Retro-focus type lens

• US 5,949,589 A
• Filed: 03/06/1998
• Issued: 09/07/1999
• Est. Priority Date: 03/18/1997
• 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: Nakazawa, Kimiaki
• Primary Examiner(s): Epps, Georgia
• Assistant Examiner(s): Lester, Evelyn A.
• Application Number:
  US09/035,753
• Time in Patent Office: 550 Days
• Field of Search: 359/649-651, 359/663, 359/749-753, 359/680-682, 359/784
• US Class Current: 359/753
• CPC Class Codes: G02B 13/04 Reversed telephoto objectives

Abstract

Negative, positive, and positive lens groups are disposed successively from the larger conjugate length side, and focal length adjustment is effected by the convex lens disposed on the smallest conjugate length side of the first lens group, thereby yielding a long back focus and a telecentric characteristic, decreasing distortion and chromatic aberration, and simplifying the focusing structure. Successively from the larger conjugate length side, a first lens group G₁ having a negative refracting power and including a convex lens L₅ disposed on the smallest conjugate length side, a second lens group G₂ having a positive refracting power and causing a luminous flux diverged by the first lens group G₁ to become substantially afocal, and a third lens group G₃ having a positive refracting power are disposed, and the convex lens L₅ disposed on the smallest conjugate length side of the first lens group G₁ is moved on an optical axis so as to effect focus adjustment.

Claims

1. A retro-focus type lens comprising, successively from a larger conjugate length side:

a first lens group having a negative refracting power and including a convex lens disposed on the smallest conjugate length side,

a second lens group having a positive refracting power and causing a luminous flux diverged by the first lens group to become substantially afocal, and

a third lens group having a positive refracting power;

wherein the convex lens disposed on the smallest conjugate length side of the first lens group is moved on an optical axis so as to effect focus adjustment.

2. A retro-focus type lens according to claim 1, wherein said first lens group comprises:

at least two convex lenses, and

at least two concave lenses each having a surface with a smaller radius of curvature directed onto the smaller conjugate length side.

3. A retro-focus type lens according to claim 1, satisfying the following conditional expression:

space="preserve" listing-type="equation">1.3<

f.sub.1R /f.sub.0 <

3.5

wherein f_1R is a focal length of the convex lens disposed on the smallest conjugate length side of the first lens group, and f₀ is a focal length of the whole system.

4. A retro-focus type lens according to claim 1, wherein the third lens group comprises, successively from the larger conjugate length side:

a cemented lens composed of a convex lens and a concave lens, and at least two convex lenses.