Abstract: A zoom lens is disclosed, comprising a front lens group having a first, a second and a third lens units, totaling at least three lens units, and whose overall refractive power for the wide angle end is positive and a rear lens group having a lens unit of positive refractive power and another lens unit of negative refractive power, wherein when zooming from the wide angle to the telephoto end, the first, the second and the third lens units all axially move in such relation that the overall refractive power of the front lens group is weaker in the telephoto end than in the wide angle end, and the lens units in the rear lens group all axially move in such relation that their separation narrows.

Abstract: A low-profile, transmissive overhead projector (OHP) employs a novel condensing lens system having a catadioptric Fresnel lens and a dioptric Fresnel lens. Each of the lenses exhibits chromatic aberration, but the dispersion of one lens is selected to cancel out the dispersion of the other, e.g., if the catadioptric lens has negative dispersion then the dioptric lens is selected to have positive dispersion. The mutual cancellation of the dispersion of each lens practically eliminates longitudinal chromatic aberration through the lens system. The lens system provides a fast condensing system which allows collimation, dispersion or concentration of light striking the lens at a high angle of incidence. This characteristic is particularly suited for a low-profile OHP as it allows the light source to be placed directly under, but very close to, the stage area.

Abstract: A bifocal lens of the Cohen lens design utilizing phase zone plate optics and a facet depth of one-half wavelength of the design wavelength, where the primary focal points are at two orders, specifically the 0.sup.th and 1.sup.st orders, and the brightness at each primary focal point is equal at about 0.40.

Abstract: A multifocal optical device is provided for focusing light traveling parallel to an optical axis of the device. The multifocal optical device comprises a multifocal phase zone plate including at least two annular zones disposed substantially concentrically about the optical axis and spaced in a radial dimension from the optical axis in proportion to the square root of q, where q is an integer zone number; and an absorbing material disposed on a portion of ones of the annular zones for absorbing a portion of the light.

Abstract: A lens system comprising, at locations in the vicinity of each other, radial type graded refractive index lens elements made of a medium having a positive refractive power and other radial type graded refractive index lens elements made of a medium having a negative refractive power. This lens system has aberrations corrected favorably with the radial type graded refractive index lens elements which are made of materials easily preparable.

Abstract: A compact high-performance telephoto lens system that has an aperture ratio of approximately 1:4 to 1:4.5, a half viewing angle of approximately 2.degree. to 3.degree. and a telephoto ratio of approximately 0.8. The lens system satisfies the following conditions:0.4f<f.sub.I <0.8f (1).nu..sub.1, .nu..sub.3 >70 (2)0.4f<.vertline.1/.PHI..sub.3 .vertline.<0.7f, .PHI..sub.3 <0 (where .PHI..sub.3 =(n.sub.2 -1).r.sub.3) (3)0.4f<.vertline.1/.PHI..sub.8 .vertline.<0.6f, .PHI..sub.8 <0 (where .PHI..sub.8 =(n.sub.5 -n.sub.4)/r.sub.8) (4)0.2f<.vertline.f.sub.III .vertline.<0.4f, f.sub.III <0 (5).nu..sub.6 <35, .nu..sub.7 >50 (6)wheref is the overall focal length; f.sub.I is the focal length of the first lens group; f.sub.III is the focal length of the third lens group; n.sub.i is the refractive index of the ith lens element at the d-line; .nu..sub.i is the Abbe number of the ith lens element; r.sub.