16.2 OPTICAL INSTRUMENTS
normal lenses (typically to F/1.2) and also requires appropriate design of the exit pupil location on long-focal-length and high-speed retrofocus lenses in order to avoid excessive vignetting.
The minimum back focal length (BFL) allowable on SLR lenses (because of the swinging mirror) is about 38.5 mm. The BFL cannot be too short on non-SLRs because of in-focus dust or cosmetic problems on optical surfaces close to the film plane. The actual limitation depends on the minimum relative aperture that would be used but is rarely less than 4 mm and usually more than 8 mm.
Since most lens accessories such as filters and lens-shades are mounted on the front of a lens, there is a practical limitation to the allowable front diameter of most lenses. Filter sizes larger than 72 mm are not desirable, and smaller is always preferred. The actual clear aperture at the front of a lens is considerably smaller than the filter size, depending on the angular field and the mounting details of the filter. Obviously there are lenses such as 600-mni F/4 telephotos for which the 72-mm limitation is not possible. In these cases, the lens can be designed to use internal filters that are incorporated into the design.
Mechanical cams are still in widespread use for the practical realization of the required motions in zoom lenses. This technology requires that the motions themselves be controlled at the design stage to be reasonably monotonic and often to have certain mutual relationships. These requirements are particularly severe for the so-called "one-touch" zoom and focus manual control found on many SLR zoom lenses.
In general, size and weight restrictions pose the biggest problems for the designer of most camera lenses. Almost any lens can be designed if there are no physical limitations. These limitations are sometimes a consequence of ergonomic considerations but can equally be an effort to achieve a marketing advantage. Size restrictions almost always adversely affect the design, and exceptionally small lenses (for a given specification) should be regarded with suspicion.
16.3 MODERN LENS TYPES
Normal (with Aspherics) and Variations
35-mm SLR normal lenses are invariably Double-Gauss types. Refer to Fig. 1. This lens form is characterized by symmetry about a central stop to facilitate the correction of coma, distortion, and lateral color. These lenses are relatively easy to manufacture and a user can expect good quality in a production lens. Total angular coverage of about 45' is typical, and speeds as fast as F/1 are achievable. Extremely good optical performance is possible, particularly if the angular field and speed are reduced somewhat. Image quality generally deteriorates monotonically from axis to corner and improves dramatically as the lens aperture is reduced by about two F-numbers. With the addition of a fixed rear group, conjugate stability can be achieved over a wide range. Refer to Fig. 2.
An interesting new wide-angle lens type is a four-component form found commonly on the so-called compact 35-mm cameras. This lens is characterized by a triplet construction followed by a rear element that is strongly meniscus-shaped, convex to the image plane. This lens has much less astigmatism than either conventional triplets or Tessars and can cover total fields of up to 75' at speeds of around F/4. Faster speeds are possible if the angular field is reduced. Most importantly, the rear meniscus component takes the burden