Zoom lens and image pickup apparatus including the same

1. A zoom lens comprising, in order from an object side to an image side:

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power; and

a rear lens group including at least two lens units,

at least the first lens unit and the second lens unit being configured to move for zooming to change an interval between each pair of adjacent lens units,

the first lens unit including, in order from the object side to the image side, a first negative lens, a second negative lens, and a third negative lens,

at least one of a lens surface on the object side or a lens surface on the image side of the first negative lens having an aspherical shape,

wherein conditional expressions

0.20< BFw/|f 1|<1.00, and

0.50< f _ BR/ft< 2.00

are satisfied where f1 represents a focal length of the first lens unit, f_BR represents a focal length of a lens unit BR arranged closest to the image side in the rear lens group, ft represents a focal length of the zoom lens at a telephoto end, and BFw represents a back focus of the zoom lens at a wide angle end.

2. The zoom lens according to claim 1 ,

wherein the lens unit BR includes a first positive lens arranged closest to the object side, and

wherein a conditional expression

ν d _ LA> 70

is satisfied where νd_LA represents an Abbe number of a material of the first positive lens.

3. The zoom lens according to claim 2 ,

wherein the lens unit BR includes a second positive lens arranged adjacent to, and on the image side of, the first positive lens, and

wherein a conditional expression

ν d _ LB> 70

is satisfied where νd_LB represents an Abbe number of a material of the second positive lens.

4. The zoom lens according to claim 3 , wherein the lens unit BR includes a fourth negative lens arranged adjacent to, and on the image side of, the second positive lens.

5. The zoom lens according to claim 2 , wherein a conditional expression

0.5 <f _ LA/ft< 1.8

is satisfied where f_LA represents a focal length of the first positive lens.

6. The zoom lens according to claim 4 , wherein a conditional expression

| f _ LC|/ft< 1.0

is satisfied where f_LC represents a focal length of the fourth negative lens.

7. The zoom lens according to claim 4 , wherein the lens unit BR includes a fifth negative lens adjacent to, and on the image side of, the fourth negative lens, the fifth negative lens having a lens surface having an aspherical shape.

8. The zoom lens according to claim 7 , wherein a biconvex air lens is formed between the fourth negative lens and the fifth negative lens.

9. The zoom lens according to claim 1 ,

wherein the lens unit BR includes a plurality of negative lenses, and

wherein a conditional expression

Nd _ BRN> 1.90

is satisfied where Nd_BRN represents an average value of refractive indices of materials of the plurality of negative lenses included in the lens unit BR.

10. The zoom lens according to claim 1 , wherein at least one of a lens surface on the object side or a lens surface on the image side of the second negative lens has an aspherical shape.

11. The zoom lens according to claim 1 , wherein the first negative lens has a lens surface having an aspherical shape in which a positive refractive power is increased from an optical axis to a periphery.

12. The zoom lens according to claim 1 , wherein at least a part of the second lens unit is configured to move from the object side toward the image side for focusing from infinity to proximity.

13. The zoom lens according to claim 1 , wherein the rear lens group consists of, in order from the object side to the image side:

a third lens unit having a positive refractive power;

a fourth lens unit having a negative refractive power; and

a fifth lens unit having a positive refractive power.

14. The zoom lens according to claim 1 , wherein the rear lens group consists of, in order from the object side to the image side:

a third lens unit having a negative refractive power; and

a fourth lens unit having a positive refractive power.

15. The zoom lens according to claim 1 , wherein the rear lens group consists of, in order from the object side to the image side:

a third lens unit having a positive refractive power;

a fourth lens unit having a positive refractive power;

a fifth lens unit having a negative refractive power; and

a sixth lens unit having a positive refractive power.

16. An image pickup apparatus comprising:

a zoom lens; and

an image pickup element configured to receive an image formed by the zoom lens,

the zoom lens including, in order from an object side to an image side:

a first lens unit having a negative refractive power;

a second lens unit having a positive refractive power; and

a rear lens group including at least two lens units,

at least the first lens unit and the second lens unit are configured to move for zooming to change an interval between each pair of adjacent lens units,

the first lens unit including, in order from the object side to the image side, a first negative lens, a second negative lens, and a third negative lens,

at least one of a lens surface on the object side or a lens surface on the image side of the first negative lens having an aspherical shape,

wherein conditional expressions

0.20< BFw/|f 1|<1.00; and

0.50< f _ BR/ft< 2.00,

are satisfied where f1 represents a focal length of the first lens unit, f_BR represents a focal length of a lens unit BR arranged closest to the image side in the rear lens group, ft represents a focal length of the zoom lens at a telephoto end, and BFw represents a back focus of the zoom lens at a wide angle end.

17. The image pickup apparatus according to claim 16 , wherein a conditional expression

ω>50°,

is satisfied where w represents an image pickup half angle of view at the wide angle end.