Lens
From Encyclopædia
A lens, in OPTICS, is a piece of transparent material shaped to form an image by bending
light rays (see REFRACTION). In ancient times lenses were called "burning glasses" because they could focus the Sun's rays to start fires. Their magnifying power has long been known (see EYEGLASSES; MICROSCOPE), but their use in TELESCOPES awaited the development of compound lenses. Other systems for focusing other kinds of beams are sometimes also called lenses (see
electron MICROSCOPE). In astronomy, whole galaxies may function as lenses for more distant
light (see
gravitational lens).OPTICS OF LENSESA simple lens has two opposing faces, at least one of which is curved. Most lenses have spherical surfaces, but other curvatures are used for special purposes.Lens ShapesA lens is called converging, or positive, if
light rays passing through it are deflected inward. It is called diverging, or negative, if the rays spread out. Converging lenses are thicker at the middle, whereas diverging lenses are thicker toward the edges. A lens surface is concave (curved inward), plane, or convex (curved outward). A meniscus lens has one concave and one convex surface.Image FormationThe focal length of a single lens is the distance from the lens to the point at which incoming parallel rays focus.
light converged in this manner can produce a real image, that is, an image that can actually be projected onto a screen. In a negative lens, rays do not actually come to a real focus, but appear to originate from a point called the virtual focus. The focal length of a diverging lens is considered to be negative.When the diverging rays from a point source, which may be an actual source of
light or a point on an extended object, are passed through a lens, the emerging rays produce an image that will be either real or virtual, depending on the type of lens and the location of the source.A simple convex lens, or magnifying glass, will produce a virtual image if the object is either at the
focal point or between the
focal point and the lens.
light from the object passes through the lens, and the eye focuses it onto the retina. The object appears enlarged behind the lens. If the object is beyond the
focal point of the same lens, a real image can be formed. A magnifying glass held in front of a
light bulb will cause an image of the filament to be projected onto a screen or wall across the room.The Lens EquationWhether real or virtual images are formed, the image distance q can be calculated from the lens equation when the distance of the object to the lens, p, is known: 1/p +1/q = 1/f. Here f is the focal length of the lens, positive for a converging lens and negative for a diverging lens. The equation also shows whether the image is real or virtual; positive values of q indicate a real image and negative values correspond to a virtual image.Image DefectsThe ability of a simple lens to form a perfect optical image is limited by certain inherent defects called ABERRATIONS. Chromatic (
color) aberration is a defect caused by different wavelengths of
light refracted at slightly different angles, so that the focal length is slightly different for each
color. This results in a colored fringe around an image. The defect can be corrected by a two-component design made with different kinds of glass.
color-corrected lenses are called achromatic lenses and are used in all
fine optical instruments. Astigmatism, coma, and spherical aberration are defects that cause an image to be blurred.The f-NumberThe f-
number of a lens is the ratio of the focal length to the lens diameter. Lenses of large diameter have small f-
numbers and hence greater
light-gathering power than lenses of small diameter or large f-
number. Aberrations generally become more serious as the f-
number decreases. This factor is particularly important in the design of
camera lenses, the development of which dates from about 1840 with the introduction of the Petzval portrait lens, a combination of three components. Protar, Dagor, and Tessar
camera lenses were developed during the latter part of the 19th century, and their basic designs are still used. Some modern
camera lenses use 12 or more components. The
Advent of electronic computers has allowed rapid advances in their design.Lens MaterialsNew kinds of optical glasses have been developed in recent years, such as rare-earth glass, having a relatively high
index OF REFRACTION. Techniques for making good-quality plastic lenses have been introduced in recent years. Such lenses have advantages in special applications, such as spectacle lenses.Lenses for use in the ultraviolet and infrared regions of the
spectrum must be made of special materials, because ordinary optical glass is opaque to these wavelengths.
quartz is the most common material for ultraviolet-transmitting lenses. Infrared lenses are made with calcium fluoride,
sodium chloride (salt) or other alkali
halides, or
silicon. GRANT R. FOWLESLENSES OF OPTICAL INSTRUMENTSThe lens of a
camera is a converging lens for forming a real image of the scene (object) that is?to be recorded photographically. A simple convex lens theoretically focuses the incoming
light beam, but in practice such a lens suffers from numerous types of defects, called aberrations, which cause blurring and distortion of the image. A lens for an optical instrument such as a
camera, ENLARGER, or PROJECTOR generally consists of up to eight or more simple lenses, or lens elements. These elements may be used either separately or cemented together in a group called a lens component. Such arrangements are designed to reduce aberrations to a minimum. The degree of reduction or enlargement of an image with respect to the size of the object depends on the focal length of the lens used.ApertureThe brightness of the image is determined by the relative
aperture, or f-
number (see
APERTURE). In the same way that the amount of
light falling on a wall opposite a circular window can be determined by calculating the ratio of the room length to the window diameter, the relative
aperture is defined as the ratio of the focal length to the diameter of the effective
aperture of the lens. This
aperture can be varied by means of a diaphragm. For given lighting conditions, a lens of small
aperture (large f-
number) gives a dim image that requires a relatively long exposure. This shortcoming, however, is compensated by the decreased aberrations, and hence increased sharpness, of the image. On the other
hand, wide-
aperture (small f-
number) lenses, which are required to give a bright and sharp image, need many lens elements to reduce aberrations and are therefore expensive and bulky.Angle of ViewThe angle of view--the amount of the field that the lens will cover--depends on the lens's focal length. The field of a
camera lens may be as small as about 15 deg or as large as about 140 deg. A standard lens may cover 60 deg; a wide-angle lens, 90 deg; and a telephoto lens, 30 deg.A wide-angle lens forms an image of a wide field of view. The scene appears smaller and more distant than it actually is. Such a lens might be used, for example, to take a close-up shot of a tall building. Such lenses, however, may introduce considerable distortion, especially at the edges. A wide-angle portrait photograph may, for example, show
hands that appear disproportionately large.A telephoto lens is a lens combination that has a long focal length, without the bulk of a standard long-focus lens. It makes distant objects appear closer and larger but covers a narrower area than a standard lens.A zoom lens is one with a focal length that may be varied; it is also called a variable-focus lens. The zoom lens consists of fixed and movable elements. The focal length is changed by moving one or more groups of variable elements mounted in a movable barrel along the axis. The variation is continuous, and in a
reflex camera the effect may be followed on the ground-glass screen. In many situations it is more convenient to use a zoom lens than to interchange several lenses, especially in CINEMATOGRAPHY. Unfortunately, although zoom lenses continue to be improved, such lenses are expensive, and the definition of the image is somewhat inferior to that produced by lenses of fixed focal length.LENS GRINDINGLens grinding is the process by which optical lenses are manufactured from glass. (Transparent
plastics, which are used to produce certain types of lenses, are molded rather than ground.)
fine lenses are made from specially prepared optical glass, which must be free of metallic impurities that might cause discoloration. Optical glass is often cast in blocks, although it is also available in strips, panes, and rods, or it may be molded roughly into lens shape. A lens blank is cut off the glass block and then rough-ground, using a diamond abrasive on a grinding
wheel, to a shape approximating its final dimensions.
fine grinding, or lapping, is accomplished by using
Carborundum or
emery abrasives in a convex or concave lapping tool made of a rigid material:
iron, for
Mass-produced lenses such as those made for eyeglasses; or glass, for high-precision optical lenses. A
number of?small lenses may be mounted together on a spherical block and lapped simultaneously. For
fine optical glass, the final polishing may take many hours, using a soft lapping tool that is shaped precisely to fit the lens. Harder polishing laps made of
resin or plastic are used for lenses of lower accuracies. Both sides of the lens are usually polished, and the lens edge is ground so that its axis is centered and it has the correct diameter.
