Imaging optical system and image pickup apparatus using the same

• US 20090052053A1
• Filed: 06/25/2008
• Published: 02/26/2009
• Est. Priority Date: 07/04/2007
• Status: Active Grant

• Current Assignee: OM Digital Solutions Corporation
• Original Assignee: Olympus Imaging Corporation (Olympus Corporation)
• Inventors: Iyama, Noriyuki
• Granted Patent: US 7,760,444 B2
• Time in Patent Office: Days
• US Class Current: 359/691
• CPC Class Codes: G02B 15/1425 the first group being negative

Abstract

An imaging optical system includes in order from an object side thereof a front lens unit having a negative refracting power, an aperture stop, and a rear lens unit having a positive refracting power, and the front lens unit having a negative refracting power includes in order from the object side, a first lens having a negative refracting power, and the rear lens unit having a positive refracting power includes in order from the object side, a second lens which is a positive lens, a third lens which is a negative lens, and a fourth lens which is a positive lens, and

• • the imaging optical system satisfies the following conditional expressions (1) and (2)

1.6<|f_G1|/F_A<5  (1)

1.2<f_G2/F_A<5  (2)

• • where,
  • F_A denotes a focal length of the overall imaging optical system,
  • f_G1 denotes a focal length of the front lens unit having a negative refracting power, and
  • f_G2 denotes a focal length of the rear lens unit having a positive refracting power.

Claims

1. An imaging optical system comprising in order from an object side thereof:

a front lens unit having a negative refracting power;

an aperture stop; and

a rear lens unit having a positive refracting power, wherein

the front lens unit having a negative refracting power includes in order from the object side, a first lens having a negative refracting power, and

the rear lens unit having a positive refracting power includes in order from the object side, a second lens which is a positive lens, a third lens which is a negative lens, and a fourth lens which is a positive lens, and

the imaging optical system satisfies the following conditional expressions (1) and (2)

1.6<

|f_G1|/F_A<

5 

 

(1)

1.2<

f_G2/F_A<

5 

 

(2)

where,

F_A denotes a focal length of the overall imaging optical system,

f_G1 denotes a focal length of the front lens unit having a negative refracting power, and

f_G2 denotes a focal length of the rear lens unit having a positive refracting power.

2. The imaging optical system according to claim 1, wherein

a surface on an image side of the fourth lens is an aspheric surface, and

toward a periphery of the aspheric surface on the image side, the aspheric surface has a shape such that the positive refracting power becomes weak, and

the fourth lens satisfies the following conditional expression (3)

0.5<

|R_4O|/|R_4I|<

1.6 

 

(3)

where,

R_4O denotes a paraxial radius of curvature of a surface on the object side, of the fourth lens, and

R_4I denotes a paraxial radius of curvature of a surface on an image side, of the fourth lens.

3. The imaging optical system according to claim 2, wherein a surface on the image side of the fourth lens has a shape such that it has a negative refracting power at an area nearest to an effective portion.

4. The imaging optical system according to claim 1, wherein the imaging optical system satisfies the following conditional expressions (4) and (5)

0.55<

d_L2/F_A<

1.2 

 

(4)

0.4<

d_L4/F_A<

1.5 

 

(5)

where,

F_A is a focal length of the overall imaging optical system,

d_L2 is a thickness on an optical axis of the second lens, and

d_L4 is a thickness on an optical axis of the fourth lens.

5. The imaging optical system according to claim 1, wherein the imaging optical system satisfies the following conditional expression (6)

65<

ν

_L1<

100 

 

(6)

where,

ν

_L1 denotes an Abbe'"'"'s number for the first lens.

6. The imaging optical system according to claim 1, wherein the first lens, the second lens, the third lens, and the fourth lens are made of glass.

8. The imaging optical system according to claim 1, wherein the first lens is a negative meniscus lens having a convex surface directed toward the object side.

9. The imaging optical system according to claim 1, wherein

each of the second lens, the third lens, and the fourth lens constituting the rear lens unit having a positive refracting power is a single lens, and

the second lens is a biconvex lens, the third lens is a biconcave lens, and the fourth lens is a biconvex lens.

10. An image pickup apparatus which picks up an image of an object by an image pickup element, comprising:

an imaging optical system described in claim 1, as an imaging optical system for imaging the object on the optical element.1.

7. An imaging optical system comprising in order from an object side thereof:

a front lens unit having a negative refracting power; and

a rear lens unit having a positive refracting power, wherein

the first lens unit having a negative refracting power includes in order from the object side, a first lens having a negative refracting power, and

the rear lens unit having a positive refracting power includes in order from the object side, a second lens which is a positive lens, a third lens which is a negative lens, and a fourth lens which is a positive lens, and

an aperture stop which determines an F number is provided between the front lens unit and the rear lens unit, and

the F number is 1.8 or less, and

when the F number changes, it assumes the minimum value.

11. The imaging optical system according to claim 7, wherein the first lens is a negative meniscus lens having a convex surface directed toward the object side.

12. The imaging optical system according to claim 7, wherein

each of the second lens, the third lens, and the fourth lens constituting the rear lens unit having a positive refracting power is a single lens, and

the second lens is a biconvex lens, the third lens is a biconcave lens, and the fourth lens is a biconvex lens.

13. An image pickup apparatus which picks up an image of an object by an image pickup element, comprising:

an imaging optical system described in claim 7, as an imaging optical system for imaging the object on the optical element.1.