Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2003-04-11
2004-11-02
Thompson, Tim (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S794000, C359S795000, C359S691000, C359S717000
Reexamination Certificate
active
06813100
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a miniature image-capturing lens which is suitable for an imaging device employing a solid-state image-capturing element such as a CCD type image sensor, a CMOS type image sensor and the like.
In recent years, due to the higher performance demands and miniaturization of imaging devices employing solid-state image-capturing elements such as a CCD (a charged coupled device) type image sensor, a CMOS (a complementary metal oxide semiconductor) type image sensor and the like, mobile phones or personal computers employing such imaging devices have become popular. Further, continual miniaturization of image-capturing lenses loaded on the imaging devices is further demanded.
A two-piece structured lens, which makes it possible to develop higher performance compared to a single lens, is suitable for the image-capturing lens for the above-mentioned usage. Well known is a retro-focus type image-capturing lens, composed of a first lens having a negative refractive power, a diaphragm, and a second lens having a positive refractive power, which are arranged in the order from the object side. The image-capturing lens composed as mentioned above is disclosed in TOKKAI 2000-321489 and TOKKAI 2001-183578.
Though an image-capturing lens of this type is suitable for a wide angle function, its back-focus is apt to be long, which makes it very difficult to have a total image-capturing lens length (a distance from a surface located closest the object side in the entire image-capturing lens to an image-side focal point, provided that in the image-capturing lens in which the aperture diaphragm is located closest to the object side, the total length of the image-capturing lens is a distance from the aperture diaphragm to the image side focal point) to be short.
SUMMARY OF THE INVENTION
The present invention has been attained in view of the above-mentioned problems. The object of the invention is to provide a miniature image-capturing lens which is smaller and is optimally corrected for aberration, though it simply consists of two separate lenses.
Concerning the size of the miniature image-capturing lens, the present invention aims at the miniaturization of the image-capturing lens to a level satisfying the following formula. By satisfying this limit, the total length of the image-capturing lens is shortened, and its external diameter is synergistically shortened. Therefore, the imaging device can be totally miniaturized and light in weight.
L
/2
Y
<1.50 Formula (5)
Where,
L: distance on an optical axis between the aperture stop and the image side focal point, and
2Y: diagonal length of an effective image area.
Image side focal point means the image point where parallel incident rays, parallel to the optical axis of the lens, are brought to focus. Further, when a plane-parallel plate is arranged between the image side surface of the image-capturing lens and the focal point, the thickness of the plane-parallel plate is treated as an air-converted distance. Here, in the case as one example that the parallel plate section has a low pass filter, the air-converted distance D
c
is obtained by the following formula: D
c
=t
, where t is a thickness of the low-pass filter and n is the refractive index of the low-pass filter.
Item 1
A miniature image-capturing lens is composed of:
an aperture stop,
a first lens, having a positive refractive power, whose convex surface faces to the image side, and
a second lens, formed to be a meniscus shape, whose concave surface faces to the object side, and
all of which are arranged in the above order from the object side,
wherein the first lens and the second lens each have at least an aspheric surface, and the following formulas are satisfied:
0.50
<f
1
/f
<0.80 Formula (1)
0.30<(
R
2
+R
1)/(
R
1
−R
2)<1.20 Formula (2)
where
f1: the focal length of the first lens,
f: the focal length of the total image-capturing lens,
R1: the radius of curvature of the object side surface of the first lens,
R2: the radius of curvature of the image side surface of the first lens.
In order to obtain the miniature image-capturing lens whose aberrations are optimally corrected, the fundamental construction of the present invention is composed of an aperture stop arranged in a total image-capturing lens system at the side nearest to the object, the positive lens with the convex surface facing the image side having a larger refractive power than the object side surface, and the second lens of a meniscus shape facing its concave surface to the object side.
By arranging the aperture stop in the image-capturing lens system at the nearest side to the object, and by having the larger positive refractive power on the image side surface of the first lens, it is possible to keep an exit pupil at a further distance from the image surface. With this arrangement, main light rays of a light flux emitted from a lens final surface becomes to be incident with an angles near perpendicular onto a solid-state image-capturing element. That is, a image side telecentric characteristic necessary for an image-capturing lens used for the solid-state image-capturing element can be secured well and a shading phenomena on a peripheral section of an image screen can be reduced. Therefore, it becomes possible to obtain a structure by which an image side telecentric characteristic is easily secured, which is necessary for miniature image-capturing lens employed in solid-state image-capturing elements. Further, by having such large negative refractive power on the object side surface of the second lens, it becomes possible to correct for any aberration.
Still further, by featuring at least one aspheric surface on each the first lens and second lens, it is possible to correct for aberration optimally. By featuring an aspheric surface on the first lens having the positive power, spherical aberration and coma are effectively corrected for. On the other hand, since the second lens is arranged nearest to the image side away from the aperture stop, there is a difference between the passing height of light flux on the axis and the passing height of light flux off the axis on the periphery area of the picture surface, and by using the aspheric surface, aberration on the peripheral area of the picture surface such as field curvature and distortion, can be optimally corrected for.
Explanation of Formula (1)
Formula (1) shows the conditions by which the refractive power of the first lens is appropriately determined. If the value of f1/f is greater than the lower limit, the refractive power of the first lens can be controlled within the required level, therefore high ordered spherical aberration, coma, and chromatic aberration of magnification are controlled to be lower, which occur on the image side surface of the first lens. If the value of f1/f is less than the upper limit, the positive refractive power of the first lens is moderately secured, and the total length of the image-capturing lens can be shortened.
Explanation of Formula (2)
Formula (2) shows the condition by which a shape factor of the first lens can be appropriately established under the condition of formula (1). If the value of (R2+R1)/(R1−R2) is greater than the lower limit, the image side surface serves mainly as the positive refractive power of the first lens, therefore the image side telecentric characteristic of the total image-capturing lens system is easily obtained. Further, if the above-mentioned value is less than the upper limit, the radius of curvature of the image side surface of the first lens does not become excessively small, and the back focus of the total image-capturing lens system is easily obtained. Therefore it is preferable from the point of view of manufacture of the first lens. It is more preferable to satisfy the following formula (2′).
0.40<(
R
2
+R
1)/(
R
1
−R
2)<0.90 Formula (2′)
Item 2
The miniature image-capturing lens is characterized by satisfying the following f
Matsui Takumi
Sato Masae
Yamaguchi Susumu
Konica Corporation
Thompson Tim
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