Television – Camera – system and detail – Optics
Reexamination Certificate
1996-12-10
2004-04-27
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Optics
C348S354000
Reexamination Certificate
active
06727949
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an image pickup apparatus, such as a video camera or the like, having an automatic focusing device and arranged to permit selection between a moving-image shooting mode and a still-image shooting mode.
2. Description of the Related Art
The automatic focusing device included in a video camera or the like is arranged to control a focusing action on the basis of a video signal coming from an image sensor for accurate focusing, to permit setting a focus detecting area as desired and to prevent focus variations and occurrence of a parallax from being caused by zooming, The automatic focusing device can be simply arranged with no special sensor additionally provided for focusing.
FIG. 1
shows in a block diagram the arrangement of the conventional automatic focusing device of the above-stated kind. Referring to
FIG. 1
, a light signal obtained by a focusing lens
1
from an object of shooting is imaged on an image sensor
2
disposed to the rear of the focusing lens
1
. A video signal is outputted from the image sensor
2
. A luminance signal obtained from the video signal is inputted to a band-pass filter
3
which is connected to the image sensor
2
. The band-pass filter
3
extracts from the luminance signal a predetermined high frequency component which varies according to the state of focus. The high frequency component is inputted to a detection circuit
4
which is connected to the band-pass filter
3
. The detection circuit
4
detects the amplitude of the high frequency component inputted (integrated). The result of the detection is taken out as a focus detection signal. The focus detection signal is inputted to a gate circuit
5
which is connected to the detection circuit
4
. The gate circuit
5
extracts from the focus detection signal only a portion of the focus detection signal obtained from a focus detecting area set within an image plane. The focus detection signal thus extracted through the gate circuit
5
is inputted to an evaluation value computing circuit
6
which is connected to the gate circuit
5
. The evaluation value computing circuit
6
computes either the integrated value or the differentiated value of the focus detection signal for each field to obtain a focus evaluating signal for the current field. To the evaluation value computing circuit
6
is connected an in-focus decision circuit
8
which is arranged to decide an in-focus state. A lens control circuit
9
is connected to the in-focus decision circuit
8
and is arranged to control the position of the focusing lens
1
. A motor
12
which is arranged to drive the focusing lens
1
is connected to the lens control circuit
9
through a lens driving circuit
10
. To the motor
12
is connected a lens position detecting circuit
11
which detects the position of the focusing lens
1
. The lens position detecting circuit
11
is connected to the lens control circuit
9
. A memory
7
is connected to the in-focus decision circuit
8
and the lens control circuit
9
.
The automatic focusing device which is arranged in the above-stated manner performs a focusing action in accordance with a so-called hill-climbing method. At the start of the focusing action, an optical path length is varied, for example, by slightly vibrating the focusing lens
1
or the image sensor
2
in the direction of an optical axis. The in-focus decision circuit
8
computes the direction in which a defocused state takes place by detecting changes in a focus evaluating signal. Then, the lens driving circuit
10
drives the motor
12
to cause the focusing lens
1
to be moved in the direction in which the focus evaluating signal becomes larger. In this case, the focusing lens
1
is moved in one and the same direction as long as the focus evaluating signal of the current field is judged to be larger than the focus evaluating signal of a preceding field. The largest value of the focus evaluating signals obtained up to now and the position of the focusing lens
1
obtained when the focus evaluating signal is at the largest value are stored in the memory
7
. Further, even when the focus evaluating signal of the current field is judged to be smaller than the focus evaluating signal of the preceding field, if the focus evaluating signal of the current field is larger than a predetermined percent of the maximum value of the focus evaluating signal obtained up to that time, the focusing lens is driven in the same direction. If the focus evaluating signal becomes smaller than the predetermined percent, the focusing lens is regarded as having passed over an in-focus point and the driving direction of the focusing lens
1
is reversed. Then, the focusing lens is moved to the lens position corresponding the maximum value of the focus evaluating signal stored in the memory
7
, so that the focusing action is completed. The in-focus decision circuit
8
generates a coincidence signal when the focus evaluating signal obtained from the focusing lens
1
which is reversely moved comes to coincide with the largest value of the focus evaluating signal stored in the memory
7
. In response to the coincidence signal, the lens control circuit
9
issues a stop instruction to bring the focusing lens
1
to a stop.
FIG. 2
is a graph showing the characteristic of a relation obtained between the position of the focusing lens and the focus evaluating signal. The peak value Vf of the focus evaluating signal which is obtained when the lens is judged to be in an in-focus state is stored in the memory
7
. Even after the focusing action is completed and the focusing lens is stopped, the focus evaluating signal continues to be detected. If the focus evaluating signal becomes smaller than a restart threshold value Vth, the movement of the focusing lens is restarted. Accordingly, the restart threshold value Vth is determined in a predetermined ratio to the peak value Vf. The focusing accuracy can be increased by having the restart threshold value Vth set at a high level. If the restart threshold value Vth is set at a high level, however, the focusing, action would be restarted even when the luminance of the object of shooting slightly changes or even when another object happens to move past the object. In such a case, the image plane fluctuates to give a disagreeable impression in viewing the video image. Thus, in the video camera, the continuity of video images is regarded as more important than the momentary image quality (focusing and exposure). To prevent the focusing action from being unnecessarily restarted in such a manner, therefore, the restart threshold value Vth is set in consideration of the diameter of an allowable circle of confusion of the image pickup system of the video camera. Unless the focus evaluating signal obtained during shooting becomes smaller than the restart threshold value Vth, the motor
12
is stopped to keep the focusing lens
1
to its position (an in-focus point) obtained when the peak value Vf which is stored in the memory
7
is obtained.
Further, in the video camera, the iris is set in such a manner to make the responsivity relative to a change of brightness slow. Therefore, a change of the picked-up image is made moderate with respect to a sudden change of brightness, so that the brightness of the image is stabilized.
The automatic focusing device used for the conventional video camera is arranged as described above. Meanwhile, there has been proposed a video camera which is arranged to be capable of operating not only in a moving-image shooting mode but also in a still-image shooting mode in which a shot of a still image can be taken and to permit switching between these two modes.
In the moving-image shooting mode, the focus is continuously adjusted until an in-focus state is attained. However, in the moving-image shooting mode, since the continuity of video images is regarded as more important than the accuracy of focusing, the video camera of the above-stated kind is arranged to continue a shooting operation and keep the focusing lens
Fujii Takashi
Mizoguchi Yoshiyuki
Murakami Taro
Saruwatari Hiroshi
Yamaguchi Toshinobu
Canon Kabushiki Kaisha
Ho Tuan
Robin Blecker & Daley
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