Zoom lens

Claim 2

A zoom lens comprising: from front to rear, a first lens group of positive refractive power for focusing; a second lens group of negative refractive power having a function of varying the image magnification; a third lens group of negative refractive power for compensating for the shift of an image plane resulting from the variation of the image magnification; a fourth lens unit of positive refractive power of which both surfaces are convex and of which the rear surface is of strong curvature; a stop; and a fifth lens group having the image formign function, said fifth lens group having a front lens sub-group comprising a first lens of which both surfaces are convex, a negative second lens whose front surface is concave and a third lens whose front surface is of strong curvature, and a rear lens sub-group comprising a negative fourth lens whose rear surface is concave, a fifth lens of which both surfaces are convex, and a positive sixth lens whose front surface is of strong curvature, and said zoom lens satisfying the following conditions: 3.9<f4/fw<4.5

Claim 3

8<f51/f2<4.2 1.25<D51-52/fw<1.39 where f4 is a focal length of said fourth lens gorup, f51 is a focal length of said front lens sub-group, fw is the shortest focal length of the entire lens system, and D51-52 is an axial air separation between said front lens sub-group and said rear lens sub-group.

Claim 4

8<f51/f2<4.2 1.25<D51-52/fw<1.39 where f4 is a focal length of said fourth lens gorup, f51 is a focal length of said front lens sub-group, fw is the shortest focal length of the entire lens system, and D51-52 is an axial air separation between said front lens sub-group and said rear lens sub-group.

Claim 5

A zoom lens according to claim 1, wherein said fourth lens group has at least one aspheric surface of such a shape that the radius of curvature gets progressively larger from the center to the margin.

Claim 6

A zoom lens comprising: from front to rear, a first lens group of positive refractive power for focusing; a second lens group of negative refractive power having a function of varying the image magnification; a third lens group for compensating for the shift of an image plane resulting from the variation of the image magnification; a fourth lens unit of positive refractive power of which both surfaces are convex and of which the rear lens surface is of strong curvature; a stop; and a fifth lens group having the image forming function, said fifth lens group having a front lens sub-group comprising a first lens of which both surfaces are convex, a second lens of which both surfaces are convex, a third lens of which both surfaces are concave, and a positive fourth lens of which the front surface is of strong curvature and a rear lens sub-group comprising a negative fifth lens of which the rear surface is concave, a sixth lens of which both surfaces are convex and a positive seventh lens of which the front surface is of strong curvature, and said zoom lens satisfying the following conditions: 3.9-f4/fw<4.5 3.8<f51/fw<4.4 0.7<|R5,5/R5,3|<1.3 1.05<D51-52/fw<1.35 where f4 is a focal length of said fourth lens group, f51 is a focal length of said front lens sub-group, fw is the shortest focal length of the entire lens system, R5,i is a radius of curvature of the i-th lens surface counting from front in said fifth lens group, and D51-52 is an axial air separation between said front lens sub-group and said rear lens sub-group.

Claim 7

A zoom lens comprising: from front to rear, a first lens group of positive refractive power for focusing; a second lens group of negative refractive power having a function of varying the image magnification; a third lens group for compensating for the shift of an image plane resulting from the variation of the image magnification; a fourth lens unit of positive refractive power of which both surfaces are convex and of which the rear lens surface is of strong curvature; a stop; and a fifth lens group having the image forming function, said fifth lens group having a front lens sub-group comprising a first lens of which both surfaces are convex, a second lens of which both surfaces are convex, a third lens of which both surfaces are concave, and a positive fourth lens of which the front surface is of strong curvature and a rear lens sub-group comprising a negative fifth lens of which the rear surface is concave, a sixth lens of which both surfaces are convex and a positive seventh lens of which the front surface is of strong curvature, and said zoom lens satisfying the following conditions: 3.9-f4/fw<4.5 3.8<f51/fw<4.4 0.7<|R5,5/R5,3|<1.3 1.05<D51-52/fw<1.35 where f4 is a focal length of said fourth lens group, f51 is a focal length of said front lens sub-group, fw is the shortest focal length of the entire lens system, R5,i is a radius of curvature of the i-th lens surface counting from front in said fifth lens group, and D51-52 is an axial air separation between said front lens sub-group and said rear lens sub-group.

Claim 8

A zoom lens according to claim 3, wherein said front lens sub-group has at least one aspheric surface of such a shape that the radius of curvature gets progressively larger from the center to the margin.

Claim 9

A zoom lens comprising: from front to rear, a first lens group of positive refractive power for focusing; a second lens group of negative refractive power having a function of varying the image magnification; a third lens group of negative refractive power for compensating for the shift of an image plane resulting from the variation of the iamge magnification; a fixed fourth lens group comprising a single lens with a positive refractive power wherein a divergent light beam from said third lens group that is incident to a front surface thereof will exit from a rear surface thereof while maintained in the divergent state; a fixed fifth lens group having the image forming function, said fifth lens group having a front lens sub-group comprising a first lens of which both surfaces are convex, a second lens of which both surfaces are convex and of which the front surface is of strong curvature, a negative third lens of which the front surface is of strong concave curvature, and a positive fourth lens of which the front surface is of strong convex curvature, and a rear lens sub-group comprising a negative fifth lens of which the rear surface is of strong concave curvature, a sixth lens of which both surfaces are convex, and a positive seventh lens of which the front surface of strong convex curvature, and said zoom lens satisfying the following conditions: 4.5<((R5,1+R5,3)/NA-1))(1f51)<7.0 0.125<(N5, 3-1)f5/R5,6<0.25 0.55<((R5,13/(N5,7-1))(1/f52)<0.65 11<(νf,6+ν5,7)/2-ν5,5<15 0.3<D5,8/f5<0.4 where Ri,j is a radius of curvature of the j-th lens surface in the i-th lens group, fi is a focal length of the i-th lens group, f51 and f52 are focal lengths of said front lens sub-group and said rear lens sub-group respectively, Ni, j and νi, j are respectively a refractive index and an Abbe number of the material of the j-th lens element in the i-th lens group, Di,j is the j-th lens thickness or air separation in the i-th lens group and NA is a means refractive index of the materials of said first and said second lenses.

Claim 10

A zoom lens comprising: from front to rear, a first lens group of positive refractive power for focusing; a second lens group of negative refractive power having a function of varying the image magnification; a third lens group of negative refractive power for compensating for the shift of an image plane resulting from the variation of the iamge magnification; a fixed fourth lens group comprising a single lens with a positive refractive power wherein a divergent light beam from said third lens group that is incident to a front surface thereof will exit from a rear surface thereof while maintained in the divergent state; a fixed fifth lens group having the image forming function, said fifth lens group having a front lens sub-group comprising a first lens of which both surfaces are convex, a second lens of which both surfaces are convex and of which the front surface is of strong curvature, a negative third lens of which the front surface is of strong concave curvature, and a positive fourth lens of which the front surface is of strong convex curvature, and a rear lens sub-group comprising a negative fifth lens of which the rear surface is of strong concave curvature, a sixth lens of which both surfaces are convex, and a positive seventh lens of which the front surface of strong convex curvature, and said zoom lens satisfying the following conditions: 4.5<((R5,1+R5,3)/NA-1))(1f51)<7.0 0.125<(N5, 3-1)f5/R5,6<0.25 0.55<((R5,13/(N5,7-1))(1/f52)<0.65 11<(νf,6+ν5,7)/2-ν5,5<15 0.3<D5,8/f5<0.4 where Ri,j is a radius of curvature of the j-th lens surface in the i-th lens group, fi is a focal length of the i-th lens group, f51 and f52 are focal lengths of said front lens sub-group and said rear lens sub-group respectively, Ni, j and νi, j are respectively a refractive index and an Abbe number of the material of the j-th lens element in the i-th lens group, Di,j is the j-th lens thickness or air separation in the i-th lens group and NA is a means refractive index of the materials of said first and said second lenses.

Claim 11

A zoom lens according to cliam 5, wherein said first lens group comprises a cemented lens of a negative meniscus-shaped first lens convex toward the front and a bi-convex second lens, and a positive meniscus-shaped third lens convex toward the front, said second lens group comprises a negative meniscus-shaped first lens convex toward front, and a cemented lens of a negative second lens of which both surfaces are concave and a positive third lens, said third lens group comprises a cemented lens of a negative first lens of which both surfaces are concave, and a positive second lens, and said fourth lens group comprises a positive lens of which both surfaces are convex.

Claim 12

A zoom lens according to cliam 5, wherein said first lens group comprises a cemented lens of a negative meniscus-shaped first lens convex toward the front and a bi-convex second lens, and a positive meniscus-shaped third lens convex toward the front, said second lens group comprises a negative meniscus-shaped first lens convex toward front, and a cemented lens of a negative second lens of which both surfaces are concave and a positive third lens, said third lens group comprises a cemented lens of a negative first lens of which both surfaces are concave, and a positive second lens, and said fourth lens group comprises a positive lens of which both surfaces are convex.