Claims
- 1. A zoom lens system that comprises, in order from the object side, a first lens group that includes at least three lens elements and that has a negative overall power and a second lens group that includes at least four lens elements, said second lens group being composed of, in order from the object side, a positive lens unit, a cemented lens having a positive lens element and a negative lens element, and a positive lens element, said second lens group element having a positive overall power, said zoom lens system being capable of changing its focal length by adjusting the overall distance between the first and the second lens groups to achieve a zoom ratio greater than 2.5, and said zoom lens system satisfying the following conditions (a) and (b):
- 0. 05<n.sub.W -n.sub.P <0.25 (a)
- 10<.nu..sub.P -.nu..sub.N <35 (b)
- where n.sub.P is the refractive index at the d-line of the positive lens element of the cemented lens in the second lens group, n.sub.W is the refractive index at the d-line of the negative lens element of the cemented lens in the second lens group, .nu..sub.P is the Abbe number of the cemented positive lens element at the d-line, and .nu..sub.N is the Abbe number of the cemented negative lens element at the d-line.
- 2. The zoom lens system according to claim 1 which further satisfies the following condition (c):
- 0.5<.vertline.r.sub.C /f2.vertline.<10 (c)
- where f2 is the focal length of the second lens group and r.sub.C is the radius of curvature of the interface between the positive and negative lens elements of the cemented lens in the second lens group.
- 3. The zoom lens system according to claim 1 wherein the first lens group comprises, in order from the object side, a negative meniscus lens element having a convex surface directed toward the object, a negative lens element and a positive lens element, said second lens group comprising, in order from the object side close to the first lens group, a positive lens unit, a cemented lens consisting of a positive and a negative lens element, and a positive lens element, said zoom lens system further satisfying the following conditions (d) and (e)
- 1.2<.vertline.f1/f.sub.W .vertline.<1.9
- (n1+n2)/2<1.7
- where f.sub.W is the focal length of the overall system at the wide-angle end, f1 is the focal length of the first lens group, and n1 and n2 are the refractive indices at the d-line of the two negative lens elements in the first lens group, with n1 referring to the lens element which is the closer to the object.
- 4. The zoom lens system according to claim 1 wherein the cemented lens of said second lens group is composed of a positive and a negative element and has a negative focal length.
Priority Claims (1)
| Number |
Date |
Country |
Kind |
| 2-102186 |
Apr 1990 |
JPX |
|
BACKGROUND OF THE INVENTION
This is a continuation of application Ser. No. 07/683,625 filed Apr. 11, 1991, now abandoned.
This application is based on and claims priority from Japanese Patent Application No. Hei. 2-102186 filed Apr. 17, 1990, the disclosure of which is incorporated by reference herein.
The present invention relates to a zoom lens system suitable for use with video cameras, electronic still cameras, etc. that has a brightness F.sub.NO of about 1:2.8 at the wide-angle end and which is capable of zooming up to a ratio of about 3.
As active efforts are being made to a reduce the size, weight and cost of video cameras and electronic still cameras, there is a growing need for correspondingly reducing the size, weight and cost of the optical system for use in those devices.
Compact and lightweight zoom lens systems have been used with still cameras for silver halide photography and they are mostly available as a "two-group" zoom lens system that comprises, in order from the object side, the first lens group having a negative power and the second lens group having a positive power, and that performs zooming by changing the distance between the two lens groups.
An example of the two-group zoom lens system for use with video cameras and electronic still cameras is described in Unexamined Published Japanese Patent Application No. 265211/1988.
However, these conventional two-group zoom lens systems are only capable of achieving a zoom ratio of up to about 2 and are unable to meet the demand of recent versions of video cameras and electronic still cameras for achieving higher zoom ratios. In order to achieve zoom ratios higher than 2, one may use a zoom lens system consisting of three or more lens groups but then the increase in the complexity of the system is inevitable and the demand for smaller size, lighter weight and lower cost cannot be fully met.
The present invention has been accomplished under these circumstances and has an object providing a compact, lightweight and low-cost zoom lens system that features a simple two-group lens configuration and which yet is capable of zooming up to a ratio of about 3 while exhibiting satisfactory optical performance.
This object of the present invention can be attained by a zoom lens system that comprises, in order from the object side, a first lens group that includes at least three lens elements and that has a negative overall power and a second lens group that includes at least four lens elements, the second lens group being composed, in order from the object side, a positive lens unit, a cemented lens having a positive lens element and a negative lens element and a positive lens element, and that has a positive overall power, the zoom lens system being capable of changing its focal length by adjusting the distance between the first and second lens groups, and the zoom lens system satisfying the following conditions (a) and (b):
In a preferred embodiment, this zoom lens system satisfies the following additional condition (c):
In another preferred embodiment of this zoom lens system, the first lens group comprises, in order from the object side, a negative meniscus lens element having a convex surface directed toward the object, a negative lens element and a positive lens element, said second lens group comprising, in order from the object side close to the first lens group, a positive lens unit, a cemented lens consisting of a positive and a negative lens element, and a positive lens element, said zoom lens system further satisfying the following conditions (d) and (e):
US Referenced Citations (8)
Foreign Referenced Citations (3)
| Number |
Date |
Country |
| 63-265211 |
Nov 1988 |
JPX |
| 2016733A |
Sep 1979 |
GBX |
| 2090999A |
Jul 1982 |
GBX |
Non-Patent Literature Citations (1)
| Entry |
| European Search Report 9107969.9-dated Jul. 19, 1991. |
Continuations (1)
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Number |
Date |
Country |
| Parent |
683625 |
Apr 1991 |
|
Patent Grant
5155629
