# US 4,394,071 A · Lens system with fill-in lens

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- Number: US 4,394,071 A
- Document ID: US4394071A
- Status: Granted
- Kind: A
- Application number: 06/296,649
- Filed: 1981-08-27
- Granted: 1983-07-19
- Earliest claimed: 1980-09-03
- Assignees: Canon Kabushiki Kaisha
- Inventors: Yasuyuki Yamada
- Web page: https://patentgrep.com/patent/US4394071A

## CPC

- G02B 9/62: having six components only
- G02B 15/02: by changing, adding, or subtracting a part of the objective, e.g. convertible objective
- G02B: OPTICAL ELEMENTS, SYSTEMS OR APPARATUS

## Abstract

A lens system having a main photo-taking lens with a fill-in lens detachably set in between the main photo-taking lens and the image plane to change the focal length of the entire system. This fill-in lens has, from front to rear, a positive 1st lens element, a negative 2nd lens element and a positive 3rd lens element. The glass of the positive 1st lens element and the lens form and glass of the positive 3rd lens element are properly chosen to minimize the bulk and size, while still achieving improvements of the image quality.

## Claims

### Claim 1

A lens system having a main photo-taking lens and a fill-in lens lying in between said main photo-taking lens and the image plane and detachable therefrom, and having the following various conditions:
said fill-in lens having, from front to rear, a positive 1st lens, a negative 2nd lens and a positive 3rd lens;
letting n.sub.1 and .nu..sub.1 denote the refractive index and Abbe number of the glass of said positive 1st lens respectively, r.sub.5 the radius of curvature of the front surface of said positive 3rd lens, d.sub.5 the axial thickness of said 3rd lens, n.sub.3 and .nu..sub.3 the refractive index and Abbe number of the glass of said 3rd lens respectively, and f the overall focal length of said main photo-taking lens and said fill-in lens attached thereto, the following relationships are fulfilled:
(1) 0.3<r.sub.5 /f<0.6
(2) 0.02<d.sub.5 /f<0.08
(3) 1.5 <n.sub.1 <1.7
(4) 30.0<.nu..sub.1 <50.0
(5) 1.45<n.sub.3 <1.75
(6) 45.0<.nu..sub.3 <85