Objective lens for endoscope

• US 5,936,778 A
• Filed: 03/09/1998
• Issued: 08/10/1999
• Est. Priority Date: 03/19/1997
• Status: Expired due to Term

• Current Assignee: Fuji Photo Optical Co. Ltd (Fujifilm Holdings Corporation)
• Original Assignee: Fuji Photo Optical Co. Ltd (Fujifilm Holdings Corporation)
• Inventors: Miyano, Hitoshi, Yamamoto, Chikara
• Primary Examiner(s): Epps, Georgia
• Assistant Examiner(s): Spector, David N.
• Application Number:
  US09/036,781
• Time in Patent Office: 519 Days
• Field of Search: 359/660, 359/644, 359/656, 359/657, 359/658, 359/659, 359/663, 359/790, 359/793, 359/795, 359/661, 359/784, 359/788, 359/645, 359/689, 359/716, 359/728, 359/781, 600/101, 600/109, 600/110, 600/112, 600/129, 600/160, 600/168, 348/45, 348/68, 356/241
• US Class Current: 359/660
• CPC Class Codes: G02B 23/243 Objectives for endoscopes
• CPC Class Codes: G02B 9/34 having four components only

Abstract

In a retro-focus type objective lens for an endoscope, its front-group concave lens system is constituted by two concave lenses, and a predetermined conditional expression is satisfied, so as to secure a sufficient length of back focus, while fully favorably correcting chromatic aberration in magnification. A four group lens configuration having five lenses comprising a front-group concave lens system composed of two concave lenses L₁ and L₂, a rear convex lens system composed of lenses L₃ and L₅, with an aperture stop interposed therebetween, in order to secure a long back focus at least three times the composite focal length of the whole system, satisfied is the following conditional expression:

dx≧3.0×|f.sub.F |           (1)

wherein f_F is a composite focal length of the front-group concave lens system, and dx is a distance between a rear-side principal point of the front-group concave lens system and a front-side principal point of the rear-group convex lens system.

Claims

1. An objective lens for an endoscope, said objective lens comprising a front-group concave lens system, a rear-group convex lens system, and an aperture stop disposed therebetween;

wherein said front-group concave lens system is constituted by two concave lenses; and

wherein said objective lens satisfies the following conditional expression (1);

space="preserve" listing-type="equation">dx≧

3.0×

|f.sub.F | (1)

wherein

f_F is a composite focal length of said front-group concave lens system; and

dx is a distance between a rear-side principal point of said front-group concave lens system and a front-side principal point of said rear-group convex lens system.

2. An objective lens for an endoscope according to claim 1, further satisfying the following conditional expressions (2) and (3):

space="preserve" listing-type="equation">ν

d>

50×

(2.6-n.sub.d) (2)

space="preserve" listing-type="equation">2.0<

f.sub.2 /f.sub.1 <

4.0 (3)

wherein

ν

d is an Abbe number of the second lens in said front-group concave lens system;

nd is a refractive index of the second lens in said front-group concave lens system at d-line;

f₁ is a focal length of the first lens in said front-group concave lens system; and

f₂ is a focal length of the second lens in said front-group concave lens system.

3. An objective lens for an endoscope according to claim 2, wherein said rear-group convex lens system comprises, successively from the object side, a third convex lens having a surface with a small radius of curvature directed onto the image surface side, and a cemented lens composed of fourth and fifth lenses one of which is constituted by a convex lens while the other is constituted by a concave lens.

10. An objective lens for an endoscope according to claim 1, wherein said rear-group convex lens system comprises, successively from the object side a third convex lens having a surface with a small radius of curvature directed onto the image surface side, and a cemented lens composed of fourth and fifth lenses one of which is constituted by a convex lens while the other is constituted of a concave lens.

4. An objective lens for an endoscope, and objective lens comprising a front-group concave lens system, a rear-group convex lens system, and an aperture stop disposed therebetween;

wherein said front-group concave lens system is constituted by two concave lenses; and

wherein said objective lens satisfies the following conditional expressions (4) and (5);

space="preserve" listing-type="equation">ν

d>

50×

(2.6-nd) (4)

space="preserve" listing-type="equation">2.0<

f.sub.2 /f.sub.1 <

4.0 (5)

wherein

ν

d is an Abbe number of the second lens in said front-group concave lens system;

nd is a refractive index of the second lens in said front-group concave lens system at d-line;

f₁ is a focal length of the first lens in said front-group concave lens system; and

f₂ is a focal length of the second lens in said front-group concave lens system.

5. An objective lens for an endoscope according to claim 4, wherein said rear-group convex lens system comprises, successively from the object side, a third convex lens having a surface with a small radius of curvature directed onto the image surface side, and a cemented lens composed of fourth and fifth lenses one of which is constituted by a convex lens while the other is constituted by a concave lens.

6. An objective lens for an endoscope, said objective lens comprising a front-group concave lens system, a rear-group convex lens system, and an aperture stop disposed therebetween;

wherein said front-group concave lens system is constituted by two concave lenses;

wherein said objective lens satisfies the following conditional expression (1);

space="preserve" listing-type="equation">dx≧

3.0×

|f.sub.F | (1)

wherein

f_F is a composite focal length of said front-group concave lens system; and

dx is a distance between a rear-side principal point of said front-group concave lens system and a front-side principal point of said rear-group convex lens system; and

further satisfying the following conditional expression (2);

space="preserve" listing-type="equation">ν

d>

50×

(2.6-nd) (2)

wherein

ν

d is an Abbe number of the second lens in said front-group concave lens system;

nd is a refractive index of the second lens in said front-group concave lens system at d-line.

8. An objective lens for an endoscope according to claim 6, wherein said rear-group convex lens system comprises, successively from the object side a third convex lens having a surface with a small radius of curvature directed onto the image surface side, and a cemented lens composed of fourth and fifth lenses one of which is constituted by a convex lens while the other is constituted of a concave lens.

7. An objective lens for an endoscope, said objective lens comprising a front-group concave lens system, a rear-group convex lens system, and an aperture stop disposed therebetween;

wherein said front-group concave lens system is constituted by two concave lenses;

wherein said objective lens satisfies the following conditional expression (1);

space="preserve" listing-type="equation">dx≧

3.0×

|f.sub.F | (1)

wherein

f_F is a composite focal length of said front-group concave lens system; and

dx is a distance between a rear-side principal point of said front-group concave lens system and a front-side principal point of said rear-group convex lens system; and

further satisfying the following conditional expression (2);

space="preserve" listing-type="equation">2.0<

f.sub.2 /f.sub.1 <

4.0 (2)

wherein

f₁ is a focal length of the first lens in said front-group concave lens system; and

f₂ is a focal length of the second lens in said front-group concave lens system.

9. An objective lens for an endoscope according to claim 7, wherein said rear-group convex lens system comprises, successively from the object side a third convex lens having a surface with a small radius of curvature directed onto the image surface side, and a cemented lens composed of fourth and fifth lenses one of which is constituted by a convex lens while the other is constituted of a concave lens.