A zoom lens unit includes, in order from an object side to an image side, a first lens group having positive refractive power and a focus function, a second lens group having negative refractive power, which moves when changing a magnification, a third lens group which moves when changing a magnification, and a fourth lens group having positive refractive power, which is fixed when changing a magnification, wherein the following condition (1) is fulfilled:
|M.sub.2W.Math.M.sub.3W.Math.M.sub.4W|<0.14(1)
where M.sub.2W represents an imaging magnification of the second lens group at a wide-angle end, M.sub.3W represents an imaging magnification of the third lens group at the wide-angle end, and M.sub.4W represents an imaging magnification of the fourth lens group at the wide-angle end.

A zoom lens unit includes, in order from an object side to an image side, a first lens group having positive refractive power and a focus function, a second lens group having negative refractive power, which moves when changing a magnification, a third lens group which moves when changing a magnification, and a fourth lens group having positive refractive power, which is fixed when changing a magnification, wherein the following condition (1) is fulfilled:
|M.sub.2W.Math.M.sub.3W.Math.M.sub.4W|<0.14(1)
where M.sub.2W represents an imaging magnification of the second lens group at a wide-angle end, M.sub.3W represents an imaging magnification of the third lens group at the wide-angle end, and M.sub.4W represents an imaging magnification of the fourth lens group at the wide-angle end.

A projection zoom lens is essentially constituted by, in order from the magnification side: a negative first lens group; a positive second lens group; a third lens group; a positive fourth lens group; a fifth lens group; and a positive sixth lens group. The distance between the first and second lens groups is shorter, the distance between the second and third lens groups is longer, the distance between the third and fourth lens groups is shorter, the distance between the fourth and the fifth lens groups is longer, and the distance between the fifth and sixth lens groups is longer at the telephoto end than at the wide angle end. A lens having a negative refractive power in the paraxial region is provided most toward the magnification side in the first lens group. The fifth lens group has a negative lens.

A projection zoom lens is essentially constituted by, in order from the magnification side: a negative first lens group; a positive second lens group; a third lens group; a positive fourth lens group; a fifth lens group; and a positive sixth lens group. The distance between the first and second lens groups is shorter, the distance between the second and third lens groups is longer, the distance between the third and fourth lens groups is shorter, the distance between the fourth and the fifth lens groups is longer, and the distance between the fifth and sixth lens groups is longer at the telephoto end than at the wide angle end. A lens having a negative refractive power in the paraxial region is provided most toward the magnification side in the first lens group. The fifth lens group has a negative lens.

A zoom lens, includes, in order from object side: a front unit including a unit including a first unit arranged closest to the object side and not moving for zooming; an Nf unit including three or more lenses and having a negative refractive power; an stop; a first rear unit moving during zooming; a second rear unit moving during zooming; and a third rear unit not moving for zooming, in which: the front unit includes four or more lenses and includes one or more units having a positive refractive power; and a difference between positions of the first rear unit at a wide angle end and a telephoto end, a difference between positions of the second rear unit at the wide angle end and the telephoto end, a focal length of the second rear unit, and a focal length of the zoom at the wide angle end are appropriately set.

A zoom optical system includes a negative first lens group having a first deflection optical element, a positive second lens group, a positive third lens group, a positive fourth lens group, and a second deflection optical element, in that order from the object side. The first lens group is provided at a fixed position relative to an imaging plane. Zooming is performed by moving at least the second and third lens groups so that distances between adjacent lens groups of the first through fourth lens groups change. The third lens group is a focusing lens group which moves along the optical axis during focusing. The following condition (1) is satisfied:

0.2<f3/f4<1.0(1),

wherein f3 designates the focal length of the third lens group, and f4 designates the focal length of the fourth lens group.

The zoom lens according to the present invention includes in order from an object side to an image side: a first lens unit; a second lens unit; a third lens unit; an aperture stop; and a fourth lens unit; wherein a partial dispersion ratio Lm of a material of a positive lens Lm formed of a material having a highest partial dispersion ratio, which is contained in a rear group, a distance d from the aperture stop to an image plane, a distance dLm from the aperture stop to a positive lens Lm, an average value Rfp of Abbe constants of materials of positive lenses contained in a front group, an average value Rfp of partial dispersion ratios thereof, an average value Rfn of Abbe constants of materials of negative lenses contained in the front group, and an average value Rfn of partial dispersion ratios thereof are each appropriately set.

An ophthalmic surgical microscope includes a beam coupler positioned along an optical path of the surgical microscope between a first eyepiece and magnifying/focusing optics, the beam coupler operable to direct the OCT imaging beam along a first portion of the optical path of the surgical microscope between the beam coupler and a patient's eye (an OCT image being generated based on a reflected portion of the OCT imaging beam). The surgical microscope additionally includes a real-time data projection unit operable to project the OCT image generated by the OCT system and a beam splitter positioned along the optical path of the surgical microscope between a second eyepiece and the magnifying/focusing optics. The beam splitter is operable to direct the projected OCT image along a second portion of the optical path of the surgical microscope between the beam splitter and the second eyepiece such that the projected OCT image is viewable through the second eyepiece.

A variable magnification optical system includes, in order from the object side: a positive first lens group, which is fixed when changing magnification; a negative second lens group that moves from the object side to the image side when changing magnification from the wide angle end to the telephoto end; and a rearward lens group having a positive refractive power throughout the entire variable magnification range that includes at least one lens group that moves when changing magnification. The first lens group includes, in order from the object side, a positive first lens group front group, a positive first lens group middle group, and a first lens group rear group constituted by a negative lens. The first lens group front group and first lens group middle group include cemented lenses formed by cementing a negative lens and a positive lens, provided in this order from the object side, together.

There are provided an optical system and an imaging device including the same. The optical system includes: in order from an object side to an image side, a first lens group having refractive power; and a second lens group having positive refractive power. A variable distance that varies during focusing between the first lens group and the second lens group is closer to an object than a variable distance that varies during focusing between other lenses. The first lens group includes a positive lens and at least one or more negative lenses in order from the image side. The second lens group includes a lens subgroup 2A having positive refractive power, an aperture stop, and a lens subgroup 2B having positive refractive power in order from the object side. The lens subgroup 2B includes a negative lens and a positive lens in order from the object side and satisfies a predetermined Conditional Expression.