Widely-adjustable lens unit

摘要

The unit comprises three or more groups of lenses. The first of these, nearest the objective, has positive refraction, as does the second, whereas the third has negative refraction. The groups are movable in the direction of the objective between positions for minimum and maximum focal length. The first group comprises a first negative lens nearest the objective, followed by second and third positive ones. The group characteristics are determined by formulae taking account of the lateral enlargement of the third group at maximum focal length, the focal length of the first group, the Abbe numbers of the first and second lenses in the first group, and the maximum focal length of the complete system. The minimum focal length of the complete system, and the resultant focal length of the first and second lenses in the first group, can also be taken into account.