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Imaging system

Imaging system description/claims

This application is a continuation of prior U.S. patent application Ser. No. 10/619,078, filed Apr. 18, 2006, and also claims foreign priority benefits under 35 U.S.C. §119 of Japanese Application No. 2002-206733, filed on Jul. 16, 2002, both of which are expressly incorporated herein by reference.

The present invention relates generally to an imaging system, and specifically to an imaging system comprising a zoom lens and an electronic image pickup device such as a CCD. More specifically, the present invention is concerned with a digital camera capable of obtaining electronic images.

So far, imaging systems incorporating a zoom lens and an electronic image pickup device, for the most part, have made use of a so-called variable stop with variable aperture diameter to control the quantity of light passing through the zoom lens.

Current image pickup devices, on the other hand, are designed with an increasing number of pixels for the purpose of achieving image quality improvements.

The more the number of pixels in an image pickup device, the higher the optical performance demanded for an optical system becomes. A problem with the use of the conventional variable stop is, however, that when it is intended to diminish the stop diameter for light quantity control, resolution drops due to the influence of diffraction. It is thus difficult to make any sensible tradeoff between light quantity control and image quality improvements.

Even when it is desired to cut down the length of the zoom lens, the thickness of the variable stop due to its mechanical makeup often imposes limitations on reducing the length of the optical system.

In view of such problems as stated above, the primary object of the present invention is to provide an imaging system wherein light quantity control as well as high image quality can be achieved while the influence of diffraction is eliminated or minimized, and the length of a zoom lens can be cut down.

According to the first aspect of the present invention, this object is achievable by the provision of an imaging system comprising a zoom lens comprising a plurality of lens groups wherein a spacing between individual lens groups is varied to vary a focal length and an aperture stop located in an optical path for limiting at least an axial light beam diameter, and an electronic image pickup device located on an image side of the zoom lens, characterized in that:

the aperture stop has a fixed shape, and

a filter for performing light quantity control by varying transmittance is located on an optical axis of a space located at a position different from that of a space in which the aperture stop is located.

Advantages of the first imaging system of the present invention are now explained. This imaging system comprises a zoom lens including a plurality of lens groups wherein the spacing between individual lens groups is varied for zooming (variation of the focal length) and an electronic image pickup device located on the image side of the zoom lens. By fixing the shape of the stop, it is possible to prevent deterioration in electronic images due to diffracted light that occurs at a reduced stop diameter.

A conventional mechanism for controlling the quantity of light at a stop position imposes limitations the flexibility in the layout of the optical system due to the use of a shape variable stop, etc. With the present invention wherein the stop of fixed shape is used, however, the stop mechanism itself can be slimmed down.

The spacing between the lens groups with the stop interposed between them can be shorter than usual, and the length of the lens group system can be cut down as well.

It is here understood that the aperture stop could be located in the lens group, i.e., said aperture stop could be located between lenses where the spacing between which remains constant upon zooming. The aperture stop is not necessarily limited to a circular one. It is here noted that for a conventional variable stop that must be constantly of circular shape, a number of stop plates are needed.

In the present invention, however, it is more preferable to use an aperture stop having a circular aperture, because so-called neatly blurred images with limited image variations at non-focused portions can be obtained irrespective of in what state the quantity of light is controlled.

It is also acceptable that the chief ray of an off-axis light beam determined by the aperture stop is shaded at other sites. In other words, the light quantity control means of the type that the area of an aperture stop is commonly decreased must be located at a position where there is no extreme decrease in the quantity of light at the marginal area of a screen upon stop-down. In the present invention, however, it is unnecessary to do so and hence the degree of freedom in design can be enhanced.

If the aperture stop of fixed shape is configured as described below, it is then possible to obtain electronic images of high resolution.

According to the second aspect of the present invention, the first imaging system is characterized in that when 1.5×103×a/1 mm<F where F is a full-aperture number at the telephoto end of the zoom lens and a is the minimum pixel pitch in mm of the electronic image pickup device, the length of the aperture stop in the vertical or horizontal direction of an image pickup plane is longer than the length of the aperture stop in the diagonal direction of the image pickup plane, or when 1.5×103×a/1 mm>F, the length of the aperture stop in the vertical or horizontal direction of the image pickup plane is shorter than the length of the aperture stop in the diagonal direction of the image pickup plane.

Advantages of the second imaging system of the present invention are now explained. For instance, as the length of the pixel pitch reaches about 2 μm, there is diffraction limited at an F-number of about 5.6. According to the present invention wherein the stop is always of fixed shape, it is possible to enhance resolution by the determination as desired of aperture shape.

• Provisional Patent
• Utility Patent

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