Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
1999-10-26
2001-12-04
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S681000, C359S685000, C359S689000
Reexamination Certificate
active
06327098
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a wide-angle zoom lens system, for single lens reflex cameras, which has an angle-of-view (=2&ohgr;) of more than 90° at the short focal length extremity.
2. Description of the Related Art
In a single lens reflex camera, it is necessary to maintain a long back focal distance because a quick-return mirror is positioned behind the photographing lens system. For this reason, a retrofocus lens system having a negative lens element and a positive lens element, in this order from is the object, is generally employed for a wide-angle lens system. However, in a retrofocus lens system, it is extremely difficult to correct distortion and field curvature in the vicinity of the short focal length extremity while securing a long back focal distance and maintaining an overall length of the lens system shorter. For example, in a wide-angle zoom lens system in which a diaphragm S is positioned between the negative front lens group and the positive rear lens group, if, an attempt is made to effectively correct distortion and field curvature, there is only one of the following two ways:
(i) positioning the positive lens element in the negative front lens group more distant from the diaphragm S; and positioning the negative lens element in the positive rear lens group more distant from the diaphragm S; or
(ii) providing the positive lens element and the negative lens element with stronger power.
However, in the former, the diameter of the front-most lens element in the front lens group has to be increased, and also the diameter of the rear-most lens element in the rear lens group has to be increased, and furthermore the entire length of the lens system has to be increased. On the other hand, in the latter, if the power of the positive lens element in the negative front lens group is increased, the power of other negative lens elements in the negative front lens group has to be increased at the same time. This causes an increase of aberration of higher-order. AS a result, the rate of change in distortion and filed curvature with respect to the change in the angle-of-view increases, in particular, in a range where the angle-of-view is larger; and coma of higher-order remains, so that maintaining optimum optical performance becomes difficult.
Due to the above-mentioned problems, in a zoom lens system, particularly, in a wide-angle zoom lens system in which the angle-of-view at the short focal length extremity is more than 90° for the purpose of miniaturization and in order to correct aberrations other than distortion and field curvature, these distortion and field curvature have to be allowed to remain to some extent.
SUMMARY OF TEE INVENTION
It is an object of the present invention to provide a zoom lens system, for single lens reflex cameras, which has a superwide angle-of-view of more than 90° at the short focal length extremity, and which is compact, and can suitably correct distortion and field curvature over the entire focal length range.
In order to achieve the above mentioned object, there is provided a wide-angle zoom lens system including a negative first lens group, a positive second lens group, a diaphragm, and a positive third lens group, in this order from the object; whereby upon zooming from the short focal length extremity towards the long focal length extremity, each lens group is arranged to move so that the space between the first and second lens groups and the space between the second and third lens groups are narrowed; and the diaphragm moves integrally with the third lens group. At least one negative lens element having a concave surface facing towards the image is provided in the first lens group, and the concave surface of the at least one negative lens element is an aspherical surface; and the wide-angle zoom lens system preferably satisfies the following condition:
0.8<X
1
/fS<
2.0 (1)
wherein
X
1
designates the distance, along the optical axis, between the aspherical surface in the first lens group and the diaphragm at the long focal length extremity; and
fS designates the focal length of the entire lens system at the short focal length extremity.
The above explained aspherical surface in the wide-angle zoom lens system is preferably formed as an aspherical surface on which the radius of curvature becomes larger than that of the paraxial spherical surface along with an increase of the distance from the optical axis towards the periphery so that negative power is weakened, and the wide-angle zoom lens system preferably satisfies the following condition:
−0.1<(&Dgr;
x/fS
)·(
N−
1)<−0.01 (2)
wherein
&Dgr;x designates the amount of asphericity at the effective radius (the distance of ‘fS×0.65’ from the optical axis); and
fS designates the focal length of the entire lens system at the short focal length extremity.
N designates the refractive index of the material forming the aspherical surface.
For the purpose of attaining both size and weight reduction, the negative front lens group preferably includes a negative lens element having a concave surface facing towards the image, another negative lens element having a concave surface facing towards the image, and a positive lens element, in this order from the object.
The positive third lens group preferably includes at least one aspherical surface, and the wide-angle zoom lens system preferably satisfies the following condition:
1.0<X
3
/
fS<
1.6 (3)
wherein
X
3
designates the distance, along the optical axis, between the aspherical surface in the third lens group and the diaphragm; and
fS designates the focal length of the entire lens system at the short focal length extremity.
In the third lens group, a lens surface on which the aspherical surface is formed is preferably a convex surface facing towards the image The aspherical surface in the third lens group is formed as an aspherical surface on which the radius of curvature becomes larger than that of the paraxial spherical surface along with an increase of the distance from the optical axis towards the periphery so that positive power is weakened, and the wide-angle zoom lens system preferably satisfies the following condition:
0.1<(&Dgr;
x/fS
)·(
N−
1)·1000<1.0 (4)
&Dgr;x designates the amount of asphericity at the effective radius (the distance of ‘fS×0.39’ from the optical axis); and
fS designates the focal length of the entire lens system at the abort focal length extremity.
N designates the refractive index of the lens material forming the aspherical surface.
According to another aspect, there is provided a wide-angle zoom lens system including a negative lens group and a positive lens group, in this order from the object. The negative lens group includes a negative lens element, another negative lens element, and a positive lens element, in this order from the object. Furthermore, at least one of the two negative lens elements in the negative lens group has an aspherical surface, and the wide-angle zoom lens system has an angle-of-view of more than 90° at the short focal length extremity.
Since the negative lens group which is closest to the object is constituted by the negative lens element, the negative lens element, and the positive lens element, in this order from the object, the angle-of-view of more than 90° at the short focal length extremity can easily be attained. Moreover, the two negative lens elements are preferably formed as negative lens elements having concave surfaces facing towards the image, and in order to suitably correct aberrations, an aspherical surface is preferably formed on at least one of the concave surfaces.
The present disclosure relates to subject matter contained in Japanese Patent Application No. Hei-10-304416 (filed on Oct. 26, 1998) which is expressly incorporated herein by reference in its entirety.
REFERENCES:
patent: 4735493 (1988-04-01), Tachihara
patent: 5329401 (1994-07-01), Sato
patent: 5724193 (1998-03-01), Hirakawa
patent: 58
Asahi Kogaku Kogyo Kabushiki Kaisha
Epps Georgia
Greenblum & Bernstein P.L.C.
Spector David N.
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