CAMERA LENSES 16.5
of field flattening away from the triplet front part. This results in considerably lower individual element powers and correspondingly lower sensitivities to tilts and decentrations of the elements. It is this problem that makes conventional triplets extremely difficult to manufacture. Refer to Fig. 3.
Inverted Telephoto (Retrofocus)
These lens types, characterized by a long back focal length, are typically used for wide-angle applications for single lens reflex cameras having a swinging viewing mirror behind the lens. Inverted telephoto implies a front negative group followed by a rear positive group, just the reverse of a telephoto construction. This type of construction tends to result in relatively large front aperture sizes, and it is not easy to design small lenses without compromising on image quality. Retrofocus designs sometimes have a zone of poorer image quality in a field area between the axis and the corner. This zone is a by-product of the struggle to balance lower- and higher-order aberrations so that the outer parts of' the field have acceptable image quality. These lenses have particularly good relative illumination both because the basic construction results in an exit pupil quite far from the image plane and also because it is possible for the size of the pupil to increase with field angle. In order to achieve conjugate stability, it is necessary to employ the use of so-called "floating elements" or variable airspaces that change with focusing. However, this feature does result in additional optomechanical complexity.
The newer forms of this lens type fall into four broad subcategories.
Very Compact Moderate Speed. These include six-element 35-mm F/2.8 with a front negative element and seven-element 28-mm F/2.8 with a leading positive element. Refer to Figs. 4 and 5, respectively. These relatively simple constructions are suitable for speeds of F/2.8 or slower and total angular coverages of up to 75º.
Highly Complex Extreme Speed. As the complexity of both the front and rear groups is increased, the inverted telephoto form can be designed to achieve speeds of F/1.4 and angular fields of 90º. The use of aspherical surfaces is essential in order to achieve these specifications. Refer to Figs. 6, 7, 8, and 9.
Highly Complex Extreme Wide-angle with Rectilinear Distortion Correction.
These arc inverted telephoto designs covering total fields of up to 120', often with speeds as fast as F/2.8. Distortion correction is rectilinear. The chromatic variations of distortion, astigmatism, and coma are usually the limiting aberrations and arc virtually impossible to correct a certain point. Refer to Figs. 10 and 11.
Extreme Wide-angle with Nonrectilinear Distortion ("Fish-eye Lenses"). Without the requirement of rectilinear correction of distortion, inverted telephoto designs can be achieved quite readily with total angular fields exceeding 180º. For these lenses, the image height h and focal length f are often related by h - 0, where 0 is the semifield angle. See, for example, USP 4,412,726.
The term telephoto strictly applies to lenses having a front vertex length less than the focal length (telephoto ratio less than one). The classic telephoto construction has a front positive group followed by a rear negative group. This can lead to telephoto ratios that are as short as 0.7 or less. The term telephoto is often loosely used to refer to any