Optical system and image pickup apparatus

1. An optical system consisting of, in order from an object side to an image side:

a front lens group;

an aperture stop; and

a rear lens group,

wherein the front lens group includes at least one positive lens and at least one negative lens, the at least one positive lens including a positive lens Lp arranged closest to the object side in the front lens group, the at least one negative lens including a negative lens Ln arranged closest to the image side among the at least one negative lens, and

wherein the following conditional expressions are satisfied:

1.0< fLp/Dps< 1.5;

0.35< Dps/Dsr< 0.85;

−10.0< fLn/Dns<− 4.0; and

1.85<Npave<2.00,

where fLp represents a focal length of the positive lens Lp, fLn represents a focal length of the negative lens Ln, Dps represents a distance on an optical axis from a lens surface on the object side of the positive lens Lp to the aperture stop, Dns represents a distance on the optical axis from a lens surface on the object side of the negative lens Ln to the aperture stop, Dsr represents a distance on the optical axis from the aperture stop to a lens surface closest to the image side in the rear lens group when the optical system is focused at infinity, and Npave represents an average value of refractive indices with respect to a d-line of materials of all the at least one positive lens included in the front lens group.

2. An optical system according to claim 1 , wherein the following conditional expression is satisfied:

1.60< Nn ave<1.75,

where Nnave represents an average value of refractive indices with respect to a d-line of materials of all the at least one negative lens included in the front lens group.

3. An optical system according to claim 1 , wherein the following conditional expression is satisfied:

0.1< fLp/ff< 0.5,

where “ff” represents a focal length of the front lens group.

4. An optical system according to claim 1 , wherein the following conditional expression is satisfied:

−1.8< fLp/fLn<− 0.8.

5. An optical system according to claim 1 , wherein the following conditional expression is satisfied:

0.1< fr/ff< 0.5,

where “ff” represents a focal length of the front lens group, and “fr” represents a focal length of the rear lens group.

6. An optical system according to claim 1 , wherein the front lens group, the rear lens group, and the aperture stop form a plurality of lens units, in which an interval between each pair of adjacent lens units among the plurality of lens units is changed during focusing.

7. An optical system according to claim 6 , wherein the plurality of lens units consist of:

a first lens unit having a positive refractive power, which is configured to move toward the object side during focusing from infinity to a closest distance; and

a second lens unit having a positive refractive power, which is arranged on the image side of the first lens unit, and is configured not to move during focusing from infinity to the closest distance.

8. An optical system according to claim 6 , wherein the plurality of lens units consist of:

a first lens unit having a positive refractive power, which is configured to move toward the object side during focusing from infinity to a closest distance; and

a second lens unit having a positive refractive power, which is arranged on the image side of the first lens unit, and is configured to move toward the image side during focusing from infinity to the closest distance.

9. An optical system according to claim 7 , wherein the following conditional expression is satisfied:

0.01< f 1/ f 2<0.30,

where f1 represents a focal length of the first lens unit, and f2 represents a focal length of the second lens unit.

10. An optical system according to claim 8 , wherein the following conditional expression is satisfied:

0.01< f 1/ f 2<0.30,

where f1 represents a focal length of the first lens unit, and f2 represents a focal length of the second lens unit.

11. An optical system according to claim 1 , wherein the front lens group includes, in order from the object side to the image side, a first lens having a positive refractive power, a second lens having a negative refractive power, a third lens having a positive refractive power, and a fourth lens having a negative refractive power.

12. An image pickup apparatus comprising:

an optical system; and

an image pickup element configured to receive light of an image formed by the optical system,

wherein the optical system is an optical system including a plurality of lens units and an aperture stop,

wherein the plurality of lens units consists of:

a front lens group arranged on an object side of the aperture stop; and

a rear lens group arranged on an image side of the aperture stop,

wherein the front lens group includes at least one positive lens and at least one negative lens, the at least one positive lens including a positive lens Lp arranged closest to the object side in the front lens group, the at least one negative lens including a negative lens Ln arranged closest to the image side among the at least one negative lens, and

wherein the following conditional expressions are satisfied:

1.0< fLp/Dps< 1.5;

0.35< Dps/Dsr< 0.85;

−10.0< fLn/Dns<− 4.0; and

1.85<Npave<2.00,

where fLp represents a focal length of the positive lens Lp, fLn represents a focal length of the negative lens Ln, Dps represents a distance on an optical axis from a lens surface on the object side of the positive lens Lp to the aperture stop, Dns represents a distance on the optical axis from a lens surface on the object side of the negative lens Ln to the aperture stop, Dsr represents a distance on the optical axis from the aperture stop to a lens surface closest to the image side in the rear lens group when the optical system is focused at infinity, and Npave represents an average value of refractive indices with respect to a d-line of materials of all the at least one positive lens included in the front lens group.