Retrofoucus photographic lens

Claim 2

A retrofocus type lens system comprising from an object side: (a) a front lens group including at least one negative lens; and (b) a rear lens group, including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, further satisfying the following conditional expressions: 25<V.sub.N <37 and 0.8<D/f<7.0 where V N represents an Abbe's number of d-line of said negative lens in the rear lens group, and D represents an equivalent air distance between a surface of the front lens group provided closest to the image side and a surface of the rear lens group provided closest to the object side.

Claim 3

The retrofocus type lens system of claim 1, wherein said rear lens group includes a cemented lens in which a positive lens and a negative lens are cemented together.

Claim 4

The retrofocus type lens system of claim 1, wherein said front lens group consists of one negative lens, said rear lens group, in the order named from the object side, consists of a positive lens and a cemented lens in which a positive lens and a negative lens are cemented together, and wherein at least one surface of the lenses made of plastic is aspherical.

Claim 5

The retrofocus type lens system of claim 2, wherein said cemented lens is plastic.

Claim 6

The retrofocus type lens system of claim 1 further comprising a diffraction type low-pass filter.

Claim 7

The retrofocus type lens system of claim 5 further comprising a fixed stop for cutting an off-axial light positioned either on the object side or the image side of the diffraction type low-pass filter.

Claim 8

The retrofocus type lens system of claim 1 further comprising an aperture-stop positioned on the object side of the positive lens of said rear lens group which is disposed closest to the object side.

Claim 9

A retrofocus type lens system comprising from an object side: (a) a front lens group including at least one negative lens; and (b) a rear lens group, Including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group consists of a plastic lens having a negative refracting power, said rear lens group includes, from the object side, a double-convex glass lens and a plastic lens subgroup composed of one or more plastic lenses, in which the plastic lens subgroup has a positive refracting power, and said rear lens group satisfies the following conditional expressions, 0.3≦(l-m.sub.p).sup.2 /(m.sub.g.sup.2 ·m.sub.p.sup.2)≦3.5 and 48≦V.sub.g where m g represents a paraxial lateral magnification of the double-convex glass lens of said rear lens group, m p represents a paraxial lateral magnification of the plastic lens subgroup of said rear lens group, and V g represents an Abbe's number of d-line of the double-convex glass lens of said rear lens group.

Claim 10

A retrofocus type lens system comprising from an object side: (a) a front lens group including at least one negative lens; and (b) a rear lens group, Including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group consists of a plastic lens having a negative refracting power, said rear lens group includes, from the object side, a double-convex glass lens and a plastic lens subgroup composed of one or more plastic lenses, in which the plastic lens subgroup has a positive refracting power, and said rear lens group satisfies the following conditional expressions, 0.3≦(l-m.sub.p).sup.2 /(m.sub.g.sup.2 ·m.sub.p.sup.2)≦3.5 and 48≦V.sub.g where m g represents a paraxial lateral magnification of the double-convex glass lens of said rear lens group, m p represents a paraxial lateral magnification of the plastic lens subgroup of said rear lens group, and V g represents an Abbe's number of d-line of the double-convex glass lens of said rear lens group.

Claim 11

The retrofocus type lens system of claim 8, wherein said rear lens group consists, from the object side, of the double-convex glass lens, the plastic lens subgroup having a positive refracting power composed of one or more plastic lenses, and a negative lens.

Claim 12

The retrofocus type lens system of claim 8, further satisfying the following conditional expression, 1.5<D/f<5.0 where D represents an equivalent air distance between a surface of the front lens group provided closest to the image side and a surface of the rear lens group provided closest to the object side.

Claim 13

The retrofocus type lens system of claim 8, wherein said plastic lens subgroup of said rear lens group comprises one negative lens and one positive lens.

Claim 14

The retrofocus type lens system of claim 8, wherein said plastic lens subgroup of said rear lens group comprises one positive lens.

Claim 15

The retrofocus type lens system of claim 8 further comprising a diffraction type low-pass filter.

Claim 16

The retrofocus type lens system of claim 13 further comprising a fixed stop for cutting an off-axial light positioned either on the object side or the image side of the diffraction type low-pass filter.

Claim 17

A retrofocus type lens system comprising from an object side; (a) a front lens group including at least one negative lens; and (b) a rear lens group including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side, and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group comprises, from the object side, a first lens having a negative refracting power and a second lens having a positive refracting power, wherein said rear lens group consists of, from the object side, a third lens having a positive refracting power, a fourth lens having a negative refracting power and a fifth lens having a positive retracting power, wherein an aperture-stop is positioned between said front lens group and said rear lens group, wherein at least one of said first through fifth lenses has an aspheric surface, said surface having a thickness in a peripheral portion of the lens is greater than that between lens surfaces formed by a curvature in a vicinity of the optical axis, and further the following conditional expressions are satisfied, ##EQU7## where v 1 represents an Abbe's number of d-line of the first lens, v 2 represents an Abbe's number of d-line of the second lens, f i (I=3, 4, 5) represents a focal length of I-th lens, and v i (I=3, 4, 5) represents an Abbe's number of d-line of the I-th lens.

Claim 18

A retrofocus type lens system comprising from an object side; (a) a front lens group including at least one negative lens; and (b) a rear lens group including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side, and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group comprises, from the object side, a first lens having a negative refracting power and a second lens having a positive refracting power, wherein said rear lens group consists of, from the object side, a third lens having a positive refracting power, a fourth lens having a negative refracting power and a fifth lens having a positive retracting power, wherein an aperture-stop is positioned between said front lens group and said rear lens group, wherein at least one of said first through fifth lenses has an aspheric surface, said surface having a thickness in a peripheral portion of the lens is greater than that between lens surfaces formed by a curvature in a vicinity of the optical axis, and further the following conditional expressions are satisfied, ##EQU7## where v 1 represents an Abbe's number of d-line of the first lens, v 2 represents an Abbe's number of d-line of the second lens, f i (I=3, 4, 5) represents a focal length of I-th lens, and v i (I=3, 4, 5) represents an Abbe's number of d-line of the I-th lens.

Claim 19

The retrofocus type lens system of claim 15, wherein said aspherical surface is used on said fifth lens.

Claim 20

The retrofocus type lens system of claim 15, wherein said aspherical surface is used on either said second or third lens.

Claim 21

The retrofocus type lens system of claim 15, wherein all lenses except for said third lens are made of plastic.

Claim 22

The retrofocus type lens system of claim 15 further comprising a diffraction type low-pass filter.

Claim 23

The retrofocus type lens system of claim 19 further comprising a fixed stop for cutting an off-axial light positioned either on the object side or the image side of the diffraction type low-pass filter.

Claim 24

A retrofocus type lens system comprising from an object side: (a) a front lens group including at least one negative lens; and (b) a rear lens group, including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side, and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group consists of, from the object side, a biconcave lens having at least one aspherical surface, wherein said rear lens group consists of, from the object side, a single lens having a positive refracting power and a cemented lens in which a positive lens and a negative lens are cemented together, and at least one surface of said rear lens group is aspherical, and the following conditional expressions are satisfied, -1.7≦r.sub.a /r.sub.b ≦0.2 and 0≦f.sub.p /f.sub.c ≦1.1 where r a represents a radius of curvature on the object side of the single lens of said rear lens group, r b represents a radius of curvature on the image side of the single lens among said rear lens group, f p represents a focal length of the single lens of said rear lens group, and f c represents a focal length of the cemented lens of said rear lens group.

Claim 25

A retrofocus type lens system comprising from an object side: (a) a front lens group including at least one negative lens; and (b) a rear lens group, including at least two positive lenses and a negative lens, wherein a positive lens of the rear lens group is provided closest to an object side, and is made of glass, and wherein more than half of the lenses of said front and rear lens groups are made of plastic, and satisfy the following conditional expression, 0.7<f.sub.G /f<4.0 where f G represents a focal length of said positive lens of the rear lens group provided closest to the object side, and f represents a focal length of the retrofocus type lens system, wherein said front lens group consists of, from the object side, a biconcave lens having at least one aspherical surface, wherein said rear lens group consists of, from the object side, a single lens having a positive refracting power and a cemented lens in which a positive lens and a negative lens are cemented together, and at least one surface of said rear lens group is aspherical, and the following conditional expressions are satisfied, -1.7≦r.sub.a /r.sub.b ≦0.2 and 0≦f.sub.p /f.sub.c ≦1.1 where r a represents a radius of curvature on the object side of the single lens of said rear lens group, r b represents a radius of curvature on the image side of the single lens among said rear lens group, f p represents a focal length of the single lens of said rear lens group, and f c represents a focal length of the cemented lens of said rear lens group.

Claim 26

The retrofocus type lens system of claim 21 further comprising an aperture-stop provided on the object side of the cemented lens of said rear lens group.

Claim 27

The retrofocus type lens system of claim 22, wherein at least one surface adjacent to the aperture stop is aspherical, and a surface located closest to the image side of said rear lens group is aspherical.

Claim 28

The retrofocus type lens system of claim 21 further comprising a diffraction type low-pass filter.

Claim 29

The retrofocus type lens system of claim 24 further comprising a fixed stop for cutting an off-axial light positioned either on the object side or the image side of the diffraction type low-pass filter.