Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2003-07-07
2004-11-02
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S772000, C359S771000, C348S340000
Reexamination Certificate
active
06813099
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to an image pickup lens preferable as an optical system of a solid state pickup element such as a CCD type image sensor or a CMOS type image sensor, an image pickup unit having the image pickup lens, and a portable terminal having the image pickup unit.
2. Description of Related Art
In recent years, with the heightened performance and the miniaturization of an image pickup device using a solid-state image pickup element such as a CCD (charge coupled device) type image sensor or a CMOS (complementary metal oxide semiconductor) type image sensor, a portable terminal and a personal computer respectively having the image pickup device have spread to users.
Also, with the miniaturization and the dense arrangement of pixels of the element due to the increase of functions in the portable terminal and the personal computer, the further miniaturization of an image pickup lens mounted on the image pickup device is strongly required for the miniaturization of the image pickup device.
In recent years, because a triplet lens structure can have higher performance as compared with a single lens structure and a doublet lens structure, the triplet lens structure composed of a first lens having the positive refractive power, a second lens having the negative refractive power and a third lens having the positive refractive power arranged in that order from an object side has been generally used as an image pickup lens of a small-sized image pickup device. This triplet type image pickup lens is disclosed in Published Unexamined Japanese Patent Application (Tokukai) No. 2001-75006.
However, in the image pickup lens disclosed in the Application No. 2001-75006, though various types aberration are preferably corrected while maintaining the wide angle of view, the image pickup lens is not appropriate to the shortening of a total lens length (that is, a distance from an aperture stop to a focal point on an image side) along an optical axis.
SUMMARY OF THE INVENTION
In an order to solve the above problem, an object of the present invention is to provide an image pickup lens which is composed of a plurality of lenses and is miniaturized, an image pickup unit and a portable terminal.
In an order to accomplish the above-mentioned object, in accordance with the first aspect of the present invention, an image pickup lens comprising four lenses arranged in an order of a first lens, a second lens, a third lens and a fourth lens from an object side,
wherein the first lens has positive refractive power and has a convex surface facing toward the object side, the second lens has the positive refractive power, the third lens has negative refractive power and has a concave surface facing toward the object side to be formed in a meniscus shape, and the fourth lens has the positive or negative refractive power and has a convex surface facing toward the object side to be formed in the meniscus shape.
In the above configuration, four lenses are arranged in an order of the first, second, third and fourth lenses. In use, the first lens is placed on the object side, and the fourth lens is placed on an image side. In this use condition, the positive lens group and the negative lens respectively having the comparatively strong refractive power are arranged in that order from the object side, and the convex surface of the first lens faces toward the object side. Therefore, a total lens length is shortened. Here, the total lens length denotes a distance on an optical axis from the object side surface of the first lens to an image side focal point of the whole image pickup lens. However, in the image pickup lens having an aperture stop arranged nearest to the object side, the total lens length denotes a distance on an optical axis from the aperture stop to the image side focal point of the whole image pickup lens. This definition of the total lens length is effective in this specification including claims.
Also, the concave surface of the third lens formed in the meniscus shape faces toward the object side, and the convex surface of the fourth lens formed in the meniscus shape faces toward the object side. Therefore, an air lens having the positive refractive power is formed between the third and fourth lenses. Accordingly, the total lens length can be further shortened, and a telecentric characteristic can be maintained in the periphery of an image screen.
Here, the fourth lens has the shape and orientation of the convex surface described above and can have the positive or negative refractive power. Hereinafter, “positive or negative refractive power” denotes that any of the positive refractive power and the negative refractive power is allowed.
Preferably, following conditional formulas (1), (2) and (3) are satisfied:
L
/2
Y
<1.60 (1)
0.40
<f
12
/
f
<0.70 (2)
25<{(&ngr;
1
+&ngr;
2
)/2}−&ngr;
3
(3),
where L denotes a distance on an optical axis from the object side surface of the first lens to an image side focal point of the whole image pickup lens, 2Y denotes the length of a diagonal line on an effective image screen, f
12
denotes a combined focal length of the first lens and the second lens, f denotes a focal length of the whole image pickup lens, &ngr;
1
denotes an Abbe number of the first lens, &ngr;
2
denotes an Abbe number of the second lens, and &ngr;
3
denotes an Abbe number of the third lens.
The conditional formulas (1) to (3) indicate conditions for obtaining a miniaturized image pickup lens in which aberration is preferably corrected. Here, L of the conditional formula (1) denotes a distance on an optical axis from the object side surface of the first lens to the image side focal point of the whole image pickup lens. The “image side focal point” denotes an image point obtained when a parallel ray parallel to the optical axis is incident on the image pickup lens. Also, when an optical member of a plane parallel plate shape such as a low pass filter is arranged in a space between the surface of the image pickup lens nearest to the image side and the image side focal point, the distance L is converted into a distance L in air, and the distance L in air satisfies the formula (1).
The conditional formula (1) indicates a condition for achieving the shortening of the total lens length. When L/2Y is lower than the upper limit of the conditional formula (1), the total lens length is directed to be shortened.
Also, as to the conditional formula (2) for appropriately setting the combined focal length of the first lens and the second lens, when f
12
/f is set to be higher than the lower limit, the combined positive refractive power of the first lens and the second lens is not excessively enlarged, the generation of both the high-order spherical aberration and the coma can be suppressed. Also, when f
12
/f is set to be lower than the upper limit, the combined positive refractive power of the first lens and the second lens is appropriately obtained, and the total lens length can be shortened.
Also, as to the conditional formula (3) for the correction of the chromatic aberration in the first and second lenses having the positive refractive power and the third lens having the negative refractive power, when {(&ngr;
1
+&ngr;
2
)/2}−&ngr;
3
is set to be higher than the lower limit, the axial chromatic aberration and the lateral chromatic aberration can be corrected.
Preferably, an aperture stop is arranged nearest to the object side. Because of the configuration of the aperture stop arranged nearest to the object side, the position of an exit pupil can be set away from an image surface. Therefore, a principal ray of a light flux emerging from the final surface of the lenses is incident on a solid-state image pickup element at an angle near a right angle. That is, the image side telecentric characteristic required for the image pickup lens of the solid-state image pickup element can be preferably obtained, and a shading phenomenon in the periphery of an image screen
Cohen & Pontani, Lieberman & Pavane
Dinh Jack
Epps Georgia
Konica Corporation
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