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16.20 OPTICAL INSTRUMENTS

and a compensating group has become academic. As a result, small internal zooming groups can serve a dual function as focusing groups under the control of an autofocusing system. Refer to Figs. 16, 17, and 18.

A recent new development in zooms is one for the so-called compact 35-mm camera. It its most basic form, this type can have as few as three elements and is characterized b, having a front positive component and a rear negative component. This lens has an inherently short back focal length at the wide end, making it not suitable for SLR camera with swinging mirrors. In more complicated versions, this idea can be extended to 28 to 160 mm or further, the main limitation being a small relative aperture at the long-focal length end. A recent practical embodiment is a four-element 38- to 90-mm F/3.5 to 7.7 having three aspherical surfaces. See, for example, USP 4,936,661.

Zoom lenses are also found on most consumer video cameras. The classic fixed front and rear-group type (with the aperture stop in the rear group) is still commonly use( because the very small format sizes can permit acceptably small lenses. This lens form is also used for motion picture and television zooms. In many of these applications, it is desirable to have an exit pupil position that does not change with zooming. Telecentricity of the exit pupil is also sometimes required. In addition, the motion picture industry still prefers zoom lenses that have conventional front-group focusing in order to easily calibrate tape-measure focus measurements.

Very long range television zooms (often 30 : 1 or more) are also of the fixed front and rear type, with a succession of cascading zooming groups in between.

16.4 CLASSIFICATION SYSTEM

A wide variety of camera lenses has been classified in Table 1 in terms of total pixel capability P and pixels per steradian AD. Pixels are defined as digital resolution elements relative to a specified modulation level and are calculated as follows:

The polychromatic optical transfer function of each lens is calculated and the spatial frequencies at which the modulation falls to 0.5 and 0.2 is noted at each of five field points The lower of the meridional and sagittal values is used.

The image field of the lens, assumed to be circular with diameter D, is divided into four annular regions. The outer boundaries of each region correspond respectively to 0.35H, 0.7H, 0.85H, and 1.OH, where H is the maximum field height. The area of each region is computed.

The average of the inner and outer boundary-limiting spatial frequency values Is assigned to each region. This is done for both the 0.5 and 0.2 modulation levels.

The area of each annular region, in square millimeters, is multiplied by the square of the spatial frequency values from the previous step to yield regional pixel counts for both 0.5 and 0.2 modulation levels.

The pixel counts are summed over all regions to yield the D data in Table 1.

The AD data in Table 1 are obtained by dividing the total pixel values by the solid angle of the lens in the object space. The solid angle S is given by the following formila:

S = 2π(1 - cos W)

where W is the semifield angle of the lens in degrees.

In general, for a given image diameter D, a larger P implies higher image quality or greater information-gathering capability. A lens designed for a smaller D will have a lower P than a lens of similar quality designed for a larger D. These same generalizations hold for AD except that, in addition, a lens designed for a smaller field angle and a given D will have a larger AD than a lens of similar image quality designed to cover a wider field for