Abstract: A display optical system has an illumination optical system for emitting illumination light, a plurality of reflection-type display devices, a projection optical system, and a plane-parallel mirror. The reflection-type display devices individually modulate the illumination light emitted from the illumination optical system and reflect the illumination light thus modulated as projection light. The projection optical system projects the projection light and has at least one optical element included therein arranged so as to be decentered with respect to the other optical elements included therein.

Abstract: An oblique projection optical system performing enlargement projection in a slanting direction from a primary image plane to a secondary image plane, or reduction projection in a slanting direction from the secondary image plane to the primary image plane. The oblique projection optical system has at least two lens units each comprising at least one lens element and having surfaces decentered with respect to each other, wherein zooming is performed by moving, of said lens units, a plurality of lens units, and wherein when a ray connecting a center of the primary image plane and a center of the secondary image plane is referred to as an image plane center ray, the predetermined conditions are satisfied without an intermediate real image being formed between the primary image plane and the secondary image plane.

Abstract: A focusing method for a decentered optical system including a decentered lens element has translationally moving and inclining at least one lens element for different object distances, in such a way that variation in a position of a primary image point is kept within a range defined by the following condition. Here &Dgr;fb represents a maximum distance traveled by the primary image point during focusing; and &THgr;max represents a maximum angle of peripheral rays traveling from a center of an object to a center of an image plane relative to a base ray: |&Dgr;fb ×sin(&THgr;max)|max≦0.8.

Abstract: A zoom lens system for focusing a finite distance object on an image plane as an image, has, in order from one conjugate side, a first lens unit having a positive optical power, the first lens unit being stationary during a zooming operation, a stop stationary during a zooming operation, a second lens unit having a negative optical power, a third lens unit having a positive optical power, and a fourth lens unit having a positive optical power. The zooming operation is performed by varying distances between the first through fourth lens units, and the first and second lens units form an afocal condition.

Abstract: An image forming optical system minimizes the effects of ghosts in an image. In one embodiment of the present invention, light is at an intermediate point of the optical system. Introducing light in this manner can result in ghosts being produced in the imaged data or in data being missing from the imaged data, both of which are undesirable conditions. The present invention minimizes ghosting and loss of data by maintaining one or more optical relationships to within set ranges.

Abstract: A focusing method for a decentered optical system including a decentered lens element has translationally moving and inclining at least one lens element for different object distances, in such a way that variation in a position of a primary image point is kept within a range defined by the following condition.

Abstract: Sequentially from the object side: a first lens unit having positive optical power; a second lens unit having negative optical power; and a third lens unit having positive optical power, wherein said first and second lens units normally move in opposite directions whereas the third lens unit moves linearly to the object side during zooming from the wide angle end to the telephoto end, and wherein the following conditional equation is satisfied:0.2<M1/Z<2.0where M1 represents the amount of movement of the first lens unit during zooming from the wide angle end to the telephoto end, Z represents the zoom ratio (fT/fW), fT represents the overall focal length of the total system at the telephoto end, and fW represents the overall focal length of the total system at the wide angle end.

Abstract: A zoom lens system has first to fifth lens units from the object side. The second lens unit has a negative optical power. The other lens units have each a positive optical power. During zooming, the first, third and fourth lens units move along the optical axis to vary the distances between the lens units. The zoom lens system fulfills the conditions 0.01.ltoreq..vertline.f2/fT.vertline..ltoreq.0.20 and 25.ltoreq.(fT/f34W).times.Z.ltoreq.300, where f2 represents the focal length of the second lens unit, fT represents the focal length of the entire zoom lens system at the longest focal length condition, f34W represents the composite focal length of the third and fourth lens units at the shortest focal length condition, and Z represents the zoom ratio.

Abstract: A zoom lens system has four lens units. The first, third and fourth lens units thereof are lens units of positive power. The second lens unit thereof has negative power. During zooming, the first lens unit is moved. During focusing, the second and third lens units are moved and a distance between the third and fourth lens units is varied. The third lens unit is moved toward an object side.