| 摘要 |
PURPOSE:To obtain three kinds of focal length with only switching of easy operation by providing an afocal converter consisting of a lens system constituted with two lenses, namely, a positive lens and a negative lens and satisfying specific conditions. CONSTITUTION:The afocal converter consists of the thin positive lens and a negative lens and satisfies conditions of formulas where r1 and r4 are the radius of curvature of the first face in the wide angle end and that of the fourth face in the wide angle end respectively and d1, d2, and d3 are the thickness of the first lens in the wide angle end, the interval between the first and second lenses in the wide angle end, and the thickness of the second lens in the wide angle end respectively and nu1 and nu2 are Abbe's numbers of the first and second lenses in the wide angle end respectively and DF is the diameter of larger one of the first and second lenses. The afocal converter is provided on the object side of a focusing master lens and is reversed or removed out of the optical path by the easy operation to switch the focal length.
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