Abstract: A lens for projection substantially consists of a negative first lens group, a positive second lens group and a positive third lens group in this order from a magnification side. A reduction side is telecentric, and a most magnification-side lens is an aspheric plastic lens. The third lens group consists of a negative front group and a positive rear group in this order from the magnification side with an air space therebetween. The front group consists of a negative lens and a positive lens in this order from the magnification side with an air space therebetween, and the air space being narrower than the air space between the front group and the rear group. Conditional formula (1): ?0.7<fFG3/f<0.7 about front focus fFG3 of the third lens group and focal length f of an entire system is satisfied.

Abstract: 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.

Abstract: A zoom lens for projection consists of a negative first lens group, which is fixed during magnification change, a middle group, which includes plural lens groups that move during magnification change, and a positive last lens group, which is fixed during magnification change, in this order from a magnification side. A reduction side is non-telecentric. Both of a lens group closest to the magnification side and a lens group closest to the reduction side in the middle group have positive refractive power, and move from the reduction side toward the magnification side during magnification change from a wide-angle end to a telephoto end. The last lens group includes at least two positive lenses and at least two negative lenses. A positive lens is arranged closest to the reduction side in the last lens group. A predetermined conditional expression about a focal length of the last lens group is satisfied.

Abstract: Composing a projection zoom lens with a first lens group disposed on the most magnification side and having a positive power, the lens group being fixed at the time of zooming, a final lens group disposed on the most reduction side and having a negative power, the lens group being fixed at the time of zooming, and a plurality of lens groups disposed between the first and final lens groups and moved at the time of zooming along an optical axis in association with each other for the zooming and correction of image plane shift arising from the zooming, in which the final lens group includes a lens having a positive power on the most reduction side.

Abstract: A variable-magnification projection optical system substantially consists of a negative first lens group that is disposed at the most enlargement-side position and is fixed during magnification change, a positive last lens group that is disposed at the most reduction-side position and is fixed during magnification change, and a plurality of lens groups that are disposed between the first lens group and last lens group and are moved during magnification change. The lens groups that are moved during magnification change substantially consist of three lens groups, wherein the most enlargement-side lens group is a negative lens group. Predetermined conditional expressions relating to a back focus of the entire system on the reduction side at the wide-angle end, a focal length of the entire system at the wide-angle end, a focal length of the last lens group, and a focal length of the first lens group are satisfied.

Abstract: A variable-magnification projection optical system substantially consists of a negative first lens group that is disposed at the most enlargement-side position and is fixed during magnification change, a positive last lens group that is disposed at the most reduction-side position and is fixed during magnification change, and a plurality of lens groups that are disposed between the first lens group and last lens group and are moved during magnification change. The most enlargement-side lens group (the second lens group) of the lens groups that are moved during magnification change has a negative refractive power. Predetermined conditional expressions relating to a back focus of the entire system on the reduction side at the wide-angle end, a focal length of the entire system at the wide-angle end, a focal length of the second lens group and a focal length of the last lens group are satisfied.

Abstract: A lens for projection substantially consists of a negative first lens group, a positive second lens group and a positive third lens group in this order from a magnification side. A reduction side is telecentric, and a most magnification-side lens is an aspheric plastic lens. The third lens group consists of a negative front group and a positive rear group in this order from the magnification side with an air space therebetween. The front group consists of a negative lens and a positive lens in this order from the magnification side with an air space therebetween, and the air space being narrower than the air space between the front group and the rear group. Conditional formula (1): ?0.7<fFG3/f<0.7 about front focus fFG3 of the third lens group and focal length f of an entire system is satisfied.

Abstract: A projection zoom lens has a negative first lens group which is fixed when changing magnification, second through fifth lens groups that move when changing magnification, and a positive sixth lens group which is fixed when changing magnification, in this order from a magnification side. The reduction side is telecentric. When changing magnification from a wide angle end to a telephoto end, the second lens group moves toward the magnification side. The first and second lenses from the reduction side of the fifth lens group are a positive lens and a negative lens, respectively. The sixth lens group is constituted by a single positive lens. The projection zoom lens satisfies a predetermined Conditional Formula.

Abstract: A zoom lens for projection substantially consists of a negative first lens group, a positive second lens group, a positive third lens group, a negative fourth lens group, a positive fifth lens group, and a positive sixth lens in this order from a magnification side. The first lens group and the sixth lens group are fixed and the second lens group through the fifth lens group move during magnification change. A reduction side is telecentric. First through fourth lenses from the magnification side in the fifth lens group are a negative meniscus-shaped single lens with its convex surface facing the magnification side, a positive single lens, a biconcave lens and a biconvex lens, respectively, and the biconcave lens and the biconvex lens are cemented together.

Abstract: A projection lens is provided having a positive refractive power as a whole and including a correction group consisting of a plurality of lens groups that corrects field curvature by moving in an optical axis direction and changing the distance between each lens group. The conjugation length of a combined optical system combining the most magnification side lens group constituting the correction group to the most reduction side lens group constituting the correction group in the projection lens is constant before and after the movement of the correction group for correcting field curvature. If the projection lens is variable in magnification or focusable by moving at least some of the lens groups, the movement of the correction group for correcting field curvature is allowed to be performed independently of the movement for either changing magnification or focusing.

Abstract: A negative first lens group, a positive second lens group, a negative third lens group, a positive fourth lens group, a negative fifth lens group, and a positive sixth lens group are disposed in this order from the magnification side. The reduction side is made telecentric. While changing magnification, the second lens group through the fifth lens group are moved, and the first lens group and the sixth lens group are fixed. Predetermined conditional expressions are satisfied.

Abstract: A projection zoom lens has a negative first lens group which is fixed when changing magnification, second through fifth lens groups that move when changing magnification, and a positive sixth lens group which is fixed when changing magnification, in this order from a magnification side. The reduction side is telecentric. When changing magnification from a wide angle end to a telephoto end, the second lens group moves toward the magnification side. The first and second lenses from the reduction side of the fifth lens group are a positive lens and a negative lens, respectively. The sixth lens group is constituted by a single positive lens. The projection zoom lens satisfies a predetermined Conditional Formula.

Abstract: A negative first lens group, a positive second lens group, a negative third lens group, a positive fourth lens group, a negative fifth lens group, and a positive sixth lens group are disposed in this order from the magnification side. The reduction side is made telecentric. While changing magnification, the second lens group through the fifth lens group are moved, and the first lens group and the sixth lens group are fixed. Predetermined conditional expressions are satisfied.

Abstract: A variable-magnification projection optical system substantially consists of a negative first lens group that is disposed at the most enlargement-side position and is fixed during magnification change, a positive last lens group that is disposed at the most reduction-side position and is fixed during magnification change, and a plurality of lens groups that are disposed between the first lens group and last lens group and are moved during magnification change. The most enlargement-side lens group (the second lens group) of the lens groups that are moved during magnification change has a negative refractive power. Predetermined conditional expressions relating to a back focus of the entire system on the reduction side at the wide-angle end, a focal length of the entire system at the wide-angle end, a focal length of the second lens group and a focal length of the last lens group are satisfied.

Abstract: A variable-magnification projection optical system substantially consists of a negative first lens group that is disposed at the most enlargement-side position and is fixed during magnification change, a positive last lens group that is disposed at the most reduction-side position and is fixed during magnification change, and a plurality of lens groups that are disposed between the first lens group and last lens group and are moved during magnification change. The lens groups that are moved during magnification change substantially consist of three lens groups, wherein the most enlargement-side lens group is a negative lens group. Predetermined conditional expressions relating to a back focus of the entire system on the reduction side at the wide-angle end, a focal length of the entire system at the wide-angle end, a focal length of the last lens group, and a focal length of the first lens group are satisfied.

Abstract: A variable magnification optical system for projection includes a first-lens-group having negative refractive power, and which is fixed when magnification of the system is changed, a second-lens-group having negative refractive power, and which is movable when magnification of the system is changed, a third-lens-group having positive refractive power, and which is movable when magnification of the system is changed, and a fourth-lens-group having positive refractive power, and which is fixed when magnification of the system is changed, which are arranged from the magnification side of the system in the order mentioned. The reduction side of the variable magnification optical system is telecentric, and formula 1.5<Bf/fw is satisfied, where Bf: back focus in air of entire system at wide angle end, and fw: focal length of entire system at wide angle end.

Abstract: Composing a projection zoom lens with a first lens group disposed on the most magnification side and having a positive power, the lens group being fixed at the time of zooming, a final lens group disposed on the most reduction side and having a negative power, the lens group being fixed at the time of zooming, and a plurality of lens groups disposed between the first and final lens groups and moved at the time of zooming along an optical axis in association with each other for the zooming and correction of image plane shift arising from the zooming, in which the final lens group includes a lens having a positive power on the most reduction side.

Abstract: A projection zoom lens includes: a first lens group that has a negative refractive power, is fixed when power varies, and has a focusing function; a second positive lens group; a third positive lens group; a fourth negative lens group; a fifth positive lens group; and a sixth lens group that has a positive refractive power, is fixed when power varies, and includes an aperture diaphragm arranged closest to a magnification side. The first to sixth lens groups are arranged in this order from the magnification side, and the second to fifth lens groups are moved when power varies. All lenses are single lenses. The ratio of the back focal length Bf of the entire system to the focal length f of the entire system at a wide angle end is more than 2.5. The Fno. is constant in the entire power variation range.

Abstract: A zoom lens for projection includes a negative magnification-side lens group fixed while magnification is changed, a plurality of lens groups that move while magnification is changed, and a positive reduction-side lens group fixed while magnification is changed which are arranged in this order from the magnification side. The magnification-side lens group is composed of a negative front-group and a positive rear-group, arranged in this order from the magnification side. The rear-group is composed of a positive lens, a negative lens and a positive lens, arranged in this order from the magnification side, and focusing is performed by moving the rear-group. Further, the following formulas (1) and (2) are satisfied: 1.73<NdRpAVE??(1); and 60<?dRn??(2), where NdRpAVE is an average value of refractive indices of the positive lenses in the rear-group, and ?dRn is the Abbe number of the negative lens in the rear-group.

Abstract: A zoom lens for projection includes a first negative lens group, a second positive lens group, a third positive lens group, a fourth lens group, and a fifth positive lens group, which are sequentially arranged from the magnification side of the zoom lens. The first lens group and the fifth lens group are fixed, and the second lens group, the third lens group and the fourth lens group move along the optical axis of the zoom lens when magnification of the zoom lens is changed. Each of the first lens group and the second lens group is composed of two lenses. Each of the third lens group and the fifth lens group is composed of a positive lens, and the fourth lens group is composed of five lenses. Further, the reduction side of the zoom lens is telecentric.