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;
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 front lens unit includes four or more lenses,
wherein the middle group includes three or more positive lenses and one or more negative lenses,
wherein the rear lens unit consists of two positive lenses and two negative lenses, and
wherein the following inequality is satisfied:
-
0.4
≤
f
/
fr
<
0.
where f is a focal length of the optical system, and fr is a focal length of the rear lens unit.
2. The optical system according to claim 1 , wherein the following inequality is satisfied:
1.85
≤
ndpl
≤
2.1
where ndpl 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 inequality is satisfied:
1.
≤
fm
/
f
≤
2.
where fm is a focal length of the middle group.
4. The optical system according to claim 1 , wherein the following inequality is satisfied:
0.
<
❘
"\[LeftBracketingBar]"
f
/
ff
❘
"\[RightBracketingBar]"
≤
0.4
where ff is a focal length of the front lens unit.
5. The optical system according to claim 1 , wherein the following inequality is satisfied:
0.
<
❘
"\[LeftBracketingBar]"
sk
/
fr
❘
"\[RightBracketingBar]"
≤
0.2
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.
7. The optical system according to claim 1 , wherein the following inequality 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 an air lens having a negative refractive power, and
wherein the following inequality is satisfied:
-
10.
≤
(
R
1
+
R
2
)
/
(
R
1
-
R
2
)
<
0.
where R1 is a radius of curvature of a surface on an object side of the air lens, and R2 is a radius of curvature of a surface on the image side of the air lens.
9. The optical system according to claim 1 , wherein the front lens unit is fixed during focusing.
10. The optical system according to claim 1 , wherein the rear lens unit is fixed during focusing.
11. 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.
12. The optical system according to claim 11 , wherein a lens unit configured to move during focusing moves toward an object side during focusing from infinity to a short distance.
13. The optical system according to claim 1 , wherein the middle group consists of one lens unit configured to move during focusing.
14. The optical system according to claim 1 , wherein the middle group includes two lens units configured to move on different trajectories during focusing.
15. 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;
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 front lens unit includes four or more lenses,
wherein the middle group includes three or more positive lenses and one or more negative lenses,
wherein the rear lens unit consists of two positive lenses and two negative lenses, and
wherein the following inequality is satisfied:
-
0.4
≤
f
/
fr
<
0.
where f is a focal length of the optical system, and fr is a focal length of the rear lens unit.
16. An optical system with a fixed focal length comprising:
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 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 middle group includes three or more positive lenses and one or more negative lenses,
wherein the rear lens unit consists of two positive lenses and two negative lenses,
wherein the two lens units in the middle group are configured to move on different trajectories during focusing, and
wherein the following inequality is satisfied:
-
0
.
4
0
≤
f
/
fr
<
0
.
0
0
where f is a focal length of the optical system, and fr is a focal length of the rear lens unit.