Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2003-06-27
2004-12-21
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
C359S717000
Reexamination Certificate
active
06833968
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image pickup lens suitable for mounting in a camera which uses a CCD or CMOS device as a pickup element.
2. Description of Related Art
One characteristic of this lens for mounting in a compact camera using a CCD or CMOS device as a pickup element is a short optical length. One lens of this type is for example the pickup lens disclosed in Japanese Patent Laid-open No. 10-206730.
However, in the pickup lens disclosed in this reference the distance from the aperture diaphragm plane to the second surface (the image-side surface) of the second lens is 5.3 mm, and the optical length is too long for use as a lens mounted in a compact camera using a CCD or CMOS device as a pickup element. In the pickup lens system disclosed in Japanese Patent Laid-open No. 10-206730, an aperture diaphragm is inserted between the first lens and the second lens. That is, the pickup lens system, disclosed in this reference employs a construction in which only one diaphragm is provided.
It is known that the position of an aperture diaphragm has important significance for lens design (see for example Fumio Kondo, Renzu no Sekkei Gihou, Kougaku Kougyou Gijutsu Kyoukai, 2nd edition Feb. 1, 1983). In other words, it is known that: (a) the entrance pupil position conjugate with the aperture diaphragm position is related to coma aberration, astigmatic aberration, distortion aberration, and similar, and is the basis for determining the third-order aberration coefficient; (b) when an aperture diaphragm is set at the position a distance t from the object-side surface (first surface) of the first lens counting from the object side (the first lens), measured along the optical axis moving toward the image side, if the value of B as defined by the equation (i) below is 0, then the basis is given for the Fraunhofer condition, according to which a sufficiently small aberration is realized;
B=C−St
(i)
where C and S are constants related to the third-order aberration coefficient; and, (c) the basis is given for the Zinken-Sommer condition, according to which the closer the value of Z as defined in equation (ii) below is to 0, the better the aberration correction is guaranteed to be;
Z=St
2−2
Ct+A
(ii)
where C, S, and A are constants related to the third-order aberration coefficient.
In this way, when conducting a quantitative examination of aberration, the position of an aperture diaphragm plays an essential role, and is an important basic parameter of the lens system.
However, a short optical length is required of an image pickup lens for mounting in a compact camera as described above. In addition, an image pickup lens mounted in a compact camera as described above must be such that distortion of the formed image is not perceived visually, and such that various aberrations are corrected to small values as required by the integration density of the pickup element.
In the following explanation, “various aberrations are corrected to amounts sufficiently small that distortion of the image is not recognized by visual perception, and sufficiently small as to satisfy the requirements of the integration density of the pickup element” is, for simplicity, represented by the phrase “various aberrations are satisfactorily corrected” or similar. An image for which various aberrations are satisfactorily corrected may be called a “satisfactory image”.
An object of this invention is to provide an image pickup lens in which various aberrations are satisfactorily corrected, the optical length is short, and moreover sufficient back focus is maintained.
SUMMARY OF THE INVENTION
An image pickup lens of this invention which achieves the above object is configured by arranging, in order from the object side, an aperture diaphragm S
1
; a first lens L
1
; a second diaphragm S
2
; and a second lens L
2
. The first lens L
1
has a meniscus shape with the concave surface facing the object side, and having positive refractive power. The second lens L
2
has a meniscus shape with the concave surface facing the image side, and having negative refractive power.
Further, in the image pickup lens, at least one surface of the first lens L
1
is aspherical, at least one surface of the second lens L
2
is aspherical, and overall at least two lens surfaces are aspherical; and the following conditions are satisfied.
0.09
<|f
1
/f
2
|<0.37 (1)
1.33
<|r
1
/f
|<47.77 (2)
3.08
<|r
1
/r
2
|<113.12 (3)
0.63
<D/f
<0.87 (4)
Here f is the focal length of the entire system (the combined focal length of the lens system comprising the first and second lenses), f
1
is the focal length of the first lens, f
2
is the focal length of the second lens, D is the distance from the aperture diaphragm plane to the second surface (image-side surface) of the second lens (lens center length), r
1
is the radius of curvature of the object-side surface of the first lens L
1
in the vicinity of the optical axis (axial radius of curvature), and r
2
is the radius of curvature of the image-side surface of the first lens L
1
in the vicinity of the optical axis (axial radius of curvature).
The aperture diaphragm S
1
of this invention is positioned between the object and the first lens L
1
. In other words, the aperture diaphragm S
1
is set on the outside of the first lens L
1
, that is, in front of the first surface (the object-side surface) of the first lens. This aperture diaphragm S
1
forms an incidence plane. A second diaphragm S
2
provided between the first lens L
1
and the second lens L
2
is inserted in order to cut out so-called flare, which is light which strikes the peripheral edge of a lens or similar and is irregularly reflected.
Next, the significance of the above condition equations (1) through (4) is explained.
The above condition equation (1) determines the power distribution of the first lens L
1
and second lens L
2
; if |f
1
/f
2
| falls below the lower limit, the power of the first lens L
1
is stronger and the power of the second lens is weaker, so that correction of the spherical aberration, coma aberration, and distortion aberration produced by the first lens becomes difficult. And if |f
1
/f
2
| exceeds the upper limit, the power of the first lens L
1
becomes weaker, and consequently the power of the second lens must be increased in order to shorten the combined focal length f and back focus b
f
(distance from the point of intersection of the image-side surface of the second surface of the second lens with the optical axis, to the point of intersection of the light-receiving surface with the optical axis) of the lens system. Hence correction of the distortion aberration and coma aberration produced by the second lens L
2
becomes difficult. As a result, if |f
1
/f
2
| falls below the lower limit or rises above the upper limit, a satisfactory image cannot be obtained. Consequently using an image pickup lens of this invention which satisfies the condition equation (1), a satisfactory image can be obtained.
The above condition equation (2) sets the range for the value of |r
1
/f| when the radius of curvature r
1
on the object side of the first lens L
1
is normalized by the combined focal length f for the pickup lens system. If |r
1
/f| falls below the lower limit, coma aberration increases, and if an attempt is made to correct this, distortion aberration results. Hence the need arises for means to cut rays which pass through the peripheral portions of lenses, and as a result the image is darker.
On the other hand, if |r
1
/f| exceeds the upper limit, astigmatic aberration and coma aberration are increased, and moreover the lens thickness is increased, so that a satisfactory image cannot be obtained over a broad angle range. That is, if the radius of curvature r
1
on the object side of the first lens L
1
is set so as to satisfy co
Burdett James R.
Epps Georgia
Harrington Alicia M.
Milestone Co., Ltd.
Venable
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