| 摘要 |
PURPOSE:To enable to vary focal length with compact constitution, by bearing prescribed relation between a master lens and auxiliary lens. CONSTITUTION:Auxiliary lens 12 provided, being insertable and extractable, on the optical path between master lens 11 and an image surface, varies the focal length of the whole lens system. With lens 12 insertd extracted, the distance between lens 11 and the image surface is unchanged and the conditions of formulas I are satisfied. In the formulas, phi1 is the power of lens 11, e1' the distance from the image-field-side principal point of lens 11 to the object-field-side principal point of lens 12, phi2 the power of lens 12, (DELTA) the principal-point interval of lens 12, and (m) the ratio between the power of only lens 11 and that of the whole system when lens 12 is equipped. Further, lens 12 has positive power and consists of positive, negative and positive lenses. |
