Optical system and image pickup apparatus
An optical system with a fixed focal length includes a front lens unit closest to an object, a rear lens unit having a negative refractive power closest to an image plane, and a middle unit having a positive refractive power as a whole, including one or more lens units, and disposed between the front lens unit and the rear lens unit. A distance between adjacent lens units changes during focusing. The rear lens unit includes two or more positive lenses. A predetermined condition is satisfied.
1. An optical system with a fixed focal length comprising:
a front lens unit closest to an object side;
a rear lens unit having a negative refractive power closest to an image plane; and
a middle group having a positive refractive power as a whole, including one or more lens units, and disposed between the front lens unit and the rear lens unit,
wherein a distance between adjacent lens units changes during focusing,
wherein the rear lens unit includes an air lens having a negative refractive power,
wherein the front lens unit includes a negative meniscus lens having a convex surface facing an image side,
wherein the rear lens unit includes two or more positive lenses, and
wherein the following conditional expressions are satisfied:
−0.40≤ f/fr≤− 0.12
65 ≤vdpl
0.00<| fpl/fr|≤ 0.60
—10.0≤( R 1+ R 2)/( R 1− R 2)<0.0
where f is a focal length of the optical system, fr is a focal length of the rear lens unit, vdpl and fpl are an Abbe number and a focal length of a positive lens having the largest Abbe number in the rear lens unit, respectively, R1 is a radius of curvature of a surface on the object side of the air lens, and R2 is a radius of curvature of a surface on the image side of the air lens.
2. The optical system according to claim 1 , wherein the following conditional expression is satisfied:
1.85 ≤ndph ≤2.10
where ndph is a refractive index of a positive lens having the largest refractive index in the rear lens unit.
3. The optical system according to claim 1 , wherein the following conditional expression is satisfied:
1.0≤ fm/f≤ 2.0
where fm is a focal length of the middle group.
4. The optical system according to claim 1 , wherein the following conditional expression is satisfied:
0.00<| f/ff|≤ 0.40
where ff is a focal length of the front lens unit.
5. The optical system according to claim 1 , wherein the following conditional expression is satisfied:
0.00<| sk/fr|≤ 0.20
where sk is a backfocus of the optical system when the optical system is in focus on an object at infinity.
6. The optical system according to claim 1 , wherein the front lens unit has a negative lens closest to the object side.
7. The optical system according to claim 1 , wherein the following conditional expression is satisfied:
25 ≤vdnl ≤50
where vdnl is an Abbe number of a negative lens having the smallest Abbe number in the rear lens unit.
8. The optical system according to claim 1 , wherein the rear lens unit includes four or more lenses.
9. The optical system according to claim 1 , wherein the front lens unit includes four or more lenses.
10. The optical system according to claim 1 , wherein the front lens unit is fixed during focusing.
11. The optical system according to claim 1 , wherein the rear lens unit is fixed during focusing.
12. The optical system according to claim 1 , wherein the front lens unit and the rear lens unit are fixed during focusing, and the middle group includes a lens unit configured to move during focusing.
13. The optical system according to claim 12 , wherein a lens unit configured to move during focusing moves toward the object side during focusing from infinity to a short distance.
14. The optical system according to claim 1 , wherein the middle group consists of one lens unit configured to move during focusing.
15. The optical system according to claim 1 , wherein the middle group includes two lens units configured to move on different trajectories during focusing.
16. The optical system according to claim 1 , wherein the front lens unit includes at least two cemented lenses where each cemented lens is a doublet.
17. The optical system according to claim 1 , further comprising an aperture stop,
wherein the aperture stop is disposed between a most object-side lens surface of the middle group and a most image-side surface of the middle group.
18. An image pickup apparatus comprising:
an optical system with a fixed focal length; and
an image sensor configured to capture an image formed by the optical system,
wherein the optical system includes:
a front lens unit closest to an object side;
a rear lens unit having a negative refractive power closest to an image plane; and
a middle group having a positive refractive power as a whole, including one or more lens units, and disposed between the front lens unit and the rear lens unit,
wherein a distance between adjacent lens units changes during focusing,
wherein the rear lens unit includes an air lens having a negative refractive power,
wherein the front lens unit includes a negative meniscus lens having a convex surface facing an image side,
wherein the rear lens unit includes two or more positive lenses, and
wherein the following conditional expressions are satisfied:
—0.40≤ f/fr≤− 0.12
65 ≤vdpl
0.00<| fpl/fr|≤ 0.60
−10.0≤( R 1+ R 2)/( R 1- R 2)<0.0
where f is a focal length of the optical system, fr is a focal length of the rear lens unit, vdpl and fpl are an Abbe number and a focal length of a positive lens having the largest Abbe number in the rear lens unit, respectively, R1 is a radius of curvature of a surface on the object side of the air lens, and R2 is a radius of curvature of a surface on the image side of the air lens.
19. An optical system with a fixed focal length comprising:
a front lens unit closest to an object side;
a rear lens unit having a negative refractive power closest to an image plane; and
a middle group having a positive refractive power as a whole, including two lens units, and disposed between the front lens unit and the rear lens unit,
wherein a distance between adjacent lens units changes during focusing,
wherein the two lens units in the middle group move on different trajectories during focusing,
wherein the front lens unit includes a negative meniscus lens having a convex surface facing an image side,
wherein the rear lens unit includes two or more positive lenses, and
wherein the following conditional expressions are satisfied:
—0.40≤ f/fr≤− 0.12
65 ≤vdpl
0.00<| fpl/fr|≤ 0.60
where f is a focal length of the optical system, fr is a focal length of the rear lens unit, and vdpl and fpl are an Abbe number and a focal length of a positive lens having the largest Abbe number in the rear lens unit, respectively.