portions of the negative let through more light and yield dark areas in the print, thus re-creating the light values of the original scene.

THE CAMERA AND LENSES

In its simplest form, the camera is a light-tight container carrying a lens, a shutter, a diaphragm, a device for holding (and changing) the film in the correct image plane, and a viewfinder to allow the camera to be aimed at the desired scene. (Digital cameras, lenses, and techniques are discussed in Chapter 8.)

BASIC CAMERA FUNCTIONS

The lens projects an inverted image of the scene in front of the camera onto the film in the image plane. The image is sharp only if the film is located at a specific distance behind the lens. This distance depends on the focal length of the lens and the distance of the object in front of the lens. To photograph near and far subjects, all but the simplest cameras have a focusing adjustment that alters the distance between the lens and the film plane to make objects at the selected distance produce a sharp image on the film. In some cameras focusing adjustment is achieved by moving only the front element or internal elements of the lens, in effect modifying the focal length.




An illustration depicts the cross section of a single-lens reflex (SLR) camera with a flip mirror.





An illustration depicts the cross section of a single-lens reflex (SLR) camera with a flip mirror.

The shutter consists of a set of metallic leaves mounted in or behind the lens or a system of blinds positioned in front of the film. It can be made to open for a predetermined time to expose the film to the image formed by the lens. The time of this exposure is one of the two factors controlling the amount of light reaching the film. The other factor is the lens diaphragm, or aperture, an opening with an adjustable diameter. The combination of the diaphragm opening and exposure time is the photographic exposure. To obtain a film image that faithfully records all the tone gradation of the object, this exposure must be matched to the brightness (luminance) of the subject and to the sensitivity or speed of the film. Light meters built into most modern cameras measure the subject luminance and set the shutter or the lens diaphragm to yield a correctly exposed image.

PRINCIPAL CAMERA TYPES

The simplest camera type, much used by casual amateurs, has most of the features listed in the previous section—lens, shutter, viewfinder, and film-holding system. The light-tight container traditionally had a box shape. Present-day equivalents are pocket cameras taking easy-load film cartridges or film disks. Typically, a fixed shutter setting gives about 1/50-second exposure; the lens is permanently set to record sharply all objects more than about five feet (1.5 metres) from the camera. Provision for a flash may be built in. Though simple to handle, such cameras are in daylight restricted to pictures of stationary or slow-moving subjects.

THE 35-MM MINIATURE CAMERA

Perforated 35-millimetre (mm) film (originally standard motion-picture film) in cartridges holding 12 to 36 exposures with a nominal picture format of 24 × 36 mm is employed in miniature cameras. Smaller image formats down to 18 × 24 mm (half frame) may be used. The 35-mm camera has a lens with a range of apertures and a shutter with exposure times typically from one second to 1/1,000 second or shorter, and it can focus on subject distances from infinity down to five feet or less. A winding lever or built-in motor advances the film from one frame to the next and at the same time tensions (cocks) the shutter for each exposure. At the end of the film load the film is rewound into the cartridge for removal from the camera in daylight.

A 35-mm camera usually has a direct-vision viewfinder, often combined with a rangefinder or autofocus system for accurate distance settings. Most current versions incorporate a light meter coupled with the exposure settings on the camera. Advanced models may have interchangeable lenses and an extended accessory system. Many 35-mm cameras are single-lens reflex types.

THE ULTRAMINIATURE OR SUBMINIATURE

This camera takes narrow roll film (16-mm or 9.5-mm) in special cartridges or film disks. The picture size ranges from 8 × 10 mm to 13 × 17 mm. These formats are used for making millions of snapshooting pocket-size cameras; special versions may be as small as a matchbox for unobtrusive use.

SHUTTER TYPES

One basic type of shutter that is found on many simple and compact 35-mm cameras is composed of overlapping leaves and is located between the front and rear elements of the lens. The leaves open and close to admit light. This is called a leaf or between-lens shutter. A second type, found on 35-mm interchangeable-lens cameras, is the focal-plane shutter, located in the back of the camera just in front of the film. With this type of shutter, two opaque blinds, or curtains, sweep in front of the film. The curtains are adjusted to create slits of various widths.

Most cameras provide a method of adjusting shutter speed, either by mechanical or electronic means. The shutter speeds may range from a full second to as little as 1/4,000 of a second. A typical range of marked shutter speeds for an average 35-mm SLR camera might be as follows: 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/500, 1/1,000. Note that each marked setting is approximately twice as fast as the preceding one and half as fast as the following one.




These aperture and shutter-speed combinations allow the same amount of light to enter the camera but result in different images. Smaller apertures extend the zone of sharp focus, and slow shutter speeds show blurred movement.





These aperture and shutter-speed combinations allow the same amount of light to enter the camera but result in different images. Smaller apertures extend the zone of sharp focus, and slow shutter speeds show blurred movement.

Shutter speed determines how effectively a moving object can be “stopped”—that is, how sharply it can be reproduced without blurring or streaking in the final image. With a fast shutter speed, the shutter is opened only briefly and the moving object has little time to change its position before the exposure is completed. With a slow shutter speed, on the other hand, the shutter remains open for a relatively long time. Thus the faster the shutter speed, the sharper a moving object will appear on the final image, and the slower the shutter speed, the more blurred the object will appear.

This principle also holds true for camera motion. The faster the shutter speed, the less noticeable any shaking of the camera will be in the final photograph. This is particularly relevant when holding a camera while taking a picture with a telephoto lens. The lens magnifies the effect of camera motion, so that if the camera is moved even slightly when the picture is taken, the final image will appear blurred. To avoid this, many photographers select a shutter speed of no slower than 1/125 of a second for hand-held shooting, though some photographers are successful with a speed of 1/60 or even 1/30 of a second.

In action pictures, the focal length also plays a role in the final image. A lens with a long focal length will magnify the image and its apparent motion. Apparent motion is greatest when the subject is moving at right angles across the line of sight, and it is least when the subject is moving directly toward or away from the camera. Much of the time photographers wish to stop all motion for a sharp, clear image. Sometimes, however, they use slower shutter speeds to produce expressive blurs of motion.

THE VIEW, OR TECHNICAL, CAMERA THE MEDIUM-SIZE HAND CAMERA

This type of camera takes sheet film (typical formats of from 2 1/2 × 3 1/2 inches to 4 × 5 inches), roll film, or 70-mm film in interchangeable magazines; it has interchangeable lenses and may have a coupled rangefinder. Special types use wide-angle lenses and wide picture formats (e.g., 2 1/4 × 4 1/2 to 2 1/4 × 6 3/4 inches [6 × 12 to 6 × 17 centimetres]). The medium-size hand camera was popular with press photographers in the first half of the 20th century. Older versions had folding bellows and a lens standard on an extendable baseboard or strut system. Modern modular designs have a rigid body with interchangeable front and rear units.

THE FOLDING ROLL-FILM CAMERA

The folding roll-film camera, now rare, resembles the 35-mm miniature camera in shutter and viewfinder equipment but has bellows and folds up to pocketable size when not in use. Generally it takes roll films holding eight to 16 exposures; typical picture sizes are 2 1/4 × 2 1/4, 2 1/4 × 3 1/4, or 1 3/4 × 2 1/4 inches. Some 35-mm cameras were also produced with bellows.

THE SINGLE-LENS REFLEX




The Contax S 35-millimetre SLR camera (1949-51), front, was the first postwar SLR to incorporate a pentaprism. The Nikon S 36-millimetre SLR camera (1951-55), back, featured range finder focusing.





The Contax S 35-millimetre SLR camera (1949–51), front, was the first postwar SLR to incorporate a pentaprism. The Nikon S 36-millimetre SLR camera (1951–55), back, featured range finder focusing.
THE TWIN-LENS REFLEX

The twin-lens reflex is a comparatively bulky dual camera with a fixed-mirror reflex housing and top screen mounted above a roll-film box camera. Its two lenses focus in unison so that the top screen shows the image sharpness and framing as recorded on the film in the lower section. The viewing image remains visible all the time, but the viewpoint difference (parallax) of the two lenses means that the framing on the top screen is not exactly identical with that on the film.

SHUTTER AND DIAPHRAGM SYSTEMS

Principal present-day shutters are the leaf shutter and the focal-plane shutter.

THE LEAF SHUTTER

The leaf, or diaphragm, shutter consists of a series of blades or leaves fitted inside or just behind the lens. The shutter opens by swinging the leaves simultaneously outward to uncover the lens opening. The leaves stay open for a fixed time—the exposure time—and then close again. A combination of electromagnets or electromagnets and springs drives the mechanism, while an electronic circuit—often coupled with a light metering system—or an adjustable escapement in mechanical shutters controls the open time. This is typically between one second and 1/500 second.

FOCAL-PLANE SHUTTER

The focal-plane shutter consists of two light-tight fabric blinds or a combination of metal blinds moving in succession across the film immediately in front of the image plane. The first blind uncovers the film and the second blind covers it up again, the two blinds forming a traveling slit the width of which determines the exposure time: the narrower the slit, the shorter the time. The actual travel time is fairly constant for all exposure times. A mechanism or electromagnet and control circuit triggers the release of the second blind. Focal-plane shutters are usually adjustable for exposure times between one second (or longer) and 1/1,000 to 1/4,000 second.

DIAPHRAGM AND SHUTTER SETTINGS

Shutter settings on present-day cameras also follow a standard double-or-half sequence—e.g., 1, 1/2, 1/4, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, 1/1,000 second, and so forth. The shorter the exposure time, the “faster” the shutter speed.

EXPOSURE VALUES

An attempt to simplify the mathematics of f-number and shutter speed-control functions led to the formulation of exposure values (EV). These run in a simple whole-number series, each step (EV interval) doubling or halving the effective exposure. The lower the EV number, the greater the exposure. Thus, EV 10 gives twice as much exposure as EV 11 or half as much as EV 9. Each EV value covers a range of aperture/speed combinations of the same equivalent exposure; for instance, f/2.8 with 1/250 second, f/4 with 1/125 second, and f/5.6 with 1/60 second. For a time some cameras carried an EV scale and coupled the aperture and speed settings; at a given EV setting in such cameras selecting various speeds automatically adjusted the aperture to compensate and vice versa. Exposure-value setting scales became obsolete with exposure automation, but the notation remains in use to indicate either exposure levels or—at specified film speeds—lighting levels requiring a given exposure.

AUTOMATIC-DIAPHRAGM SYSTEMS

On a camera with a viewing screen (view camera or single-lens reflex) viewing and focusing are carried out with the lens diaphragm fully open, but the exposure is often made at a smaller aperture. Reflex cameras (and increasingly also view cameras) therefore incorporate a mechanism that automatically or semiautomatically stops down (reduces) the lens to the working aperture immediately before the exposure.

METHODS OF FOCUSING AND FRAMING

The ground-glass (now mostly grained plastic) screen is the most direct way of viewing the image for framing and for sharpness control. The screen localizes the image plane for observation. The image is also visible without a screen, but then the eye can locate the image plane of maximum sharpness only with a precisely focused high-power magnifier. This aerial focusing method avoids interference of the ground-glass structure with sharpness assessment.

FOCUSING AIDS

The focusing screen is often overlaid by a pattern of fine concentric lens sections. Called a Fresnel screen, it redirects the light from the screen corners toward the observer's eye and makes the image evenly bright.

Cameras without a screen generally are equipped with a distance scale, the lens being set to the estimated object distance. More advanced cameras have an optical rangefinder as a distance-measuring aid; it consists of a viewfinder and a swinging mirror a few inches to one side of the viewfinder axis. As the eye views an image of the object, the mirror superimposes a second image from a second viewpoint. On turning the mirror through the correct angle, which depends on the object distance, the two images are made to coincide. The mirror movement can be linked with a distance scale, or coupled with the lens focusing adjustment. When the lens is incorrectly focused, the rangefinder shows a double or split image. In place of a rotating mirror, the rangefinder may use swinging or rotating optical wedges (prisms).

AUTOFOCUS SYSTEMS
RANGE FINDER

A range finder is any of several instruments used to measure the distance from the instrument to a selected point or object. One basic type is the optical range finder modeled after a ranging device developed by the Scottish firm of Barr and Stroud in the 1880s. The optical range finder is usually classified into two kinds, coincidence and stereoscopic.

The coincidence range finder, used chiefly in cameras and for surveying, consists of an arrangement of lenses and prisms set at each end of a tube with a single eyepiece at its centre. This instrument enables the user to sight an object by correcting the parallax resulting from viewing simultaneously from two slightly separated points. The object's range is determined by measuring the angles formed by a line of sight at each end of the tube; the smaller the angles produced, the greater is the distance, and vice versa. The stereoscopic range finder operates on much the same principle and resembles the coincidence type except that it has two eyepieces instead of one. The design of the stereoscopic instrument makes it more effective for sighting moving objects. It was widely used for land-gunnery ranging during World War II.

Since the mid-1940s, radar has supplanted optical range finders for most military target-ranging operations. This nonoptical ranging device determines the distance to a target by measuring the time it takes radio pulses to reach the object, bounce off, and return.

Advances in laser technology led to the development in 1965 of another kind of ranging instrument known as the laser range finder. It has largely replaced coincidence range finders for surveying and radar in certain military applications. The laser range finder, like radar, measures distance by timing the interval between the transmission and reception of electromagnetic waves, but it employs visible or infrared light rather than radio pulses. Such a device can measure distances of up to 1 mile (1.61 km) to an accuracy of 0.2 inch (0.5 cm). It is especially useful in surveying rough terrain where remote points have to be located between rocks and brush.

While these devices measure distance automatically, single-lens reflex cameras may incorporate electronic image-analysis systems to measure sharpness. The signal output of such systems actuates red or green LEDs in the camera finder system to show whether the image is sharp or not. The same signal can control a servomotor in the lens for fully automatic focusing. These devices are limited at low lighting and contrast levels—where the human eye also finds sharpness assessment difficult.

VIEWFINDERS

The sighting devices in cameras lacking screens are called viewfinders; they show how much of the scene will appear on the film. The simplest viewfinder is a wire frame above the camera front, with a second frame near the back to aid the eye in correct centring. Most present-day finders are built into the camera and are compact lens systems. Bright-frame finders show a white frame reflected into the view to outline the field recorded on the film. An alternative form is the reflecting viewfinder in which the photographer looks down into a field lens on top of the camera. The upper section of a twin-lens reflex camera is such a reflecting finder.

As the viewfinder axis in a camera other than a single-lens reflex does not usually coincide with the lens axis, the finder's and the lens's views do not exactly match. This parallax error is insignificant with distant subjects; with near ones it is responsible for the familiar fault of a portrait shot of a head that appears partly cut off in the picture even though it was fully visible in the finder. Camera viewfinders may have parallax-compensating devices.

The optical finder gives a direct upright and right-reading view of the subject with the camera held at eye level. The traditional reflex camera, held at waist level, showed a laterally reversed view. Modern reflexes have a pentaprism arrangement that permits upright, right-reading, eye-level viewing by redirecting the image from the horizontal screen on top of the camera.

EXPOSURE-METERING SYSTEMS

Exposure meters, or light meters, measure the light in a scene to establish optimum camera settings for correct exposures. A light-sensitive cell generates or controls an electric current according to the amount of light reaching the cell. The current may energize a microammeter or circuit controlling LEDs to indicate exposure settings. In most modern cameras the current or signal acts on a microprocessor or other circuit that directly sets the shutter speed or lens aperture. The cell usually is a silicon or other photodiode generating a current that is then amplified. In older cadmium sulfide cells the light falling on the cell changed the latter's resistance to a current passing through it. Selenium cells, still used in some cameras, also generate a current but are larger and less sensitive.

View cameras may use a photocell on a probe that can be moved to any point just in front of the focusing screen, thus measuring image brightness at selected points of the image plane. This takes place before the exposure, and the probe is then moved out of the way. Professional photographers also use hand-held separate exposure meters and transfer the readings manually to the camera.

AN EXPOSURE METER

An exposure meter (also called a light meter) measures the intensity of light and indicates proper exposure (i.e., the combination of aperture and shutter speed) for film or image sensors of a specific sensitivity. Traditional exposure meters are separate handheld devices, though almost every modern camera, both film and digital, comes with a built-in meter.




A light meter measures the amount of light in the area and the result is used in setting the camera before clicking the picture.





A light meter measures the amount of light in the area and the result is used in setting the camera before clicking the picture.

Exposure meters incorporated into cameras measure reflected but not incident light. In some meters, the light-sensitive element is set on the exterior of the camera, but in other cameras, particularly single-lens reflex (SLR) cameras, they are set internally. The latter meters are of the “through-the-lens” (TTL) type, reading light as it is focused by the camera's lens and strikes the film or sensor. Many of the capabilities of handheld meters are found in built-in meters. Exposure correction can be done either semiautomatically or automatically. In a semiautomatic model, the operator adjusts the aperture and shutter speed until the camera's display indicates a correct exposure. In fully automatic cameras, the exposure is corrected by the camera mechanism itself.

FLASH SYSTEMS

Flash is a widely used artificial light source for photography, providing a reproducible light of high intensity and short duration. It can be synchronized with an instantaneous exposure. Being battery powered, small flash units are self-contained.

ELECTRONIC FLASH

The most common flash system depends on a high-voltage discharge through a gas-filled tube. A capacitor charged to several hundred volts (by a step-up circuit from low-voltage batteries or from the line voltage supply) provides the discharge energy. A low-voltage circuit generating a high-voltage pulse triggers the flash, which lasts typically 1/1,000 second or less. Small electronic flash units may be built into or clipped onto the camera. Larger units are attached with brackets. Large professional units with floodlight and spotlight fittings are used in studio photography. Even small flashes often have adjustable reflectors, for example, to illuminate an indoor subject by the flash reflected from the ceiling or walls.

AUTOMATIC AND DEDICATED FLASH

With certain camera–flash combinations OTF metering inside the camera can control the flash duration by suitable contacts made when the flash is attached to the camera. These “dedicated” flashes (so named because their control circuitry has to match that of specific cameras) may also signal in the camera finder when the flash is ready to operate and to set the camera automatically to its synchronizing shutter speed.

HAROLD E. EDGERTON

Harold E. Edgerton (1903–1990) was an American electrical engineer and photographer who was noted for creating high-speed photography techniques that he applied to scientific uses.

In 1926, as a graduate student, Edgerton began to experiment with flash tubes. He developed a tube using xenon gas that could produce high-intensity bursts of light as short as 1/1,000,000 second. Edgerton's tube remains the basic flash device used in still photography. The xenon flash could also emit repeated bursts of light at regular and very brief intervals and was thus an ideal stroboscope. With his new flash Edgerton was able to photograph the action of such things as drops of milk falling into a saucer, a tennis racket hitting a ball, and bullets hitting a steel plate or traveling at speeds of up to 2,800 feet (853 metres) per second. The resulting images often possessed artistic beauty in addition to their value to industry and science.

Edgerton explored many uses for his new photographic equipment. During World War II he constructed stroboscopic units to photograph the night operations of enemy troops. After the war he and his associates photographed nuclear test explosions. He later devised methods and equipment to photograph sea life at unprecedented depths.

FLASHBULBS

An older type of flash is an oxygen-filled glass envelope containing a specific amount of aluminum or zirconium wire and means for igniting the wire in the bulb. The wire burns away with a brilliant flash lasting typically about 1/100 to 1/50 second. Each flashbulb can, however, yield only one flash. Current flashbulb systems use four to 10 tiny bulbs, each in its own reflector, arranged in cube or bar carriers that plug into cameras designed for them. The individual flashes are fired in turn by a battery and circuit in the camera through mechanically generated current pulses or other means. In view of the greater convenience of electronic flash, flashbulbs in their various forms are largely obsolescent.

FIRING AND SYNCHRONIZATION

Flash units are usually fired with a switch in the camera shutter to synchronize the flash with the shutter opening. A contact in the camera's flash shoe (hot shoe) or a flash lead connects the unit with this shutter switch. The shutter contact usually closes the instant the shutter is opened. A focal plane shutter must fully uncover the film (generally at a shutter speed of 1/60 second or slower) for flash synchronization. With flashbulbs the shutter must also stay open while the flash reaches its peak brightness—about 1/50 second.

SYSTEM CAMERAS

From the development of the 35-mm miniature camera in the 1930s evolved the concept of the system camera that could be adapted to numerous jobs with a range of interchangeable components and specialized accessories. Today, most moderately advanced 35-mm miniatures take interchangeable lenses, close-up and photomicrographic attachments, filters, flash units, and other accessories. The most elaborate camera systems also include such accessories as alternative finder systems; interchangeable reflex screens, film backs, and magazines; and remote-control and motor-drive systems. Modular professional roll-film and view cameras are built up from a selection of alternative camera bodies, film backs, bellows units, lenses, and shutters. This is the nearest approach to the universal camera, assembled as required to deal with practically every type of photography.

CHARACTERISTICS AND PARAMETERS OF LENSES

FOCAL LENGTH AND IMAGE SCALE

The image scale, or scale of reproduction, is the ratio of the image size to the object size; it is often quoted as a magnification. When the image is smaller than the object, the magnification of the object is less than 1.0. If the image is 1/20 the size of the object, for example, the magnification may be expressed either as 0.05 or as 1:20. For an object at a given distance, the scale of the image depends on the focal length of the lens. A normal camera lens usually has a focal length approximately equal to the diagonal of the picture format covered. A lens of longer focal length gives a larger scale image but necessarily covers less of the scene in front of the camera. Conversely, a lens of shorter focal length yields an image on a smaller scale but—provided the angle of coverage is sufficient—takes in more of the scene. Many cameras, therefore, can be fitted with interchangeable lenses of different focal lengths to allow varying the image scale and field covered. The focal length of a lens in millimetres (sometimes in inches) is generally engraved on the lens mount.

APERTURE ANGLE OF COVERAGE

A lens must cover the area of a camera's film format to yield an image adequately sharp and with reasonably even brightness from the centre to the corners of the film. A normal lens should cover an angle of at least 60°. A wide-angle lens covers a greater angle—about 70° to 90° or more for an ultrawide-angle lens. A long-focus lens covers a smaller angle.

The angle of coverage depends on the lens design. Designations like “wide angle” or “narrow angle” are not necessarily synonymous with “short focus” and “long focus,” as the latter terms refer to the focal length of the lens relative to the picture format.

OPTICAL PERFORMANCE

Lenses usually consist of optical glass. Transparent plastics also have come into use, especially as they can be molded into elements with aspheric surfaces. They are, however, more sensitive to mechanical damage.

ABERRATIONS
CHROMATIC ABERRATION




In chromatic aberration, different wavelengths of light have different focal points.





In chromatic aberration, different wavelengths of light have different focal points.
RESOLVING POWER AND CONTRAST-TRANSFER FUNCTION

One way of testing lens performance is to observe the image it forms of patterns of increasingly closely spaced black lines separated by white spaces of line width. The closest spacing still recognizable in the image gives a resolving power value, expressed in line pairs (i.e., black line plus white space) per millimetre. Photographs of such line patterns, or test targets, show the resolving power of the lens and film combination. For example, a resolution of 80–100 line pairs per millimetre on a fine-grain film represents very good performance for a normal miniature camera lens.

SPECIAL LENS TYPES

Apart from general-purpose camera lenses of various focal lengths, there are lenses of special characteristics or design.

TELEPHOTO LENSES

Long-focus lenses are bulky, because they comprise not only the lens itself but also a mount or tube to hold it at the appropriate focal distance from the film. Telephoto lenses are more compact; their combinations of lens groups make the back focus (the distance from the rear lens element to the film) as well as the length of the whole lens appreciably shorter than the focal length. Strictly, the term telephoto applies only to a lens of this optically reduced length; in practice long-focus lenses of all types tend to be called indiscriminately telephoto or “tele” lenses.

If a camera lens is interchangeable, an accessory teleconverter lens group can be positioned between the prime lens and the camera. This turns a normal lens into an even more compact telephoto system, which is less costly than a telephoto lens but which reduces the speed of the prime lens and usually impairs sharpness performance.

WIDE-ANGLE AND RETROFOCUS LENSES

Short-focus, wide-angle lenses are usually mounted near the film. Single-lens reflex cameras need a certain minimum lens-to-film distance to accommodate the swinging mirror. Wide-angle (and sometimes normal-focus) lenses for such cameras therefore use retrofocus designs. In these the back focus is appreciably longer than the focal length. Both a telephoto and a retrofocus lens must be specially designed for its particular use to ensure optimum image performance.

FISH-EYE LENSES

For image angles greater than 110°, it becomes difficult to bring the lens close enough to the film to allow the rays between the lens and film to diverge sufficiently. The fish-eye lens overcomes this difficulty by making the rays diverge less behind the lens than they do in front. The resulting image shows appreciable distortion, with image details near the edges and corners progressively compressed. Fish-eye lenses usually cover angles between 140° and 210° and are used for unusual wide-angle effects where the distortion becomes a deliberate pictorial element. They also have certain scientific applications, for instance, to cover a horizon-to-horizon view of the sky in recording cloud formations.

MIRROR LENSES VARIABLE-FOCUS LENSES

In variable-focus lenses the focal length can be varied by movement of some of the elements or groups within the lens system. One lens can thus replace a range of interchangeable lenses.

The variable-focus, or zoom, lens was originally developed for motion-picture photography, in which adjustment of the focal length during a shot produced a zooming-in or zooming-out effect (hence the name). It is now widely used in single-lens reflex cameras where the reflex finder permits accurate continuous assessment of image coverage. In a true zoom lens the image changes in scale but not in sharpness during zooming; some varifocal lenses, however, need refocusing at different focal lengths. Due to correction requirements over a range of focal lengths, zoom lenses are complex systems containing from 12 to 20 elements. Zoom lenses for still cameras have focal-length ratios from 2:1 to 4:1 or more (e.g., 35–135 mm for a 35-mm reflex).

LENS-CHANGING SYSTEMS

Miniature and roll-film cameras hold interchangeable lenses in screw or quick-change bayonet mounts. In a focal-plane shutter camera the usable range of focal lengths is practically unlimited. In cameras with leaf shutters, either the lens is mounted in front of the shutter or the lens is changed with the shutter. Some designs use convertible lenses with the rear components built into the camera together with the shutter; interchangeable front groups then provide different focal lengths in combination with the fixed rear group. View-camera lenses—usually with their own shutters—are mounted on lens boards that clip into and out of the front camera standard.

Afocal attachments provide the effect of alternative focal lengths with a fixed camera lens. They are magnifying or reducing telescopes without a focal length (hence afocal), yielding a virtual image that the camera lens projects onto the film. Their designated magnification factor indicates the effect on the image scale; e.g., a 1.5× tele attachment magnifies the image on the film 1 1/2 times, while a 0.7× wide-angle attachment reduces the image scale to 0.7 times that of the prime camera lens.

LENS COATING

To reduce such losses, the air-to-glass surfaces of modern lenses typically carry a microscopically thin coating of metallic fluorides. The coating eliminates most reflected rays. Complete elimination can occur only for light of one wavelength if the coating thickness and refractive index are exactly right. In practice a coated lens surface reflects about 0.5 percent of incident white light—1/10 of the light lost by an uncoated lens. Multiple coatings can reduce reflections over a wider wavelength range.