This is one of the most powerful design forms and is the basis of most high-speed (over f/2) camera lenses. The Cooke triplet is probably the most widely used photographic lens, either in its original form or in one of its many derivatives. At the other extreme, high-index crown glasses make speeds above f/2 feasible for short-focal length, narrow-angle applications. At f/5.6, compact triplets can cover fields up to 60°. As a result, it is a popular and economical choice for a wide range of applications. Consisting of two positive crown elements on either side of and spaced away from a negative flint element, this lens gives the designer the necessary freedom (shapes, powers and spacings) to do a reasonably complete correction job. The Cooke triplet anastigmat is the simplest lens in which all the primary aberrations can be corrected. This design form is also used extensively in high-quality commercial process lenses and in enlarging lenses. It is an excellent general-purpose photographic lens and can be designed with a wider-than-normal field. Historically known as the Celor, this lens was derived from the Dagor, although it can be regarded as a triplet with a split flint element.
NOFLEXAR SYSTEM LENS CONSTRUCTION SCHEMATICS FULL
Although limited to speeds of f/8 to f/4.5 by heavy residual spherical aberration, these anastigmats are still useful as moderately wide-angle lenses, and are capable of excellent definition when stopped a bit below their full aperture. Correction of chromatic and spherical aberration was accomplished by the cemented surfaces. The first true anastigmats used a thick meniscus construction to achieve the wide separation of convex and concave surfaces necessary to flatten the field. Known as the rapid rectilinear, this was once a very popular lens for medium-priced cameras. However, the remaining spherical aberration and the field curvature limit the lens to a speed of about f/8. If the two menisci are achromatized, the chromatic aberration is eliminated and the spherical is somewhat reduced. Spherical aberration is slightly better than in the single meniscus, but axial color and field curvature are slightly worse. Symmetrical about the stop, it has greatly reduced coma, lateral color and distortion. The arrangement of the two simple meniscus lenses on either side of the aperture stop is called a periscopic lens. In a wide-angle configuration there is usually a fairly complex positive member closely enveloping the stop with symmetrically arranged meniscus negative elements on either side. A new class of wide-angle lens has evolved from this form, as the bending of ray bundles outward by the negative member lends itself to this application. In its early form the rear member was a standard objective, but modern versions tend to be designed as a unit and are difficult to classify. Because of its relatively large back-focus and size, it is most useful for short-focal-length, wide-angle applications. The retrofocus, or reversed telephoto lens, is usually composed of an ordinary objective with a negative component near its front focal point. Diffractive surfaces have been used in inexpensive cameras, eyepieces for headmounted displays and several other applications.
Such a surface can be molded on a plastic lens.
A diffractive surface (which is a Fresnel surface with a step height of λ/(n-1)) can be used to achromatize, athermalize or correct the aberrations of a single element lens. If the meniscus lens is compounded, chromatic aberration can be corrected and spherical reduced slightly, but this is not an economical solution. The stop may be before or behind the lens, although it is always on the concave side. Spherical and chromatic aberrations and field curvature are totally uncorrected, but are rendered tolerable by the low speed -about f/10 to f/15. The stop also selects the pencil of off-axis rays that passes through the lens this controls astigmatism and eliminates coma. To get an image of good quality it must be used with a separate aperture stop, reducing the aperture of the lens and the aberrations and their effects. The simple meniscus lens is almost universally used in inexpensive cameras.
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