Provided is a zoom lens including a plurality of lens units, in which an interval between each pair of adjacent lens units is changed for zooming. The plurality of lens units consist of, in order from an object side to an image side: a first lens unit having a negative refractive power; a second lens unit having a positive refractive power; and a rear lens group including at least two lens units. The first lens unit and the second lens unit are configured to move during zooming. The first lens unit includes, in order from the object side to the image side, a negative lens G1, a negative lens G2, and a negative lens G3.

Provided is a zoom lens including: a lens unit Ln1 having a negative refractive power; a lens unit Ln2 having a negative refractive power; a lens unit Lp1 having a positive refractive power; and a rear lens group including one or more lens units, the lens unit Ln1, the lens unit Ln2, the lens unit Lp1, and the rear lens group being successively arranged in order from an object side to an image side, in which the lens unit Ln1 and the lens unit Lp1 are moved along the same locus during zooming, and in which the lens unit Ln2 is moved toward the object side during focusing from infinity to proximity.

Provided is a zoom lens including: a lens unit Ln1 having a negative refractive power; a lens unit Ln2 having a negative refractive power; a lens unit Lp1 having a positive refractive power; and a rear lens group including one or more lens units, the lens unit Ln1, the lens unit Ln2, the lens unit Lp1, and the rear lens group being successively arranged in order from an object side to an image side, in which the lens unit Ln1 and the lens unit Lp1 are moved along the same locus during zooming, and in which the lens unit Ln2 is moved toward the object side during focusing from infinity to proximity.

An objective lens including a first lens group immovable in changing magnification from a normal observation state to a close-up observation state, a second lens group arranged nearer to the image side than the first lens group and moving along the optical axis in the change of magnification, a third lens group arranged nearer to the image side than the second lens group and immovable in the change of magnification, and an aperture stop arranged between the second lens group and an image plane and immovable in the change of magnification, and the objective lens satisfying the following condition:
1.24<|F.sub.1f/F.sub.n|<1.8
where F.sub.1f denotes the focal length of a lens that is placed nearest to the object side in the first lens group and F.sub.n denotes the focal length of the whole system in the normal observation state.

A projection lens includes a compound aspherical lens in which a resin layer is formed on a surface of a glass lens and a lens surface of the resin layer on the air contacting surface side has an aspherical shape. If the glass transition temperature of the resin layer is taken as Tg and its unit is taken as ° C., Tg of at least one of the resin layers is 150<Tg<280. The projection lens is configured to satisfy a given conditional expression with respect to the at least one of the resin layers.

A projection zoom lens is essentially constituted by, in order from the magnification side: a negative first lens group, which is fixed when changing magnification; a positive second lens group, which moves when changing magnification; a plurality of other lens groups; and a final lens group, which is fixed when changing magnification. The distances among all adjacent lens groups change when changing magnification. The first through third lenses from the magnification side within the first lens group are a first single lens having an aspherical surface with a concave surface toward the magnification side and a negative refractive power in the paraxial region, a second single lens having an aspherical surface, and a third lens. A first lens group front group constituted by the first through third lenses has a negative refractive power. Predetermined conditional formulae are satisfied.

A projection zoom lens is essentially constituted by, in order from the magnification side: a negative first lens group, which is fixed when changing magnification; a positive second lens group, which moves when changing magnification; a plurality of other lens groups; and a final lens group, which is fixed when changing magnification. The distances among all adjacent lens groups change when changing magnification. The first through third lenses from the magnification side within the first lens group are a first single lens having an aspherical surface with a concave surface toward the magnification side and a negative refractive power in the paraxial region, a second single lens having an aspherical surface, and a third lens. A first lens group front group constituted by the first through third lenses has a negative refractive power. Predetermined conditional formulae are satisfied.

A projection system includes a first lens group to an n-th lens group with n being 6 or 7 sequentially arranged from a enlargement side. The first lens group includes a first-first lens group and a first-second lens group sequentially arranged from the enlargement side. The two lens groups are separated from each other by a variable distance for image plane correction. The first lens group and the n-th lens group are fixed and the second lens group to the (n−1)-th group are moved when the magnification is changed. The following conditional expression is satisfied: 2<BF/fw<2.8, where fw represents the focal length of the projection system operating at the wide angle end, and BF represents the air conversion length of the back focal distance of the projection system.

A zoom lens includes a first lens group having negative refractive power; a second lens group having positive refractive power; a third lens group having negative refractive power and a fourth lens group having positive refractive power. Upon varying magnification, distances between the first lens group and the second lens group, between the second lens group and the third lens group and between the third lens group and the fourth lens group vary. The second lens group includes a front lens group, an aperture stop, and a rear lens group. Each of the front lens group and the rear lens group includes at least one positive lens. At least a part of the lenses in the second lens group is moved as a group to have a component in a direction perpendicular to an optical axis.

A zoom lens includes a first lens group having negative refractive power; a second lens group having positive refractive power; a third lens group having negative refractive power and a fourth lens group having positive refractive power. Upon varying magnification, distances between the first lens group and the second lens group, between the second lens group and the third lens group and between the third lens group and the fourth lens group vary. The second lens group includes a front lens group, an aperture stop, and a rear lens group. Each of the front lens group and the rear lens group includes at least one positive lens. At least a part of the lenses in the second lens group is moved as a group to have a component in a direction perpendicular to an optical axis.