Imaging lens

Details Imaging lens Imaging lens

1999-07-15

2001-01-30

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

C359S749000, C359S750000, C359S751000

Reexamination Certificate

active

06181477

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a small-sized imaging lens which improves the assembling property without deteriorating the performance thereof.
2. Description of the Related Art
A conventional small-sized imaging lens has been designed for use with a simple camera in the form of what is called a film with a lens. Japanese Unexamined Patent Publication Nos. 4-211215, 5-281465 and 6-258574, for example, disclose an imaging lens configured of two pieces of lenses.
Also, other applications of the imaging lens having two lenses as described in the above-mentioned publications include those described in Patent Publication No. 8-5910 used for a read optical system such as a facsimile and Patent Publication No. 5-341185 for the endoscope.
By the way, the conventional small-sized imaging lenses described above have the following problems.
Specifically, the imaging lenses described in Patent Publication Nos. 4-211215 and 5-281465, in which the image surface is corrected by the curving of the surface of a silver salt film providing a photosensitive material and the open F-number is as dark as about 9 to 11, is not suitable as an imaging lens for an image-pickup equipment using an imaging device (hereinafter called the CCD) in place of the silver salt film.
The imaging lens described in Patent Publication Nos. 6-258574 and 8-5910 is what is called of the telephoto type configured of a lens having a positive refracting power and a lens having a negative refracting power sequentially from the object side in order to reduce the size. Therefore, the angle of the emitted light ray is so large that a color shift is caused by the use of the CCD as a photosensitive member. Further, it is difficult to secure a space for filters for cutting the infrared light and a low-pass filter essential to the image-pickup equipment using the CCD.
Also, the imaging lens described in Patent Publication No. 5-341185 designed for the endoscope is configured of lenses having a negative refracting power and a positive refracting power arranged sequentially from the object side. An iris is arranged between these lenses and the CCD is used as a photosensitive member. Nevertheless, the distortion is caused by the asymmetric arrangement of refractive power with respect to the iris, and the eccentricity of the two lenses deteriorates the optical performance, thus requiring a high assembly accuracy.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to provide an imaging lens of two-lens configuration, in which the deterioration of the optical performance due to the eccentricity is suppressed for improving the assembling property, while at the same time making possible the correction of various aberrations satisfactorily.
Another object of the invention is to provide an small-sized imaging device configured of two lenses in which the deterioration of the optical performance is suppressed as described above.
In order to solve the problems described above, an imaging lens system according to this invention comprises, as arranged from the object side toward the image surface, a first lens in the shape of meniscus having a weak refracting power and a second lens having a positive refracting power in the shape of meniscus with the convex surface thereof faced to the image surface side, wherein the conditions f
2
/f>0.9, nd
2
>1.55 are satisfied, where f is the focal length of the whole lens system, f
2
the focal length of the second lens and nd
2
the refractive index of the second lens on the d line, respectively.
Thus, a small-sized imaging lens can be configured of two lenses with the distortion corrected in proper way by two lenses.


REFERENCES:
patent: 5812326 (1998-09-01), Yamada
patent: 5999324 (1999-12-01), Kohno

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