| 摘要 |
PURPOSE:To obtain a lens which has a prescribed quantity of back focusing and satisfactorily corrects aberrations over the entire variable power range with a smaller number of lens elements by selecting the optical specifications of an optical system including lenses for forming respective groups so as to satisfy prescribed conditions. CONSTITUTION:The zoom lens having the focal length Fw of the entire system at a wide angle end is formed of a 1st lens group having a negative refracting power and focal length F1 and the 2nd lens group having a positive refracting power and focal length F2. The 1st lens group is formed of a negative meniscus lens the convex face of which is directed to an object side, a negative lens the concave face of which is directed to an image plane side and the image plane side curvature on the image plane side is R12b and a positive lens the convex face of which is directed to the object side. The 2nd lens group is formed of a lens which has convex faces on both faces and N21 refractive index, a positive lens which has R22a image plane side curvature on the object side and has a large curvature on the image plane side, a negative lens which has a large curvature on the image plane side and f23 focal length, and a lens which has convex faces on both faces. The respective optical specifications are so selected as to satisfy the conditions expressed by the equations. The zoom lens which has the prescribed quantity of back focusing and satisfactorily corrects aberrations in the variable power range with a smaller number of lens is thus obtd.
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