An optical system includes, in order from an object side toward an image side, a first lens unit having positive refractive power, and a second lens unit having negative refractive power. A distance between consecutive ones of the lens units changes when focusing is performed. The first lens unit is stationary during focusing. The second lens unit is moved toward the image side when focus is changed from an object at infinity to an object at a short distance. Lateral magnification β2 of the second lens unit when focusing on the object at infinity, focal length f1 of the first lens unit, and focal length f2 of the second lens unit are set appropriately to satisfy predetermined mathematical conditions.

An optical system includes, in order from an object side toward an image side, a first lens unit having positive refractive power, and a second lens unit having negative refractive power. A distance between consecutive ones of the lens units changes when focusing is performed. The first lens unit is stationary during focusing. The second lens unit is moved toward the image side when focus is changed from an object at infinity to an object at a short distance. Lateral magnification β2 of the second lens unit when focusing on the object at infinity, focal length f1 of the first lens unit, and focal length f2 of the second lens unit are set appropriately to satisfy predetermined mathematical conditions.

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.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.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 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.

An optical system is provided. The optical system includes a first lens group that has a positive refractive power, a second lens group that has a negative refractive power, a third lens group that has a negative refractive power, a fourth lens group that has a positive refractive power, and a fifth lens group that includes one or more lens groups, in sequence from an object, wherein the first lens group and the fifth lens group are fixed and the third lens group moves parallel to an optical axis to perform focusing when adjusting a magnification of the optical system.

An optical system is provided. The optical system includes a first lens group that has a positive refractive power, a second lens group that has a negative refractive power, a third lens group that has a negative refractive power, a fourth lens group that has a positive refractive power, and a fifth lens group that includes one or more lens groups, in sequence from an object, wherein the first lens group and the fifth lens group are fixed and the third lens group moves parallel to an optical axis to perform focusing when adjusting a magnification of the optical system.

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.