In general, a macro-photographing or -filming lens experiences difficulties in compensating for aberration since a variation in the aberration becomes greater as a magnification is higher. To cope with it, there has been developed a lens of the so-called floating system in which more than one groups of lens pieces are moved during focusing.
One of a variety of lenses suitable for macro-photographing or -filming in the prior art is that which has the foremost lens group closest to an object that stay still during focusing, the succeeding middle lens groups at least two or more of which are moved for focusing, the further succeeding image stabilization lens group of negative refractivity that are moved to points indicated by coordinates along an axis perpendicular to the optical axis to shift a position of imaging in directions perpendicular to the optical axis, and the rearmost lens group closest to the image plane and of positive refractivity; and the rearmost lens group includes a plurality of lens pieces of positive and negative refractivities. The imaging lens meets the requirements as follows:1.7<Nbna Nbpa<1.71.85<|fa/f|<2.80where Nbpa is the average of refractive indices of the lens pieces of positive refractivity, Nbna is the average of refractive indices of the lens pieces of negative refractivity, fa is a composite focal length from the foremost lens group to the anti-vibration lens group, and f is a focal length of the entire optical system (see Patent Document 1 listed below).
Another of the prior art lenses suitable for macro-photographing or -filming is that which includes the foremost or first lens group closest to an object and of positive refractivity, the second foremost lens group of negative refractivity, the succeeding third lens group of positive refractivity, the fourth lens group of positive refractivity, and the rearmost or fifth lens group of negative refractivity. During focusing on the object from a point at infinity to a close range, the first lens group stays still, the second lens group is moved toward the image plane, the third lens group is moved toward the object, the fourth lens group is moved toward the object along a trajectory different from that of the third lens group, and the fifth lens group stays still. The imaging lens meets the requirements as defined in the following formulae:0.89≦f4/f3<2.0where f3 is a focal length of the third lens group, and f4 is a focal length of the fourth lens group (see Patent Document 2).
An additional one of the prior art lenses suitable for macro-photographing or -filming is an optical system capable of photographing the object at a close range, which has its first focusing lens group moved along the optical axis for focusing on the object from a point of infinity to a close range, its second focusing lens group moved by a displacement different from that of the first focusing lens group during focusing, and its third focusing lens group moved by a displacement different from those of the first and second focusing lens groups during focusing; and at least one of the focusing lens groups has a single lens piece (see Patent Document 3).
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