Television – Camera – system and detail – Optics
Reexamination Certificate
1996-06-11
2003-11-25
Moe, Aung S. (Department: 2612)
Television
Camera, system and detail
Optics
C348S354000
Reexamination Certificate
active
06654061
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an automatic focus adjusting apparatus and method utilized by an image sensing apparatus which performs focusing by adjusting draw-out amount of an object lens by using image signals obtained from an image sensor.
A conventional automatic focus adjusting apparatus discriminates whether an image of an object is in focus or out of focus on the basis of high-frequency component signals extracted from image signals by using a band-pass filter, and the like, and on signals representing edge parts of the image extracted by using a differential circuit. More specifically, the image signals representing an image in focus are obtained by moving the position of a lens so that the above extracted signals have the maximum intensity (power). The method adopted by the above conventional apparatus is explained in detail in “Automatic Focus Adjustment of a Television Camera in Climbing Servo Method” (Ishida et al.) published in “NHK Technical Research, Vol. 17, 1st issue” (1965). Referring to
FIG. 1
, the climbing servo method will be briefly described below.
In
FIG. 1
, reference numeral
1
denotes a lens;
2
, an image sensor;
3
, a preamplifier for amplifying voltages of image signals detected by the image sensor
2
;
4
, a signal processing circuit;
5
, a band-pass filter (abbreviated as “BPF”, hereinafter) which transmits an image signal component in a predetermined frequency band;
6
, a detector;
7
, a microcomputer responsible for focus control;
8
, a motor driver; and
9
, a motor.
An image of an object is projected on the photosensing surface of the image sensor
2
by the lens
1
, and electric signals converted from the optical image are obtained by the image sensor
2
. The image signal is amplified to an appropriate level by the preamplifier
3
, then, converted into a standardized image signal, such as NTSC, by the signal processing circuit
4
. The output from the preamplifier
3
also enters the BPF
5
, where a high-frequency component included in the image signal is extracted. Further, the high-frequency component is inputted to the detector
6
, thereby obtaining output corresponding to “power” of the high-frequency components.
The resolution of the image projected on the photosensing surface of the image sensor
2
by the lens
1
depends on how well the image is focused. More specifically, when the focal point of the lens
1
projecting the image is on the photosensing surface of the image sensor, the resolution reaches its maximum, and, as the distance between the photosensing surface and the focal point of the lens
1
(the distance is referred as “defocused amount”, hereinafter) becomes larger, the resolution drops.
FIG. 2
is a graph showing changes of MTF (Modulation Transfer Function) for states of an image projected by the lens
1
when the image is focused, lightly defocused and largely defocused. As shown in
FIG. 2
, as the state of the image approaches the focused state (i.e., the defocused amount becomes smaller), high resolution is achieved over a frequency range extending to higher spatial frequencies. In contrast, as the state of the image is more defocused (i.e., the defocused amount becomes larger), the spatial frequency to which the image is possibly resolved becomes low. This relationship corresponds to that between the amplitude and the frequency of an image signal. In other words, as the state of image approaches the focused state, the amplitude of a high frequency component of an image signal becomes large, whereas, as the state of the image approaches the highly defocused state, an amplitude of the high frequency component of the image signal becomes small.
Therefore, as shown in
FIG. 3
, the output from the detector
6
alters depending upon the draw-out amount of the lens
1
, and has its peak at a specific point. This is because the focusing state of an image projected on the photosensing surface of the image sensor
2
changes as the draw-out amount of the lens
1
alters, and the intensity of a high-frequency component in a signal which is converted from the image by the image sensor
2
reaches its maximum since the projected image is clearest in the focused state.
The microcomputer
7
inputs the output from the detector
6
, calculates the driving direction and driving velocity, which are to be applied to the motor
8
, which maximize the output from the detector
6
. Then the microcomputer
7
controls the rotational direction and the rotational velocity of the motor
9
through the motor driver
8
. Thereafter, the microcomputer
7
controls to stop the motor
8
at a position where the output from the detector
6
reaches its maximum. As described above, an image is focused by controlling the draw-out amount of a lens of a video camera.
In this method, if a lens has been drawn out initially to a position illustrated by the point A in
FIG. 3
, for example, in order to draw out the lens to the point B at which the image is focused, the lens has to be moved in either the direction toward the point at which the focal distance is infinite or in the direction toward the point at which the focal distance is the minimum after deciding the driving direction. Note, the term, “climbing method” is named since the output from the detector
6
has a locus as if “climbing a mountain” with respect to the draw-out position, as shown in FIG.
3
.
In such a case, when the lens is moved from the point A toward a point at which the focal distance is infinite, it can be simply moved to the point B as shown in
FIG. 3
, however, when the lens is moved toward a point at which the focal distance is the shortest, the direction of the movement must be reversed after confirming that the output from the detector
6
drops when the lens
1
is moved in the original direction.
Further, even though the lens is moving toward the point B, it is impossible to determine that the output from the detector
6
has reached its maximum at the point B when the lens has arrived at the point B. Therefore, it is necessary to move the lens in such a manner that the lens once passes the point B, and the drop of the output from the detector
6
is confirmed at a point C, then the lens is moved back to the point B.
The aforesaid operations have to be performed since one cannot know if the lens are currently in the focused state or not, or if the lens pass the peak and are being drawn out in the direction toward the point where the focal length is infinite (referred as “rear-focused state”, hereinafter), or if the lens pass the peak and are being drawn out in the direction toward the point at which the focal length is the shortest (referred as “front-focused state”, hereinafter) without monitoring the change of the output from the detector
6
. However, the aforesaid operation is not preferable so as to find the focused state and set the lens in the focused state automatically, effectively and smoothly.
In contrast with the “climbing” method as described above, a “wobbling” method has been proposed. In the “wobbling” method, how well an image of an object is focused (referred as “focusing state”, hereinafter) on the photosensing surface is determined by slightly vibrating a part of a lens system or an image sensor (referred as “wobbling operation”, hereinafter) in the direction of the optical axis at a predetermined period by using a driving system other than the driving system used for focusing. The wobbling method is disclosed in detail in the Japanese Patent Laid-Open No. 58-188965 by Kitamura et al., for instance. In the wobbling method, whether the image is focused or not, or whether the image is in a rear-focused state or in a front-focused state can be discriminated without driving the lens system by a motor. Accordingly, an image can be focused effectively and smoothly comparing to the aforesaid “climbing method”. However, since the mechanism for vibrating the lens system or the image sensor is complicated, it is expensive.
In contrast, a “step driving” method which uses a driving mechanism such as a steppin
Canon Kabushiki Kaisha
Moe Aung S.
Morgan & Finnegan , LLP
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