Ojective lens for endoscope

• US 4,984,878 A
• Filed: 08/08/1989
• Issued: 01/15/1991
• Est. Priority Date: 09/29/1988
• 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
• Primary Examiner(s): Sugarman, Scott J.
• Application Number:
  US07/390,758
• Time in Patent Office: 525 Days
• Field of Search: 350/414, 350/469
• US Class Current: 359/783
• CPC Class Codes: G02B 13/22 Telecentric objectives or lens systems
• CPC Class Codes: G02B 23/243 Objectives for endoscopes
• CPC Class Codes: G02B 9/34 having four components only

Abstract

An objective lens particularly suitable for use in endoscope, employing a lens system which is composed of four single lenses, namely, composed of, from the object side, a first lens L₁ consisting of a negative lens having the concave surface thereof on the object side, a second lens L₂ consisting of a positive lens having a surface of smaller radius of curvature disposed on the object side, a third lens L₃ consisting of a positive lens having a surface of smaller radius of curvature on the image side, and a fourth lens L₄ consisting of a positive lens. In this lens construction, the respective lenses L₁ to L₄ are adapted to satisfy the conditions of n₁ >1.70, γ₁ >40.0, γ₃ <42.5, n₅ >1.60, γ₅ >50.0, n₇ >1.60, and γ₇ >50.0 (where n₁, n₅ and n₇ are refractive indices of the first lens L₁, third lens L₃ and fourth lens L₄ relative to line d, respectively, and γ ₁, γ₃, γ₅ and γ₇ are Abbe'"'"'s numbers of the first to fourth lenses L₁ and L₄, respectively.)

Claims

1. An objective lens for an endoscope, having a lens system consisting of four single lenses designated as first lens L₁, second lens L₂, third lens L₃ and fourth lens L₄ from the object side, wherein:

said first lens L₁ consists of a negative lens having a concave surface on the image side;

said second lens L₂ consists of a positive lens having a surface of a smaller radius of curvature on the object side;

said third lens L₃ consists of a positive lens having a surface of a smaller radius of curvature on the image side; and

said fourth lens L₄ consists of a positive lens;

said first to fourth lenses L₁ to L₄ of said lens system being arranged to satisfy the following conditions (1) to (4). ##EQU1## where n₁, n₅ and n₇ are refractive indices of said first lens L₁, third lens L₃ and fourth lens L₄ relative to line d, and γ

₁, γ

₃, γ

₅ and V₇ are Abbe'"'"'s numbers of said first lens L₁, second lens L₂, third lens L₃ and fourth lens L₄, respectively.

2. An objective lens as defined in claim 1, wherein said first to fourth lenses L₁ to L₄ are arranged to satisfy further the following conditions (5) and (6):

space="preserve" listing-type="equation">(5) d.sub.1 +d.sub.2 +d.sub.3 <

2.5f, and

space="preserve" listing-type="equation">(6) 2f<

r.sub.7 /(n.sub.7 -1.0)<

3f

where d₁ is the center thickness of said first lens L₁, d₂ is the airspace between said first and second lenses L₁ and L₂, d₃ is the center thickness of said second lens L₂, f is the resultant focal length of said system, and r₇ is the radius of curvature of the object-side surface of said fourth lens L₄.

3. An objective lens as defined in claim 1, further comprising a stop interposed between said second and third lenses L₂ and L₃.

4. An objective lens as defined in claim 1, wherein said lens system has the image plane thereof located to coincide with the image-side surface of said fourth lens L₄.

5. An objective lens as defined in claim 4, wherein said the focus of said lens system is adjusted by shifting said fourth lens L₄ in the direction of the optical axis.

6. An objective lens as defined in claim 1, wherein said lens system is arranged to have an imaging position located rearward of said fourth lens L₄.

7. An objective lens as defined in claim 5, wherein the focus of said lens system is adjusted by varying the distance between said fourth lens L₄ and said image plane.