Imaging-optical system

• US 7,158,314 B2
• Filed: 03/09/2005
• Issued: 01/02/2007
• Est. Priority Date: 03/10/2004
• Status: Expired due to Fees

• Current Assignee: Pentax Corporation (Hoya Corporation)
• Original Assignee: Pentax Corporation (Hoya Corporation)
• Inventors: Fujii, Hiroaki
• Primary Examiner(s): Sugarman; Scott J.
• Application Number:
  US11/074,716
• Publication Number:
  US 20050200971A1
• Time in Patent Office: 664 Days
• Field of Search: 359/676, 359/683, 359/689
• US Class Current: 359/689
• CPC Class Codes: G02B 13/04 Reversed telephoto objectives

Abstract

An imaging-optical system includes a negative first lens group, a positive second lens group and a positive third lens group.

A zooming operation is performed by moving the second and third lens groups while an object distance to the object is being varied without varying a distance from the most object-side surface of the first lens group to the imaging plane.

An in-focus state is maintained while the focal length of the imaging-optical system is being varied through the zooming operation.

The first lens group includes cemented lens elements, and the cemented lens elements are positioned at the most object-side of the first lens group.

The cemented lens elements satisfy the following condition:
0.9<Ra/fw<2.0  (1)

wherein

Ra designates the radius of curvature of a bonding surface of the cemented lens elements of the first lens group; and

fw designates the focal length of the imaging-optical system at the short focal length extremity.

Claims

1. An imaging-optical system comprising a negative first lens group, a positive second lens group and a positive third lens group, in this order from an object,

wherein a zooming operation is performed by moving said positive second lens group and said positive third lens group while an object distance to said object is being varied without varying a distance from the most object-side surface of said negative first lens group to the imaging plane;

wherein an in-focus state of said imaging-optical system is maintained while the focal length of said imaging-optical system is being varied through said zooming operation;

wherein said negative first lens group comprises cemented lens elements, and said cemented lens elements are positioned at the most object-side of said negative first lens group; and

wherein said cemented lens elements satisfy the following condition;

0.9<

Ra/fw<

2.0

wherein

Ra designates the radius of curvature of a bonding surface of said cemented lens elements of said negative first lens group; and

fw designates the focal length of said imaging-optical system at the short focal length extremity.

2. The imaging-optical system according claim 1, satisfying the following condition:

0.6<

Ra/ft<

2.0

wherein

Ra designates the radius of curvature of said bonding surface of said cemented lens elements of said negative first lens group; and

ft designates the focal length of said imaging-optical system at the maximum magnification position.

3. The imaging-optical system according claim 1, satisfying the following conditions:

ν

12<

30

n11>

1.8

wherein

ν

12 designates the Abbe number of an image-side lens element of said cemented lens elements of said negative first lens group; and

n11 designates the refractive index of an object-side lens element of said cemented lens elements of said negative first lens group.

4. The imaging-optical system according claim 1, wherein said negative first lens group consists of said cemented lens elements having a negative refractive power.

5. The imaging-optical system according claim 1, satisfying the following conditions:

−

3.0<

f1/fw<

−

1.5

2<

f3/fw<

3

wherein

f1 designates the focal length of said negative first lens group;

f3 designates the focal length of said positive third lens group; and

fw designates the focal length of said imaging-optical system at the short focal length extremity.

6. The imaging-optical system according claim 1, wherein said positive third lens group comprises cemented lens elements comprising a negative lens element and a positive lens element;

and

wherein said cemented lens elements satisfy the following conditions;

1.2<

|Rb|/fw<

2.5

0.8<

|Rb|/ft<

2.3

wherein

Rb designates the radius of curvature of a bonding surface of said cemented lens elements of said positive third lens group;

fw designates the focal length of said imaging-optical system at the short focal length extremity; and

ft designates the focal length of said imaging-optical system at the maximum magnification position.

7. An endoscope comprising an imaging optical system according to claim 1.

8. An imaging-optical system comprising a negative first lens group, a positive second lens group and a positive third lens group, in this order from an object,

wherein a zooming operation is performed by moving said positive second lens group and said positive third lens group while an object distance to said object is being varied without varying a distance from the most object-side surface of said negative first lens group to the imaging plane;

wherein an in-focus state of said imaging-optical system is maintained while the focal length of said imaging-optical system is being varied through said zooming operation;

wherein said negative first lens group comprises cemented lens elements, and said cemented lens elements are positioned at the most object-side of said negative first lens group; and

wherein said cemented lens elements satisfy the following condition;

−

1.1<

Ra/f1<

−

0.5

wherein

Ra designates the radius of curvature of a bonding surface of said cemented lens elements of said negative first lens group; and

f1 designates the focal length of said negative first lens group.

9. The imaging-optical system according claim 8, satisfying the following condition:

0.6<

Ra/ft<

2.0

wherein

Ra designates the radius of curvature of said bonding surface of said cemented lens elements of said negative first lens group; and

ft designates the focal length of said imaging-optical system at the maximum magnification position.

10. The imaging-optical system according claim 8, satisfying the following conditions:

ν

12<

30

n11>

1.8

wherein

ν

12 designates the Abbe number of an image-side lens element of said cemented lens elements of said negative first lens group; and

n11 designates the refractive index of an object-side lens element of said cemented lens elements of said negative first lens group.

11. The imaging-optical system according claim 8, wherein said negative first lens group consists of said cemented lens elements having a negative refractive power.

12. The imaging-optical system according claim 8, satisfying the following conditions:

−

3.0<

f1/fw<

−

1.5

2<

f3/fw<

3

wherein

f1 designates the focal length of said negative first lens group;

f3 designates the focal length of said positive third lens group; and

fw designates the focal length of said imaging-optical system at the short focal length extremity.

13. The imaging-optical system according claim 8, wherein said positive third lens group comprises cemented lens elements comprising a negative lens element and a positive lens element;

and

wherein said cemented lens elements satisfy the following conditions;

1.2<

|Rb|/fw<

2.5

0.8<

|Rb|/ft<

2.3

wherein

Rb designates the radius of curvature of a bonding surface of said cemented lens elements of said positive third lens group;

fw designates the focal length of said imaging-optical system at the short focal length extremity; and

ft designates the focal length of said imaging-optical system at the maximum magnification position.

14. An endoscope comprising an imaging optical system according to claim 8.