Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2003-10-20
2004-10-12
Lester, Evelyn A. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S680000, C359S685000, C359S689000, C359S716000, C359S749000, C359S753000
Reexamination Certificate
active
06804064
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a zoom lens, more specifically, to the zoom lens utilizing a plastic aspheric lens.
2. Description of the Prior Art
In recent years, a market for a digital camera is expanding greatly, and the user's demand for the digital camera have been ranged in scope accordingly. That is, needless to say that not only a high image quality and a miniaturization are demanded, but also the demand for a weight saving of a zoom lens becomes greater in recent years.
There are considered various types of zoom lenses for the digital camera. As a type suited for widening an angle of view and for the miniaturization, there is a zoom lens of type which has a first group of lenses having a negative focal length, a second group of lenses having a positive focal length and a third group of lenses having a positive focal length which are arranged subsequently from a side of an object side to a side of an image, and has an aperture stop disposed at the object side of the second group of lenses that moves integrally with the second group of lenses, and in a change of magnification from a short focal end to a long focal end, the second group of lenses monotonously moves from the image side to the object side and the first group of lenses moves so as to correct a displacement of a position of image plane in accordance with the change of magnification.
For example, in Japanese Patent Laid Open No. H10-039214, there is disclosed a small-size zoom lens which a position of an exit pupil can be separated from an image plane sufficiently, has the wide angle of view and is blight, has a good performance, and yet, capable of carrying out a large change of magnification ratio. According to the small-sized zoom lens disclosed in Japanese Patent Laid Open No. H10-039214, it is constituted by arranging a first group of lenses to a third group of lenses subsequently from the object side to the image side, and the first group of lenses has a negative refractive power, the second group has a positive refractive power and the third group of lenses has the positive refractive power. It also has an aperture stop at the object side of the second group of lenses that moves integrally with the second group of lenses when carrying out a zooming, and the third group of lenses is a fixed group relating to the zooming. At the time of carrying out the zooming from a wide angle end to a telephoto end, the first group of lenses is moved to the image side on an optical axis firstly and then a direction of its movement is reversed to the object side on the way so that the first group of lenses is moved to the image side in such a manner as to draw a convex-like arc shape to correct a displacement in an focusing position, and the second group of lenses is moved monotonously to the object side on the optical axis to carry out the change of magnification.
Also, in Japanese Patent Laid Open 2000-267009, a small sized, high image quality and inexpensive zoom lens which arranges a plastic lens for a zoom lens which has a negative-positive two-element zoom lens effectively and which is particularly suited for a digital still camera, is disclosed. According to the zoom lens disclosed in Japanese Patent Laid Open 2000-267009, at least one of lenses constitutes each group of lenses in a zoom lens having negative-positive-positive three-element lens seen from the object side is the plastic lens, and it also has a structure which a following conditional formula is satisfied: −0.8<Cp×(N′−N)/&phgr;W<0.8−0.45<M3/M2<0.90 (where &phgr;T/&phgr;W>1.6) where Cp is a curvature of the plastic lens, &phgr;W is a power of entire system at the wide angle end, N and N′ are the object side of an aspheric surface and an refraction index of d-line of medium at the image side, M2 and M3 show amounts of movements of the second group of lenses and the third group of lenses respectively (using the wide angle end as the base and setting the object side as −), and &phgr;T is the power of entire system at the telephoto end.
The above mentioned Japanese Patent Laid Open No. H10-039214 was filed at the earliest time as the above zoom lens of such type, and a basic structure of the above zoom lens of such type is all disclosed. However, since all the lenses are made from glass, it cannot be said that the sufficient weight saving is achieved.
Also, Japanese Patent Laid Open 2000-267009 provides the zoom lens utilizing a plastic aspheric lens. However, the second group of lenses having a function of the change of magnification and an imaging function which are most influential on an aberration correction function is a triplets type in the embodiment described in Japanese Patent Laid Open 2000-267009, therefore there is a problem that it has no sufficient aberration correction function.
SUMMARY OF THE INVENTION
The present invention has been made in view of the aforementioned problems, and therefore it is an object of the present invention to provide a zoom lens which is small sized, light weighted and has a wide angle of view by using a plastic aspheric lens while having high performance equivalent to or more than conventional zoom lenses.
To accomplish the aforementioned object, a zoom lens according to a first aspect of the present invention comprises a first group of lenses having a negative refractive power, a second group of lenses having a positive refractive power and a third group of lenses having the positive refractive power, and an aperture stop provided at an object side of the second group of lenses. In addition, a first positive lens, a negative lens, a second positive lens and a third positive lens are subsequently arranged from the object side to the image side in the second group of lenses. Furthermore, when a zooming from a short focal end to a long focal end is carried out, the second group of lenses is monotonously moved from an image side to the object side, and the first group of lenses is moved so as to correct a displacement of a position of image plane in accordance with the zooming, and at least the third positive lens positioned nearest to the image side is a plastic aspheric lens.
Also, according to a second aspect of the present invention, the negative lens is formed by a negative lens in a meniscus shape which faces its convex surface toward the object side. In addition, the second positive lens is formed by a positive lens in a meniscus shape which faces its convex surface toward the object side.
In the zoom lens of the present invention, the first group of lenses to the third group of lenses are arranged subsequently from the object side to the image side. The first group of lenses has the “negative refractive power”, the second group of lenses has the “positive refractive power” and the third group of lenses has the “positive refractive power.” The aperture stop provided at the object side of the second group of lenses moves integrally with the second group of lenses when carrying out the zooming. In addition, when carrying out the zooming from a wide angle end to a telephoto end, the first group of lenses is moved to the image side on an optical axis firstly and then a direction of its movement is reversed to the object side on the way so that the first group of lenses is “moved to the image side in such a manner as to draw a convex-like arc shape” to correct a displacement in an focusing position, and the second group of lenses is moved monotonously to the object side on the optical axis, to carry out the change of magnification. Also, since the aperture stop is moved integrally with the second group of lenses when carrying out the zooming, the movement of the second group of lenses is not hindered by the aperture stop. The most characteristic of the present invention is that at least the third positive lens positioned nearest to the image side is the plastic aspheric lens.
According to this invention, because the plastic aspheric lens is used for the second group of lenses with a conventional s
Lester Evelyn A.
Ricoh & Company, Ltd.
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