Abstract: A zoom lens comprising, from front to rear, a first group of positive power for focusing, a second group of negative power for varying the image magnification, a third group of negative power for compensating for the image shift, a fourth lens group of positive power in the form of a bi-convex lens whose rear surface is stronger in curvature than the front surface, a stop, and an image forming fifth group, the fifth groug being divided into two parts of which the front part comprises a bi-convex first lens, a negative second lens having a concave front surface and a positive third lens of stronger front curvature and the rear part comprises a negative fourth lens having a concave rear surface, a bi-convex fifth lens and a positive sixth lens of stronger front curvature, and the zoom lens satisfying the following conditions:3.9<f4/fw<4.53.8<f51/fw<4.21.05<R5,2/R5,3<1.21.25<D51-52/fw<1.

Abstract: A zoom lens including at least two lens units of which the front one is of positive refractive power and the rear one is of negative refractive power, the units being axially movable to decrease their separation in zooming from the wide angle to the telephoto end. At least one of the lens units is provided with at least one refractive index distribution type lens arranged on a common optical axis of the other lenses in order to achieve a great advance in compactness.

Abstract: A zoom lens comprises, from an object side to an image plane side, a positive first lens unit for focusing, a negative second lens unit as the variator, a negative third lens unit as the compensator, a positive fourth lens unit for producing an almost diverging light beam, and an image forming fifth lens unit. The fifth lens unit includes a bi-convex first lens, a negative second lens having a concave surface facing the object side, a bi-convex third lens having a strong refracting surface facing the object side, a negative fourth lens having a concave surface facing the image plane side, a bi-convex fifth lens having a strong refracting surface facing the image plane side, and a bi-convex sixth lens having a strong refracting surface facing the object side. The zoom lens satisfies the following conditions:1.05<f4/L<1.320.4<.vertline.R5,4/fw.vertline.<30.60<fw/R5,7<0.0761.58<(N5,1+N5,3+N5,5+N5,6)/4<1.

Abstract: A zoom lens comprising, from front to rear, a positive first group for focusing, a negative second group as the variator, a negative third group as the compensator, a positive fourth group and a fifth group as the relay lens, the fourth group in the form of a bi-convex singlet and the fifth group in two parts with a widest spacing therebetween, of which the front part comprises a bi-convex first lens, a negative second lens whose front surface is concave toward front, and a positive third lens whose front surface is of strong curvature, and the rear part comprises a negative lens whose rear surface is concave toward rear, a bi-convex lens and a positive lens whose front surface is of strong curvature, satisfying the following conditions:3.9<f4/fw<4.53.7<f5F/fw<5.21.0<R5,2/R5,3<1.61.4<(D5F-D5R)/fw<1.

Abstract: An objective of variable focal length includes a plurality of lens units of which the first, counting from front, is of negative power and the second is of positive power. At least the first and second lens units are moved with variation of the air separation therebetween to vary the focal length of the entire system. The objective is characterized in that at least one of the lens units is included with at least one refractive index distribution type lens arranged on the common optical axis of the other lenses.

Abstract: In the photographic optical system comprised of a plurality of lenses and having a filter in between the lenses, a light beam emerging from a lens on the object side of the filter is arranged to diverge to prevent the light reflected from a solid state image pickup element on a predetermined image plane from being reflected again by the filter to re-focus on the predetermined image plane.

Abstract: A compact zoom lens comprises, from front to rear, a first lens unit for focusing having a positive refractive power, a second lens unit having a negative refractive power and axially movable for varying the image magnification, a third lens unit axially movable for compensating for the shift of an image plane resulting from the variation of the image magnification, and a fourth lens unit of positive refractive power having an image forming function which remains stationary relative to a focal plane. The fourth lens unit includes, from front to rear, a first lens of positive refractive power, a second lens of negative refractive power, a third lens of negative refractive power, a fourth lens of positive refractive power and a fifth lens of positive refractive power. And, in particular, a stop for determining the F-number is arranged in a space between the second and third lens units.

Abstract: A variable focal length objective having a plurality of lens units with at least one unit separation being varied to vary the focal length of the entire system, in which at least one of the plurality of lens units has at least gradient-index lens having a refractive index distribution of the form: N(h)=N.sub.0 +N.sub.1 h.sup.2 +N.sub.2 h.sup.4 +N.sub.3 h.sup.6 + . . . where h is the height from the optical axis, N.sub.0 is the refractive index on the optical axis, and N.sub.1, N.sub.2, N.sub.3, . . . are constants, satisfying the following conditions:.phi..sub.G .multidot.N.sub.1 <0K.multidot..phi..sub.G .multidot.N.sub.2 <0where .phi..sub.G is the refractive power of that lens unit which has that gradient-index lens, and K is +1 or -1 when the stronger of the curvatures of both surfaces of that gradient-index lens is convex or concave to the air respectively.

Abstract: A lens system has a lens unit comprising a positive lens and a negative lens. The positive lens has an index distribution in the direction of the optic axis. The value of the index gradient for a light having a short wavelength in the vicinity of the vertex of at least one convex surface of the positive lens is smaller than the value of the index gradient for a light having a long wavelength.

Abstract: A compact zoom lens comprising, from front to rear, a first lens unit of positive power for focusing, a second lens unit of negative power monotonously movable for zooming, a third lens unit held stationary during zooming, and a fourth lens unit of positive power to move in a locus convex toward the front to maintain the constant position of an image plane which shifts as the image magnification varies, a fixed stop for determining F-numbers being located between the second and third lens units.

Abstract: A variable focal length objective of which at least one of a pulrality of lens units is moved to vary the image magnification, wherein at least one of the plurality of lens units has at least one refractive index distribution type lens provided with a condition of N.sub.1 >0 as, taking the distance from the optical axis to a radial direction as h, the refractive index distribution is expressed by N(h)=N.sub.0 +N.sub.1 h.sup.2 +N.sub.2 h.sup.4 +N.sub.3 h.sup.6 +. . . (N.sub.0, N.sub.1, N.sub.2 . . . are constants), and satisfies the following conditions: ##EQU1## where fmax: the maximum focal length;fmin: the minimum focal length;Lmin: the optical total length when the total length is shortest;Y: the maximum image height;N.sub.1 *: the coefficient of h.sup.2 of the refractive index distribution type lens having a refractive index distribution of N.sub.1 >0;D*: the lens thickness of the refractive index distribution type lens having the refractive index distribution of N.sub.1 >0.