Cameras, Lens Designs: Wide Angle and Zoom

The function of camera lenses is to refract light rays and bend them to form an image inside the camera of a subject outside the camera, to be captured on a photosensitive surface such as film, or more recently, on photosensitive surfaces in digital cameras. Lens design strategies have always been dedicated to improving the quality and fidelity of the camera’s images because basic lenses generally are afflicted by various limitations and imperfections, called aberrations.

The story of lens design has been largely a tale of the struggle to maximize lens performance—seeking to increase the effective aperture size (or speed) and improve resolving power and sharpness— while eliminating the aberrations or imperfections which degrade the images they produce. Probably the first camera lens to be used in nineteenth century photography was the singlet landscape lens suggested by William Hyde Wollaston for camera obscuras in 1812.

The lens, produced in a meniscus shape, had some of its faults corrected by experimentally locating the aperture at the optimum distance from the lens, and it was used on simple cameras for many decades. The first design improvement for this basic photographic type was the Chevalier lens for daguerreotype cameras, consisting of two glass elements cemented together.

The first lens of high relative aperture that facilitated portraiture with light-sensitive materials of extremely low sensitivity, such as the daguerreotype, was designed by Josef Petzval in 1840. It contained a telescope objective at the front, widely spaced from another telescope objective at the rear. This basic design was a classic type, utilized widely throughout the history of photography for still cameras, motion picture projectors, as well as microscopes. Originally used for portraiture, the Petzval design was eventually supplanted by other designs.

The most popular camera lens of the latter nineteenth century until 1910 was the symmetrical ‘‘duplet’’ consisting of two identical lens groups on either side of a central diaphragm, because this design eliminated many types of lens aberrations. Other activity in nineteenth century lens design included the use of new types of glass and experimentation with both symmetrical and asymmetrical systems.

By 1893 a new basic design, the ‘‘triplet,’’ was introduced, consisting of three airspaced singlets or three air-spaced lens groups. In the twentieth century enormous quantities of triplet lenses were manufactured for moderately priced folding cameras and other cameras. Considerable research went into methods of modifying the triplet design to produce faster, higher-aperture lenses.

The relationship between the focal length of a camera lens and image size has been somewhat arbitrarily codifed over the years. A ‘‘standard’’ lens is usually considered to be one whose focal length approximates the diagonal dimension of the rectangular image produced by the camera, on the theory that this produces ‘‘normal’’ perspective. This is a simplistic notion, as the perception of perspective is more complex than many realize, but the standard remains.

Thus a 35 mm camera is routinely equipped with a lens of about 45 to 55 mm. A lens of shorter focal length is considered ‘‘wide angle,’’ although it became fashionable at the end of the twentieth century for photographers who prided themselves on selecting a wide-angle lens as their personal ‘‘standard’’ workhorse. A specially constructed lens of extreme wide angle, called a fisheye lens, has scientific applications, but it was popularized as an experimental tool for seemingly distorted and intentionally bizarre effects.

A lens of greater than standard focal length is called ‘‘long- focus’’ or ‘‘telephoto.’’ A telephoto lens denotes a particular design type, however, which is intended to produce a lens significantly shorter than its effective focal length would suggest; that is, the total length from the front lens to the image plane is actually less than the focal length. The second principal plane lies outside the positive end of the lens system, or the rear principal plane will be in front of the front component. Not every long-focus lens is a true telephoto, despite the confusing tendency of photographers to use such nomenclature uncritically.

Telephoto construction requires a positive front element that is widely separated from a negative rear component. As it is more difficult to correct aberrations in a telephoto lens, a long-focus lens of conventional construction may be preferable; a telephoto is used when the need for compactness equals or outweighs the concern for optimum quality.

Perhaps the single most important contribution of the twentieth century to photographic lenses, especially since it came to be utilized almost universally, was the application of hard, permanent coatings on lenses to reduce the surface reflectivity of the glass. The fact that coatings could reduce reflections was discovered by H. Dennis Taylor in 1896 when he worked with tarnished lenses, but attempts to reproduce such tarnish with precision were unsuccessful. In 1936 John Strong suggested depositing a thin layer of a low-index material such as calcium fluoride onto lens surfaces,.

The thin film reduces surface reflections through interference, in which beams of light passing through the coated lens interfere with each other and eliminate the reflected light. Commercial lens coatings were offered in December 1938, and the method was subsequently greatly improved. In addition to the very real technological advance that lens coating represented, the esthetic appeal of coated lenses evidently had an impact on the market.

Advertising images of cameras with gleaming lens coatings of subtle, attractive colors—browns, blues, magentas, and purples—helped to make amateur photography one of the most important hobbies of the twentieth century. Lens coatings tended to connote precision and drew upon a ready market of gadget collectors.

A ‘‘zoom’’ lens has some means for continuously varying the focal length of the lens while retaining image focus at the film plane. Originally popular for motion-picture photography for the obvious advantage of both continuous and seamless zoom effects, as well as the ability to change the apparent camera-to-subject distance rapidly without the need to change lenses. They were extremely difficult to design and manufacture with adequate corrections, however, and they did not become plentiful until the mid-twentieth century.

The best known zoom lens at that time, the Zoomar, had 22 separate lens elements in the 16 mm motion-picture model and an even larger number in its 35 mm counterpart. In the late twentieth century low-cost manufacture of zoom lenses made them more available to the amateur market, and many 35 mm cameras were routinely sold with variable-focus lenses. Eventually small ‘‘point-and-shoot’’ 35 mm cameras became ubiquitous, and most were equipped with zoom lenses in lieu of interchangeable lens capability.

All modern lenses are corrected achromatically, which means that two different colors will focus at the same distance between the lens and focal plane, but the difficulty increases with faster telephoto and zoom lenses. Apochromatic (APO) lenses are corrected to focus three different colors in the same plane. Such designs were expensive until the 1990s.

Engineers rely upon computers to develop improved optical and glass manufacturing technology to provide high-quality low-dispersion glass. The availability of computerized lens design undoubtedly constitutes one of the most important advances in the history of twentieth century photographic optics.

Unusual lenses have been manufactured for special purposes. One of the most striking is the anamorphic lens developed for widescreen CinemaScope motion pictures in the 1950s. This lens could be used to compress laterally a widescreen image into the normal dimensions (1:1.85 aspect ratio) of a conventional 35 mm movie frame, then expand the image to fill the CinemaScope screen, with its extreme aspect ratio (1:2.35).

Another exciting late twentieth century innovation relating to camera lenses actually has only an indirect relationship to optics. This was the advent of automatic focus or autofocus camera systems: properly speaking, these methods involve the use of a motor to focus the lens for the user, operating via signals received through such technologies as SONAR, infrared pulses, and computer analysis.