| 摘要 |
PURPOSE:To correct satisfactorily an aberration by constituting a zoom lens having four lens groups, so that the fourth lens group is stuck to a lens 4B of a positive refractive power, whose refractive index is lower than that of a lens 4A of a negative refractive power, and its convex surface is turned to an image surface side. CONSTITUTION:In case of variable power, an incident height of a velocity of a light of out-of- axis to the fixed fourth lens group is the lowest at the wide angle end, and becomes the highest extending from a middle focal distance to a telephoto end. Accordingly, the fourth lens group is constituted of a cemented lens of a meniscus shape, which has stuck a lens 4A of a negative refractive power and a lens 4B of a positive refractive power consisting of glass whose refractive index is lower than that of the lens 4A, and whose convex surface has been turned to an image surface side. In this way, a meridional image surface of a correction shortage extending from the middle focal distance to the telephoto end is corrected appropriately, and a variation of an astigmatism is reduced as a whole, and a variation of a spherical aberration can also be reduced. In this regard, when leading an aspherical surface into the third lens group, when a focal distance of the third lens group and a focal distance in a telephoto end of the whole system are denoted as f3 and fT, respectively, they are set so as to become 0.12fT<f3<0.35fT, and a variation of an aberration caused by variable power is reduced. |
