Optical: systems and elements – Lens – Multiple component lenses
Reexamination Certificate
2002-01-23
2002-11-05
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
Multiple component lenses
Reexamination Certificate
active
06476982
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a high-performance and compact photographing lens system to be utilized in a small imaging apparatus, such as digital still cameras, surveillance cameras, and PC cameras (an imaging apparatus for a personal computer), in which an imaging device, such as a CCD (charged coupled device), is used.
2. Description of the Prior Art
In recent years, general-use digital still cameras (hereinafter, DSC) have rapidly spread in the market. From a structural point of view, a DSC is an imaging apparatus which electrically picks up a still image formed by a photographing lens system through an imaging device such as a CCD or the like (hereinafter, CCD), and records the picked-up image in a built-in memory or a memory card. When DSCs were first available in the market, they obtained popularity, since a liquid crystal monitor thereof was able to be used either as a finder for a photographing operation or as a monitor on which recorded images were reproduced. With these features, i.e., promptness and convenience, DSCs, attracted more users, than cameras with a silver halide film did. On the other hand, compared with the silver-halide-film cameras, the DSCs have low resolution of the photographic image, which is considered to be a drawback thereof. Along with rapid spread of DSCs, CCDs having a large number of pixels have been supplied at low cost. As a result, although under some limited conditions such as the size of print paper used, etc., DSCs have been improved to the extent that the resolution thereof is closer to that of silver-halide-film cameras, and these improved DSCs have been produced and sold as products.
Here, a conventional photographing lens system of a DSC is discussed. Since an image is picked up by a CCD with a large number of pixels, it is understood that the structure of the photographing lens system is similar to a photographing lens system of a video tape recorder (VTR). However, higher performance is required with respect to resolution and quality of image, so that it is often the case that the structure of the conventional photographing lens system of a DSC is complicated. Furthermore, with respect to the size of the optical system, the photographing lens system of a DSC inevitably becomes larger than that of a VTR, even under the condition that the size of the image pick-up surface of CCD of a DSC is the same as that of a VTR. The characteristics of a conventional photographing lens system of a DSC are itemized as follows:
1. High Picture Quality
In recent years, even for general-use DSCs, the number of pixels has reached 3 to 4 millions. If the difference in the dimensions of the pick-up surfaces between DSCs and VTRs is ignored, the number of pixels of a DSC is ten times larger than that of a VTR. Similarly, it is understood that the precision (difficulty) on the correcting of aberrations, required for the photographing lens systems of a DSC and a VTR, may have such an amount of difference therebetween.
In order to increase the number of pixels in a CCD, an attempt has been made to make the size of the pixels themselves smaller, i.e., reducing a pitch of pixels, without increasing the dimensions of the image pick-up surface as much as possible. For example, in a recently released CCD, having the effective pixel number of a 1.3 million, to be used for a DSC, the pixel pitch in the CCD is about 4.2 &mgr;m. Accordingly, assuming that the diameter of the minimum circle of confusion is double the pixel pitch, the diameter of the minimum circle of confusion is 8.4 &mgr;m. Since the diameter of the minimum circle of confusion of a 35 mm-format silver-halide-film camera is about 33 &mgr;m, it can be said that the resolution required for the photographing lens system of a DSC is about four times higher than that of a silver-halide-film camera.
2. Suitable Telecentricity on the Image Side
Telecentricity on the image side means a state in which principal rays of light bundles, with respect to each image point, become substantially parallel with the optical axis after being emitted from the final lens surface of an optical system, i.e., the principal rays are substantially normal to the image plane. In other words, the position of the exit pupil of the optical system is sufficiently distant from the image plane. This arrangement is necessary in order to avoid the reduction of the effective aperture efficiency (shading). More specifically, since a color filter on the CCD is slightly distant from the image pick-up surface, if light rays inclined with respect to the optical axis are incident on the color filter, the aperture efficiency is decreased, i.e., shading occurs. In particular, in the case of recent CCDs with high-sensitivity, many of the CCDs are provided with a micro-lens array in front of the image pick-up surface. However, in such a case as well, if the exit pupil is not sufficiently distant the image plane, the aperture efficiency around the periphery is lowered.
3. Long Back Focal Distance Necessary
Not limited to a space for a protective glass plate required according to the structure of a CCD, spaces for some optical elements are generally required between the photographing lens system and the CCD. Such optical elements are, for example, an optical low-pass filter (OLPF) which is inserted for the purpose of preventing a moire effect and the like due to the superlattice (grid pattern) structure of the CCD, and an infrared absorption filter inserted between the photographing lens system and the CCD for the purpose of lowering the sensitivity of the CCD in the infrared wavelength range so that the sensitivity thereof become closer to the spectral luminous efficiency of the human eye.
As explained, the photographing lens system of a conventional DSC has the above-described three characteristics (conditions). In regard to item 2 (Suitable Telecentricity on the Image Side), the arrangement of the color filter and micro-lens array of the CCD has been reexamined. In regard to item 3 (Long Back Focal Distance Necessary), materials for the OLPF and other optical elements have been reexamined, and the structure of a CCD has also been reexamined from the fundamental viewpoint. According to these activities, there has been a symptom of improvement on the photographing lens system of a DSC. Due to room for design which is obtained by the above activities, an engineering environment in which development of the photographing lens system of a DSC can be made by taking compactness and cost-effectiveness into consideration.
SUMMARY OF THE INVENTION
In view of the above-mentioned drawbacks, an object of the present invention is to provide a compact photographing lens system which attains high resolution, and smaller number of lens elements.
As a preferable aspect of the present invention, a photographing lens system includes an aperture stop, a first lens group, a second lens group, and a third lens group, in this order from the object side. The first lens group includes a positive first lens element having positive refractive power, and a negative second lens element having a negative refractive power which is cemented to or separate from the positive first lens element. The second lens group includes a positive third lens element having an aspherical surface formed on at least one refractive surface thereof. The third lens group includes a negative fourth lens element having an aspherical surface formed on at least one refractive surface thereof. The photographing lens system satisfies condition (1) with respect to the power of the first lens group, and satisfies condition (2) with respect to the dimensions of the entire photographing lens system along the optical axis. By satisfying conditions (1) and (2), a compact photographing lens system, which attains high resolution, and smaller number of lens elements, can be obtained.
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Casio Computer Co. Ltd.
Frishauf Holtz Goodman & Chick P.C.
Lester Evelyn A
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