Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2002-11-27
2004-08-10
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S747000, C359S683000, C359S784000
Reexamination Certificate
active
06775074
ABSTRACT:
This application claims benefit of Japanese Application No. 2001-362268 filed in Japan Nov. 28, 2001, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to an image pickup lens and an image pickup system comprising the same, and more particularly to an image pickup optical system best suited for miniature image pickup systems used with cellular phones, etc.
An optical system for miniature image pickup systems used with cellular phones, etc. must have a wide view angle and be as small and thin as possible while the necessary back focus is kept. An image pickup device used on such miniature image pickup systems becomes steadily small and definite, and the associated optical system must have a resolving power consistent therewith.
In such an optical system for miniature image pickup systems, the focal length of an image pickup system becomes short by virtue of image pickup device size. While that optical system must be as small and thin as possible, there is a certain limit to thickness reductions by reducing the number of lenses used. If the optical system is turned back somewhere, then much more thickness reductions are achievable. To cut back on the cost of such an optical system, the number of lenses used should preferably be as small as possible.
With an optical system having such short a focal length, however, it is difficult to make correction for chromatic aberration of magnification and obtain sufficient resolving power as far as off-axis areas, using a reduced number of lenses.
With a conventional five-lens arrangement like an image pickup lens set forth in JP-A 09-189856, it is possible to obtain satisfactory resolution over a wide view angel. However, that arrangement is expensive by virtue of too many lens elements, and compactness is difficult to achieve because the overall length of the lens system tends to become long. To bend an optical path, there must be large space for such bending; however, the increased number of lens elements makes it difficult to ensure large space.
The image pickup lens set forth in JP-A 11-95096 is made up of four lens elements, providing a wide-aperture, wide-angle image pickup lens system. As the focal length of such an arrangement is further reduced, however, it is difficult to correct chromatic aberration of magnification and it is impossible to obtain adequate resolving power as far as off-axis areas.
SUMMARY OF THE INVENTION
In view of such problems with the prior art, an object of the present invention is to provide a compact optical system that is made up of a reduced number of lens elements with a bendable optical path and so is compatible with miniature image pickup equipments used with cellular phones, etc.
According to the present invention, this object is achieved by the provision of an image pickup lens, characterized by comprising, in order from an object side thereof, a first lens element having negative refracting power, a second lens element having positive refracting power, a stop, a third lens element having negative refracting power and a fourth lens element having positive refracting power, wherein lens elements having refracting power due to refraction are defined only by said first to fourth lens elements and the following conditions (1) and (2) are satisfied:
50>&ngr;
d1
−&ngr;
d2
>10 (1)
2
>f/f
3
>0.35 (2)
where &ngr;
d1
is the d-line reference Abbe number of the first lens element, &ngr;
d2
is the d-line reference Abbe number of the second lens element, f is the focal length of the image pickup lens, and f
3
is the composite focal length of the third and fourth lens elements.
In what follows, why the aforesaid arrangement is used, and how the same works is explained.
With the lens arrangement of the present invention, it is possible to make adequate correction for chromatic aberration of magnification while making sure of a back focus at a short focal length, using as small as four lens elements. With the second lens element located on the image side of the image pickup lens with respect to the stop, no chromatic aberration of magnification can be corrected even when the Abbe number of the second lens element is kept small. To slim down the optical system, the optical path must be bent between lens elements located as close to an object as possible. With the lens arrangement of the present invention, however, it is possible to make the space between the first lens element and the second lens element so wide that the lens system can be bent with no deterioration of aberrations.
In the present invention, chromatic aberration of magnification is corrected by reducing the Abbe number of the second lens element located on the object side with respect to the stop. Condition (1) gives a definition of the difference in Abbe number between the first lens element and the second lens element. As the lower limit of 10 to this condition is not reached, it is impossible to make adequate correction for the chromatic aberration of magnification. Because there is some limit to the Abbe numbers of optical glasses, combinations of optical glasses that exceed the upper limit of 50 are hardly available.
To make sure of the back focus at a short focal length, the optical system must be of the retrofocus type wherein the object side thereof is constructed of a lens element having negative refracting power and the image side thereof is made up of a lens element having positive refracting power. To make the focal length short as contemplated herein, the negative refracting power and positive refracting power must be much more increased. Condition (2) gives a definition of the ratio of the focal length of the image pickup lens to the composite focal length of the third and fourth lens elements. As the lower limit of 0.35 to this condition is not reached, it is difficult to make sure of any back focus, and as the upper limit of 2 is exceeded, the composite focal length of the third and fourth lens elements becomes too short for correction of aberrations.
In the image pickup lens of the present invention, the third and fourth lens elements may be cemented together.
While the third and fourth lens elements may be constructed as separate, individual lens elements, it is understood that if they are positioned at a narrow space, it is possible to make better correction for chromatic aberrations. Further, if the third lens element having negative refracting power is cemented to the fourth lens element having positive refracting power, there is then obtained an arrangement favorable for fabrication because aberrations ensuing from mutual decentration of both lens elements can be held back.
Preferably in the present invention, the second lens element should be a positive meniscus lens element concave on its image side.
This is because when there is a wide space between the first lens element and the second lens element, it is easy to locate the principal point of the second lens element on the first lens element side.
Preferably in this case, the second lens element should comply with the following conditions (A) and (B).
0.45
>f/f
2
>0.15 (A)
−1.1>(
r
21
+r
22
)/(
r
21
−r
22
)>−10.0 (B)
Here f
2
is the focal length of the second lens element, r
21
is the axial radius of curvature of the object-side surface of the second lens element, and r
22
is the axial radius of curvature of the image-side surface of the second lens element.
In consideration of a balance between correction of aberrations and size reductions, it is preferable that the refracting power and shape of the second lens element comply with the aforesaid conditions (A) and (B).
As the lower limit of 0.15 to condition (A) is not reached, it is necessary to move the first lens element toward the object side thereby making sure of a view angle and an F-number, and as the upper limit of 0.45 is exceeded, it is difficult to make sure of the space between the first lens element and the second lens element.
As the
Hasan M.
Olympus Corporation
Sugarman Scott J.
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