A zoom lens system is an optical system that includes, in order from an object side to an image side, a first lens group having one first lens element with positive optical power, a second lens group having one second lens element with negative optical power, a third lens group with negative optical power, and a subsequent lens group including at least two lens groups. When zooming, distances between the lens groups change, and condition: 1.65<|f2|/fw<2.20 (where f2: Focal length of the second lens group, and fw: Focal length of the entire system at a wide-angle end) is satisfied.

A zoom lens includes a first lens group arranged closest to an object side, configured not to move for zooming, and having a positive refractive power, a lens group GR arranged closest to an image side, configured not to move for zooming, and having a positive refractive power, and a lens group GP arranged adjacent to the object side of the lens group GR, configured to move for zooming, and having a positive refractive power, wherein the lens group GR consists of a subgroup GRN having a negative refractive power and a subgroup GRP having a positive refractive power, which are arranged with a longest air gap on an optical axis in the lens group GR, and configuration of the subgroup GRN and a lateral magnification of the lens group GR are appropriately set.

An extender lens unit to be inserted into and removed from an optical path in a relay lens unit included in a zoom lens consists of a front unit having a positive refractive power, a middle unit having a negative refractive power, and a rear unit having a negative refractive power. The extender lens unit includes three negative lenses. The middle unit includes a negative lens. An average refractive index of positive lenses included in the extender lens unit, an average refractive index of negative lenses included in the extender lens unit, a focal length of the negative lens included in the middle unit, and a focal length of the middle unit are appropriately set.

A zoom lens includes in order from an object side, a zoom lens unit that moves during zooming, and an aperture stop; and a relay lens unit that does not move for zooming, and the relay lens unit includes an extender lens unit changing a focal length range of the zoom lens by entering into and leaving from an optical path of the zoom lens, and that unit includes front, middle, and rear lens sub-units each including one positive lens and one negative lens. The following expressions are satisfied:
0.250<NnNp;
|fbp/fb|<0.200; and
|fbn/fb|<0.200,
where Np and fbp denote a refractive index and a focal length of the positive lens, Nn and fbn denote a refractive index and a focal length of the negative lens, and fb denotes a focal length of the middle lens sub-unit.

A five-group zoom lens including, in order from the object side, positive, negative, negative, positive, and positive groups. The first group includes a negative meniscus lens with the concave surface toward the image side, a positive lens and a positive lens. The second group includes a negative lens with the image-side surface having an absolute value of curvature radius smaller than that of the object side surface, a biconcave lens, and a biconvex lens. The third group includes a negative lens with the object side surface having an absolute value of curvature radius smaller than that of the image-side surface, and is moved during focusing on a closer object. The fifth group includes a positive fifth-A group including a biconvex lens, and a negative fifth-B group including a biconcave lens and a biconvex lens, wherein condition expression (1) is satisfied:
4<f5/f5B<0.7(1).

The zoom lens according to the present invention includes, in order from an object side to an image side: a first lens unit having a positive refractive power that does not move for zooming; a second lens unit having a negative refractive power that moves during zooming; a third lens unit having a negative refractive power that moves along a locus convex toward the object side during zooming from a wide angle end to a telephoto end; a fourth lens unit having a positive refractive power that moves during zooming; an aperture stop; and a fifth lens unit having a positive refractive power that does not move for zooming, in which the fourth lens unit has a lens surface having an aspherical shape with which the positive refractive power is weakened.

The projection zoom lens essentially consists of a positive first lens group fixed while changing magnification and the second lens group through the fourth lens group which move while changing magnification, and a fifth lens group fixed while changing magnification; and an aperture stop positioned between adjacent moving lens groups or within one moving lens group, wherein the numerical number of the zoom lens is set to be constant over the entire zoom range, the reduction side is configured to be telecentric, and the projection zoom lens satisfies conditional formulas (4) and (5):
2.0<Bf/Im(4)
L/Im<12(5), where, Bf: the back focus (air converted length) at the reduction side of the entire system at the wide angle end, Im: the maximum effective image circle diameter, and L: the distance between the most-magnification-side lens surface and the most-reduction-side lens surface along the optical axis when the projection distance is infinite.

A zoom lens includes in order from an object side, a zoom lens unit that moves during zooming, and an aperture stop; and a relay lens unit that does not move for zooming, and the relay lens unit includes an extender lens unit changing a focal length range of the zoom lens by entering into and leaving from an optical path of the zoom lens, and that unit includes front, middle, and rear lens sub-units each including one positive lens and one negative lens. The following expressions are satisfied:

0.250<NnNp;

|fbp/fb|<0.200; and

|fbn/fb|<0.200,

where Np and fbp denote a refractive index and a focal length of the positive lens, Nn and fbn denote a refractive index and a focal length of the negative lens, and fb denotes a focal length of the middle lens sub-unit.

In an optical system (ZL) capable of imaging in a plurality of object side media (for instance, air and water) whose refractive indexes are different from each other, the optical system is composed of, in order from an object, a front lens group (G1) whose position in an optical axis fixed, and a rear lens group (for instance, G2 to G4), and the rear lens group comprises at least two moving lens groups (for instance, G2 and G3), and at least two moving lens groups in the rear lens group are moved along the optical axis upon changing between object side media.

The zoom lens consists of a first lens group, a front group, a middle group, and a rear group, in order from an object side. The first lens group has a positive refractive power and includes a negative lens and a positive lens, successively in order from a position closest to the object side to an image side. The front group consists of one or more lens groups that move during zooming and has a negative refractive power as a whole throughout an entire zoom range. The middle group includes only one lens group as a lens group. The rear group consists of a plurality of lens groups. An aperture stop is disposed between a lens surface closest to the image side in the front group and a lens surface closest to the object side in the rear group. The zoom lens satisfies predetermined conditional expressions.