Optical system and image pickup apparatus having the optical system
An optical system includes, in order from an object side to an image side, a first lens unit, a stop, and a second lens unit. The first lens unit includes a first negative lens having a meniscus shape and a negative refractive power at a position closest to the object side and a negative lens Gn having a negative refractive power at a position closer to the image side than the first negative lens. The second lens unit includes a positive lens Gp having a positive refractive power. A material of each of the negative lens Gn and the positive lens Gp satisfies the following condition: θ gF −(−0.001682·ν d +0.6438)≧0.01 where “νd” denotes an Abbe number of a lens material and “θgF” denotes a relative partial dispersion with respect to g-line light and F-line light.
1. An optical system comprising, in order from an object side to an image side:
a first lens unit;
a stop; and
a second lens unit,
wherein the first lens unit includes:
a first negative lens having a meniscus shape and a negative refractive power at a position closest to the object side; and
a negative lens Gn having a negative refractive power at a position closer to the image side than the first negative lens,
wherein the second lens unit includes a positive lens Gp having a positive refractive power,
wherein a material of each of the negative lens Gn and the positive lens Gp satisfies the following condition:
θ gF −(−0.001682·ν d+ 0.6438)≧0.01
where “νd” denotes an Abbe number of a lens material and “θgF” denotes a relative partial dispersion with respect to g-line light and F-line light, and
wherein an angle (θ) to an optical axis of an off-axis principal ray incident on a lens surface of the first negative lens facing the object side at an arbitrary image height y and a focal length (f) of the entire optical system satisfy the following condition:
1.8≦( y/f sin(θ/2))≦2.1.
2. The optical system according to claim 1 , wherein a focal length of the first negative lens (f 1 ) and the focal length of the entire optical system (f) satisfy the following condition:
−4.5≦ f 1/ f− 1.8.
3. The optical system according to claim 1 , wherein a focal length of the negative lens Gn (fex) and a focal length of the first negative lens (f 1 ) satisfy the following condition:
0.3≦ fex/f 1≦2.1.
4. The optical system according to claim 1 , wherein radiuses of curvature of surfaces of the first negative lens on the object side and on the image side, respectively (r 1 , r 2 ) and a shape factor of the first negative lens (SH 1 (=(r 1 −r 2 )/(r 1 +r 2 )) satisfy the following condition:
0.4 ≦SH 1≦1.0.
5. The optical system according to claim 1 , wherein a refractive index of a material of the first negative lens (Nd) and an Abbe number of the material of the first negative lens (νd) satisfy the following conditions:
1.63 ≦Nd≦ 1.89
40 ≦νd≦ 61.
6. The optical system according to claim 1 , wherein a part of the first lens unit moves in a direction of the optical axis during focusing.
7. The optical system according to claim 1 , wherein the first lens unit includes a cemented lens including a negative lens GCn having a negative refractive power and a positive lens GCp having a having a positive refractive power cemented to each other, and
wherein an Abbe number of a material of the negative lens GCn (νdn) and a relative partial dispersion of the material of the negative lens GCn with respect to g-line light and F-line light (θgFn) satisfy the following condition:
θ gFn −(−0.001682·ν dn+ 0.6438)≧0.01.
8. The optical system according to claim 7 , wherein an Abbe number of a material of the positive lens GCp (νdp) and a relative partial dispersion of the material of the positive lens GCp with respect to g-line light and F-line light (θgFp) satisfy the following condition:
θ gFp≦− 0.001682·ν dp+ 0.6438.
9. The optical system according to claim 1 , wherein the second lens unit includes a positive lens having an aspheric surface whose positive refractive power becomes more intense from a center of the positive lens towards a periphery thereof at a position closest to the object side.
10. The optical system according to claim 1 , wherein the optical system has a zooming function for zooming from a wide-angle end to a telephoto end by both the first lens unit and the second lens unit moving towards the object side in such a way that a distance between the first lens unit and the second lens unit becomes narrower.
11. The optical system according to claim 10 , wherein a focal length (fa) of the entire optical system at the wide-angle end or at the telephoto end and the angle (θ) to the optical axis of an off-axis principal ray incident on the lens surface of the first negative lens facing the object side at the arbitrary image height y satisfy the following condition:
y≦ 1500· fa ·sin(θ/2)/(750−2· fa ·sin(θ/2)).
12. An image pickup apparatus comprising the optical system according to claim 1 .