Television – Camera – system and detail – Optics
Reexamination Certificate
1998-02-20
2003-04-22
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Optics
Reexamination Certificate
active
06552748
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an auto focus camera. More specifically, the present invention relates to an auto focus camera in which a lens of a digital still camera is driven to an in-focus position.
2. Description of the Background Art
Conventional photographic cameras adopt optical auto focusing such as phase difference detecting method. In a digital still camera, a video signal from an image pickup device such as a CCD (Charge Coupled Device) is used for evaluating a state of focus control, as it is advantageous in that there is no parallax essentially, and that highly accurate focusing is possible even when the depth of field is shallow or when an object is at a long distance. Further, any special sensor for auto focusing is not required, which leads to very simple structure.
An example of such an auto focus camera is disclosed in Japanese Patent Laying-Open No. 3-68280. This reference discloses a technique for speeding up focusing operation in which a focus lens is moved between an infinite focusing position and a close focusing position step by step, with each step being relatively large, focus evaluating value of each step is obtained, and the focus lens is moved close to a position corresponding to the maximum focus evaluating value over the distance to the object. Thereafter, the focus lens is moved step by step with each step being smaller, near the optimal object distance. The maximum evaluating value is found among the focus evaluating values of respective small steps, whereby the speed of focusing operation is increased.
FIG. 5
shows relation between the lens position and the focus evaluating value. As shown in
FIG. 5
, generally, the focus evaluating value is highest at the in-focus position, and it decreases as the amount of defocus increases.
As already described, in auto focusing operation, the focus lens is normally driven such that the maximum focus evaluating value is attained. However, where the object to be photographed has low illuminance and there is a portion having high brightness in the photographing field, as shown in
FIG. 6
, the focus evaluating value at the in-focus position tends to be low, while the focus evaluating value tends to be higher at a position further out of focus.
More specifically, when there is a clear and bright object at a position of FIG.
6
(
a
) while there are objects which are not sharp at positions of FIGS.
6
(
b
) and (
c
), rising edges of video signals corresponding to portions having high brightness become acute and high frequency component increases, as the amount of defocus increases, at objects
6
(
b
) and
6
(
c
). When the position of the focus lens is set so as to attain the maximum focus evaluating value in such situation, the lens is focused on the position (b) or (c) of
FIG. 6
, and not well focused on the desired object.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide an auto focus camera capable of driving a focus lens to an in-focus position even when there are two peaks of focus evaluating values in a situation where illuminance is low and a highly bright portion exists in the photographing field.
Briefly stated, the present invention provides an auto focus apparatus for driving the focus lens to a position where high frequency component of a video signal output from an image pickup device is maximized, in which evaluating values generated based on the high frequency component are detected by a evaluating value detecting circuit, and in response to detection of two peak evaluating values, the focus lens is set by a lens driving circuit at a position where the lens is focused at a prescribed distance.
Therefore, according to the present invention, focus evaluating values of the entire focusing area are measured and when there are two peaks of focus evaluating values, the lens is driven such that the lens is focused at a prescribed distance. Therefore, significant defocus can be avoided.
In a preferred embodiment, a position corresponding to the lowest evaluating value between the two peak values detected by the evaluating value detecting circuit is regarded as the in-focus position, and the lens driving circuit drives the lens to the in-focus position.
More preferably, the video signal output from the image pickup device is converted by an A/D converting circuit to a digital signal, brightness signal component is extracted from the video signal which has been converted to the digital signal, by a brightness signal generating circuit, high frequency component is extracted from the provided brightness signal component by a filter circuit, and applied to the evaluating value detecting circuit.
In a preferred embodiment, the lens driving circuit drives the lens first to an infinite focusing end in the initial state and thereafter moves the lens to the closest focusing point, and in response to detection of two peak values therebetween, sets the lens at the in-focus position.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
REFERENCES:
patent: 4392726 (1983-07-01), Kimura
patent: 4422097 (1983-12-01), Inuiya
patent: 4814889 (1989-03-01), Covey
patent: 5235375 (1993-08-01), Yamana et al.
patent: 5249058 (1993-09-01), Murata et al.
patent: 5430483 (1995-07-01), Haruki
patent: 5534923 (1996-07-01), Suda
patent: 5539462 (1996-07-01), Lee et al.
patent: 5923371 (1999-07-01), Iijima
patent: 6222588 (2001-04-01), Yamazaki et al.
patent: 0 390 091 (1990-03-01), None
patent: 57-161708 (1982-10-01), None
patent: 3-68280 (1991-03-01), None
patent: 04-318772 (1992-11-01), None
patent: 08-122627 (1996-05-01), None
patent: 08-334683 (1996-12-01), None
patent: 10-336508 (1998-12-01), None
Ho Tuan
Michaelson Peter L.
Michaelson & Wallace
Sanyo Electric Co,. Ltd.
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