Imaging lens system and imaging apparatus

1. An imaging lens system comprising:

a negative first lens that is disposed on a most object side, has a concave surface directed toward the object side and has a meniscus shape;

a cemented lens that is disposed on a most image side, a most object side surface of the cemented lens being a convex surface; and

a stop that is disposed just in front of the object side of the cemented lens, wherein

the following conditional expressions are satisfied:

0.05<( R 2− R 1)/( R 1+ R 2)<0.25   (1)

vd 1− vd 2>15   (2)

where R 1 denotes a radius of curvature of an object side surface of the first lens,

R 2 denotes a radius of curvature of an image side surface of the first lens,

vd 1 denotes an Abbe number of a lens, which is located on the most object side among lenses constituting the cemented lens, at the d-line, and

vd 2 denotes an Abbe number of a lens, which is located on the most image side among the lenses constituting the cemented lens, at the d-line.

2. The imaging lens system according to claim 1 , wherein all the lenses constituting the cemented lens are positive lenses.

3. The imaging lens system according to claim 1 , wherein the following conditional expressions are satisfied:

0.10<( R 2− R 1)/( R 1+ R 2)<0.20   (1-1)

where R 1 denotes a radius of curvature of the object side surface of the first lens, and

R 2 denotes a radius of curvature of the image side surface of the first lens.

4. The imaging lens system according to claim 1 , wherein an absolute value of a radius of curvature of each surface of the cemented lens is larger as approaching the image side from the object side.

5. The imaging lens system according to claim 1 , wherein refractive indexes of all the tenses at the d-line are larger than 1.75.

6. The imaging lens system according to claim 1 , wherein the following conditional expression is satisfied:

vd 1− vd 2>20   (2-1),

where vd 1 denotes an Abbe number of a lens, which is located on the most object side among lenses constituting the cemented lens, at the d-line, and

vd 2 denotes an Abbe number of a lens, which is located on the most image side among the lenses constituting the cemented lens, at the d-line.

7. An imaging apparatus comprising:

the imaging lens system according to claim 1 ; and

an imaging device that converts an optical image formed by the imaging lens system into an electric signal.

8. An imaging lens system comprising, in order from an object side:

a negative first lens that has a concave shape directed toward the object side and has a meniscus shape;

a positive second lens that has a convex surface directed toward the object side;

a positive third lens that has a meniscus shape with a convex shape directed toward the object side;

a negative fourth lens that has a meniscus shape with a convex surface directed toward the object side;

a stop; and

a cemented lens including a positive fifth lens and a positive sixth lens, the cemented lens having a convex surface directed toward the object side.

9. The imaging lens system according to claim 8 , wherein the following conditional expressions are satisfied:

0.10<( R 2− R 1)/( R 1+ R 2)<0.20   (1-1)

where R 1 denotes a radius of curvature of the object side surface of the first lens, and

R 2 denotes a radius of curvature of the image side surface of the first lens.

10. The imaging lens system according to claim 8 , wherein an absolute value of a radius of curvature of each surface of the cemented lens is larger as approaching the image side from the object side.

11. The imaging lens system according to claim 8 , wherein refractive indexes of all the lenses at the d-line are larger than 1.75.

12. The imaging lens system according to claim 8 , wherein the following conditional expression is satisfied:

vd 1− vd 2>20   (2-1),

where vd 1 denotes an Abbe number of a lens, which is located on the most object side among lenses constituting the cemented lens, at the d-line, and

vd 2 denotes an Abbe number of a lens, which is located on the most image side among the lenses constituting the cemented lens, at the d-line.

13. An imaging apparatus comprising:

the imaging lens system according to claim 8 ; and

an imaging device that converts an optical image formed by the imaging lens system into an electric signal.

14. An imaging lens system comprising:

a negative meniscus lens that is disposed on a most object side and has a concave surface directed toward the object side;

a cemented lens that is disposed on a most image side, a most object side surface of the cemented lens being a convex surface, the cemented lens being formed by cementing positive lenses; and

a stop that is disposed just in front of the object side of the cemented lens, wherein

an incident angle of an axial marginal ray to the most object side surface of the cemented lens is 8° or more, and

an exit angle of the axial marginal ray from the most image side surface of the cemented lens is 8° or more.

15. The imaging lens system according to claim 14 , wherein in air-exposed surfaces of all the lenses of the whole system, respective incident angles of the axial marginal rays from air to the surfaces are 6° or more, and exit angles of the axial marginal rays from the surfaces to air is 6° or more.

16. The imaging lens system according to claim 14 , wherein refractive indexes of all the lenses at the d-line are larger than 1.8.