Zoom lens

• US 4,909,613 A
• Filed: 05/02/1989
• Issued: 03/20/1990
• Est. Priority Date: 05/02/1988
• Status: Expired due to Fees

• Current Assignee: Ricoh Company Limited
• Original Assignee: Ricoh Company Limited
• Inventors: Kikuchi, Shuichi
• Primary Examiner(s): Arnold, Bruce Y.
• Assistant Examiner(s): Gass, Rebecca D.
• Application Number:
  US07/347,221
• Time in Patent Office: 322 Days
• Field of Search: 350/423, 350/426
• US Class Current: 359/689
• CPC Class Codes: G02B 15/142 having two groups only
• CPC Class Codes: G02B 15/1421 the first group being positive

Abstract

A zoom lens comprises a front lens group having a positive focal length and a rear lens group having a negative focal length, with the distance between the front and rear lens groups being variable for varying the magnification of the zoom lens. The front lens group comprises a first front lens group having a negative focal length and a second front lens group having a positive focal length, the first and second front lens groups being successively arranged in the order specified from an object side. The first front lens group comprises, in the order named from the object side, at least a positive lens and a negative lens. The second front lens group comprises at least one negative lens and at least two positive lenses. The rear lens group comprises, in the order specified from the object side, a positive lens, a negative lens, a negative lens, and a positive lens. The distance between the first and second front lens groups varies during zooming movement of the zoom lens.

Claims

1. A zoom lens comprising a front lens group having a positive focal length and a rear lens group having a negative focal length, with the distance between the front and rear lens groups being variable for varying the magnification of the zoom lens, said front lens group comprising a first front lens group having a negative focal length and a second front lens group having a positive focal length, the first and second front lens groups being successively arranged in the order specified from an object side, said first front lens group comprising, in the order named from the object side, at least a positive lens and a negative lens, said second front lens group comprising at least one negative lens and at least two positive lenses, said rear lens group comprising, in the order specified from the object side, a positive lens, a negative lens, a negative lens, and a positive lens, said zoom lens meeting the following conditions:

space="preserve" listing-type="equation">0.6<

f.sub.1 /f.sub.W <

0.9 (i)

space="preserve" listing-type="equation">0.15<

|f.sub.2 |/f.sub.T <

0.3 (ii)

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

.sub.1ap <

ν

.sub.1an (III)

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

.sub.1bp >

ν

.sub.1bn (IV)

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

.sub.23 >

ν

.sub.24 (V)

where f₁ is the focal length of the front lens group, f₂ is the focal length of the rear lens group, f_W is the focal length of the zoom lens at the wide angle end of the zoom lens stroke, f_T is the focal length of the zoom lens at the telephoto end of the zoom lens stroke, ν

_1ap is the average of Abbe numbers of the positive lens in the first front lens group, ν

_1an is the average of Abbe numbers of the negative lens in the first front lens group, ν

_1bp is the average of Abbe numbers of the positive lenses in the second front lens group, ν

_1bn is the average of Abbe numbers of the negative lens in the second front lens group, ν

₂₃ is the Abbe number of the third lens in the rear lens group, and ν

₂₄ is the Abbe number of the fourth lens in the rear lens groups.

2. A zoom lens comprising a front lens group having a positive focal length and a rear lens group having a negative focal length, with the distance between the front and rear lens groups being variable for varying the magnification of the zoom lens, said front lens group comprising a first front lens group having a negative focal length and a second front lens group having a positive focal length, the first and second front lens groups being successively arranged in the order specified from an object side, said first front lens group comprising, in the order named from the object side, at least a positive lens and a negative lens, said second front lens group comprising at least one negative lens and at least two positive lenses, said rear lens group comprising, in the order specified from the object side, a positive lens, a negative lens, a negative lens, and a positive lens, said zoom lens meeting the following conditions:

space="preserve" listing-type="equation">0.6<

f.sub.1 /f.sub.W <

0.9 (I)

space="preserve" listing-type="equation">0.15<

|f.sub.2 |/f.sub.T <

0.3 (II)

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

.sub.1ap <

ν

.sub.1an (III)

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

.sub.1bp >

ν

.sub.1bn (IV)

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

.sub.23 >

ν

.sub.24 (V)

where f₁ is the focal length of the front lens group, f₂ is the focal length of the rear lens group, f_W is the focal length of the zoom lens at the wide angle end of the zoom lens stroke, f_T is the focal length of the zoom lens at the telephoto end of the zoom lens stroke, ν

_1ap is the average of Abbe numbers of the positive lens in the front front lens group, ν

_1an is the average of Abbe numbers of the negative lens in the first front lens group, ν

_1bp is the average of Abbe numbers of the positive lenses in the second front lens group, ν

_1bn is the average of Abbe numbers of the negative lens in the second front lens group, ν

₂₃ is the Abbe number of the third lens in the rear lens group, and ν

₂₄ is the Abbe number of the fourth lens in the rear lens group, the distance between said first and second front lens groups being variable during zooming movement.