Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2003-02-14
2004-12-07
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S689000
Reexamination Certificate
active
06829101
ABSTRACT:
This application claims benefits of Japanese Application No. 2002-36267 filed in Japan on Feb. 14, 2002, the contents of which are incorporated by this reference.
BACKGROUND OF THE INVENTION
The present invention relates generally to a zoom lens and an image pickup apparatus using the same, and more particularly to a high-zoom-ratio zoom lens that is well fit for cameras, especially video cameras and digital still cameras.
Home-video cameras are now increasingly diminished, and the market is fairly mature. About 10:1 power zoom lenses are mainly used for phototaking lenses. Recently, on the other hand, attention has been focused on digital still cameras (electronic still cameras) as the coming generation of cameras that are taking the place of silver-halide 35 mm-film (usually called Leica size) cameras. Designed to provide stills, the digital still cameras must satisfy higher image quality levels than do home-video cameras, and so even general user-oriented digital cameras rely primarily on image pickup devices having 2,000,000 pixels. It is thus difficult to design compact, high-zoom-ratio zoom lenses unlike the case of home-video cameras, and so about 3:1 power zoom lenses are mainly used. The market for digital still cameras having such a 3:1 power zoom lens are coming of age, and so the advent of higher-zoom-ratio zoon lenses is desired. However, most of compact, high-zoom-ratio zoom lenses currently available for electronic image pickup apparatus are oriented for home videos having a small number of pixels. When such zoom lenses are used with digital cameras, it is impossible to allow image pickup devices to take full advantage of their own resolving power.
For instance, if an image pickup device having about 2,000,000 pixels is used with a zoom lens of the construction exemplified in the inventive examples given later, that image pickup device may take full advantage of its own resolving power. However, the spectral sensitivity properties of that image pickup device including a color filter, unlike those of silver-halide color film, are not faithful for the specific features of the human eyes (which have higher sensitivity to wavelengths of about 400 to 430 nm than required, low sensitivity to wavelengths of about 600 nm, and high sensitivity to wavelengths of 700 nm or higher not originally in existence). For this reason, those spectral sensitivity properties are largely affected by chromatic aberrations of the lens in near-ultraviolet and near-infrared ranges, resulting in chromatic blurring and, hence, considerable damage to image quality. Color reproducibility to plants and many other subjects having strong reflection spectra in the infrared range in general, too, is considerably damaged.
The chromatic blurring of a phototaken image is particularly noticeable at a subject site having a large brightness difference. Referring to the boundary between a high brightness site and a low brightness site, this is explained as follow. The high brightness site is whitened beyond the latitude of an image pickup device due to over-exposure whereas light of wavelengths of particularly about 400 to 430 nm on a high brightness side aberrates as chromatic aberrations on a low brightness side. In view of light quantity, on the other hand, the aberrating light comes within the range of the latitude of the image pickup device. In addition, the relative sensitivity of this wavelength range becomes much higher as compared with silver-halide color film. Consequently, a portion of the low brightness side near to that boundary is strikingly colored in purple. When it comes to an optical system having a high zoom ratio and a largely asymmetric power profile wherein the stronger the power of each lens element becomes due to compactness, the more noticeable chromatic aberrations due to secondary spectra becomes, the wavelengths of 400 to 430 nm that are main chromatic blurring components are achromatized with wavelengths in the vicinity of 550 nm having the highest sensitivity. However, chromatic aberrations in the range of 500 to 600 nm having relatively high sensitivity start to occur gradually, and so the possibility of improvements in image quality due to an increase in the number of pixels of the image pickup device is spoiled by the image-formation capability of the zoom lens. Accordingly, to eliminate a chromatic blurring problem while sharpness is maintained, there is no option but to make correction for secondary spectra or removing light components of 400 to 430 nm wavelengths that are main chromatic blurring components by means of filters, etc. without detrimental to color reproduction.
SUMMARY OF THE INVENTION
In view of such situations that the number of pixels of an image pickup device continues to increase whereas the pixel pitch becomes increasingly narrow, the object of the present invention is to provide a digital still camera which is compatible with the resolving power of a coming image pickup device having as many pixels as about 3,000,000 or greater and uses a zoom lens having a high zoom ratio yet compact, simple construction, wherein chromatic aberrations of the zoom lens are reduced and the zoom lens is kept against the influences of chromatic aberrations.
According to the first aspect of the invention, this object is achieved by the provision of a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
a doublet component of said two doublet components, which component is located on an image side thereof, has a meniscus form concave on an image side thereof.
According to the second aspect of the invention, there is provided a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
a doublet component of said two doublet components, which component is located on an image side thereof, has negative refracting power.
According to the third aspect of the invention, there is provided a zoom lens including a negative lens group having negative refracting power and at least one positive lens group located on an image side thereof and having positive refracting power, characterized in that:
a spacing between said negative lens group and said positive lens group changes upon zooming,
said positive lens group includes two doublet components in each of which a positive lens element and a negative lens element are cemented together in order from an object side thereof, and
said two doublet components satisfy the following conditions (1) and (2):
0.000
<AVE
[(&Dgr;&thgr;
gF
)
C
1
p,
(&Dgr;&thgr;
gF
)
C
2
p
]<0.080 (1)
−0.030
<AVE
[(&Dgr;&thgr;
gF
)
C
1
n,
(&Dgr;&thgr;
gF
)
C
2
n
]<0.003 (2)
where
(&Dgr;&thgr;
gF
)C
1
P is the anomalous dispersibility of a medium forming the positive lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(&Dgr;&thgr;
gF
)C
1
n
is the anomalous dispersibility of a medium forming the negative lens element in the doublet component of said two doublet components, which component is located on the object side of said positive lens group,
(&Dgr;&thgr;
gF
)C
2
p
is the anomalous dispersibility of a medium forming the positive lens element of the double
Hozumi Kouki
Mihara Shin'ichi
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