A zoom lens includes, in order from a magnified side to a minified side, a first lens group, an aperture stop, and a second lens group. The second lens group has at least one aspheric surface. The zoom lens satisfies the conditions: 0.1<Ic/TTLw<0.15 and TTLw/EFLw≦10, where Ic denotes a radius of an image circle, TTLw denotes a total track length of the zoom lens in the wide configuration, and EFLw denotes an effective focal length of the zoom lens in the wide configuration.

A zoom lens system includes: a first lens group having a negative refractive power; a second lens group having a positive refractive power; and an aperture between the first and second lens groups, wherein the first lens group and the second lens group are sequentially arranged from an object side to an image side, wherein the zoom lens system performs zooming and focusing by moving the second and first lens groups, respectively, along an optical axis, and satisfies
2.5<(f.sub.w/f.sub.t)×(T.sub.w/Y)<4.5, where f.sub.w denotes an entire focal length at a wide-angle end, f.sub.t denotes an entire focal length at a telephoto end, and Tw denotes a distance between an image plane and an object side lens surface of a lens closest to the object side of the first lens group at the wide-angle end, and Y denotes a radical axis image height.

A zoom lens includes a first lens group and a second lens group. A relative movement between the first lens group and the second lens group is allowed, whereby the zoom lens zooms between a wide-angle end and a telephoto end. While the zoom lens is switched from the wide-angle end toward an intermediate state, the first lens group is moved toward the image side. While the zoom lens is switched from the intermediate state toward the telephoto end, the first lens group is moved toward the object side.

A zoom lens includes a first lens group and a second lens group. The first lens group has a negative refractive power and includes a first lens, a second lens, and a third lens arranged in sequence from a magnified side to a minified side, refractive powers of which are negative, negative, and positive, respectively. The second lens group has a positive refractive power and is disposed between the first lens group and the minified side. The second lens group includes a fourth lens, a fifth lens, a sixth lens, a seventh lens, an eighth lens, a ninth lens, and a tenth lens arranged in sequence from the magnified side to the minified side, refractive powers of which are positive, negative, positive, negative, positive, negative, and positive, respectively. The first lens and the tenth lens are aspheric lenses, and the second lens to the ninth lens are spherical lenses.

A variable magnification optical system consists of a first lens-group having negative refractive-power, a stop and a second lens-group having positive refractive-power in this order from an object-side. The first lens-group includes an L11 negative meniscus lens, an L12 negative lens and a C11 cemented lens, in which a biconcave lens and a positive lens are cemented together in this order from the object-side, in this order from the object-side. The second lens-group includes an L21 positive lens that is arranged closest to the object-side and includes at least one aspheric surface, and an object-side surface of which is convex, and only two cemented lenses toward an image-side of the L21 positive lens, and each of which consists of a negative lens and a positive lens cemented together in this order from the object-side. The following conditional expressions (1) and (2) are satisfied:
−1.0<Rf11/Rf12<0.7  (1); and
55.0<νdave1  (2).

A zoom lens consisting of, in order from an object side to an image side, a first lens unit having a negative refractive power, and a rear lens unit having one or more lens units and having a positive refractive power as a whole. Intervals between adjacent lens units change during zooming. The first lens unit has at least three meniscus lenses each having a negative refractive power and a convex shape toward the object side. At least one of the at least three meniscus lenses includes an aspherical surface having a positive aspherical amount. A predetermined condition is satisfied.

A stereoscopic vision optical system includes a first lens group having a negative refractive power, disposed nearest to an object, a second lens group having a positive refractive power, and a rear-side lens group having a positive refractive power. The rear-side lens group includes a first rear group and a second rear group. The optical axis of the first lens group, an optical axis of the first rear group, and an optical axis of the second rear group is positioned on the same plane. The optical axis of the first lens group is positioned between the optical axis of the first rear group and the optical axis of the second rear group, and the following conditional expression (1) is satisfied:
0.08≤((−L/2)×(f1/f2))×(1/WD)≤0.25  (1).

According to an embodiment, a projection optical system for an image projection device, that includes a plurality of lenses. The projection optical system satisfies conditional expression (1): where fw is a focal length of a lens having the weakest refractive power in the lenses and f is a focal length of the whole projection optical system. $\begin{array}{cc}0.01\le .Math.\frac{f}{f_w}.Math.<0.34& \left(1\right)\end{array}$

The zoom lens according to the present invention includes, in order from an object side to an image side, a first lens unit with a negative refractive power and a rear lens group including one or more lens units, in which intervals between adjacent ones of the lens units are changed during zooming and focusing. The first lens unit includes three or more negative lenses. The rear lens group includes a focus lens unit with a negative refractive power that moves during focusing. A focal length of the first lens unit, a shortest focal length of the zoom lens, curvature radii of object-side and image-side lens surfaces of a lens in the first lens unit disposed closest to the object side, and a focal length of the focus lens unit are appropriately set.

A zoom lens includes, in order from an object side to an image side, a first lens unit having a negative refractive power and a second lens unit having a positive refractive power, a distance between the first lens unit and the second lens unit changing during zooming. A predetermined condition is satisfied.