Television – Camera – system and detail – Optics
Reexamination Certificate
1996-03-12
2004-03-16
Vu, Ngoc-Yen (Department: 2612)
Television
Camera, system and detail
Optics
C348S229100, C348S347000
Reexamination Certificate
active
06707500
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the exposure control of an image pickup apparatus.
2. Description of the Related Art
A method of controlling the aperture of an iris, the storage time of an image sensor (electronic shutter) and the gain of an AGC (automatic gain control) circuit for keeping the level of video signals at a predetermined value is employed in general for exposure control in image pickup apparatuses such as video cameras or the like. An image pickup apparatus of this kind is arranged and operates as described below with reference to
FIG. 1
which is a block diagram.
Referring to
FIG. 1
, a zoom lens
1
which has a variable focal length as a lens group is arranged to form the image of an object of shooting. An iris mechanism
2
is arranged either to be of an iris blade structure to control the quantity of incident light or to be in the form of liquid crystal means to control the quantity of transmission light. In this case, an iris is employed. An image sensor
3
is arranged to photo-electrically convert the incident light. A driving motor
6
is an IG meter or the like arranged to drive the iris mechanism
2
. An image sensor control circuit
7
is arranged to control the image sensor
3
in such a way as to read out a photo-electrically converted signal and to control the signal storage time in a manner called an electronic shutter function.
A sample-and-hold (S/H) circuit
8
is arranged to sample the signal photo-electrically converted by the image sensor
3
. An automatic gain control circuit (hereinafter referred to as AGC circuit)
9
is arranged to electrically amplify a signal. A camera signal processing circuit
10
is arranged to form a standard television signal by carrying out a gamma correcting process, a color separating process, a color-difference matrixing process, etc., and, after that, by adding synchronizing signals. The camera signal processing circuit
10
may be either an analog signal processing circuit which processes a signal in the form of an analog signal or a digital signal processing circuit which is arranged to analog-to-digital convert a video signal, to process the digital-converted signal and, after that, to digital-to-analog convert the signal. In this case, however, the camera signal processing circuit is an analog signal processing circuit.
A video tape recorder (hereinafter referred to as VTR)
11
is arranged to record a video signal on a tape. An electronic viewfinder (hereinafter referred to as EVF)
11
is provided for monitoring video images being picked up.
An exposure detecting circuit
13
is arranged to detect the state of exposure for the shooting object through a signal outputted from the AGC circuit
9
. An exposure control circuit
22
is arranged to control the iris mechanism
2
, the shutter speed of an electronic shutter which is controlled by the image sensor control circuit
7
, the gain of the AGC circuit
9
, etc., on the basis of a detection signal outputted from the exposure detecting circuit
13
. An iris driving circuit
16
is arranged to operate, according to the output of the exposure control circuit
22
, the motor
6
which drives the above-stated iris mechanism
2
. A trigger key
14
is provided for inputting a trigger signal to perform an AE lock action by which an exposure control state is maintained.
The exposure control circuit
22
contains therein an exposure control computing part
22
a
which is arranged to compute control information for control over the AGC gain, the shutter speed of the electronic shutter and the amount of aperture of the iris according to the signal outputted from the exposure detecting circuit
13
indicating a state of exposure, such as an integrated value of luminance level, a data holding part
22
h
composed of an AGC data holding part
22
b
, an electronic shutter data holding part
22
c
and an iris data holding part
22
d
which are arranged respectively to hold, in response to the operation of the trigger key
14
, control information on the AGC gain, the shutter speed of the electronic shutter and the amount of aperture of the iris outputted from the exposure control computing part
22
a
, and an AGC control part
22
e
, an electronic shutter control part
22
f
and an iris control part
22
g
which are arranged respectively to control the gain of the AGC circuit
9
, the speed of the electronic shutter of the image sensor
3
and the amount of aperture of the iris mechanism
2
according to data of varied kinds supplied through the AGC data holding part
22
b
, the electronic shutter data holding part
22
c
and the iris data holding part
22
d.
In order to obtain optimum video images by a simple shooting operation under various conditions, in the image pickup apparatus, the exposure detecting circuit
13
is arranged to detect changes in luminance of the object of shooting from a video signal, and the exposure control circuit
22
is arranged to select and correct exposure control parameters such as the aperture of the iris mechanism
2
, the electronic shutter controlling the storage time of the image sensor
3
, the gain of the AGC circuit
9
, etc., on the basis of the detection signal from the exposure detecting circuit
13
. The image pickup apparatus is thus arranged to perform control for making an optimum exposure always in a stable manner. Such an arrangement enables the automatic exposure control means to automatically carry out optimum exposure control, without recourse to any manual operation by the operator, even when the luminance of the object varies.
However, according to the above-stated automatic exposure control means, since the state of exposure is controlled always according to the luminance of the object image incident thereon through the zoom lens
1
, the state of exposure control varies when the luminance of objects located around the main object of shooting happens to change due to a change taking place in focal length of the zoom lens
1
, a motion of the main object or the like, even if the luminance of the main object does not vary.
Therefore, if the luminance of peripheral objects changes even when the state of exposure is in an optimum state for the main object of shooting, the state of control over the exposure comes to change and eventually becomes inapposite to the main object.
To cope with such a shooting condition, a means for holding a state of exposure called “AE lock means” have been developed to hold the apparatus in an apposite state of exposure control. The AE lock means is operated as follows. The operator watches the state of exposure on the image plane of the EVF
12
which is a monitoring means and inputs a trigger signal from the trigger key
14
when the operator wishes to keep the current state of exposure unvarying. In response to the trigger signal input, the data holding part
22
h
of the exposure control circuit
22
is brought into a holding state, in which amounts of correction computed by the exposure control computing part
22
a
at that time for correction of the exposure control parameters of varied kinds including the aperture of the iris mechanism
2
, the speed of the electronic shutter controlling the storage time of the image sensor
3
and the gain of the AGC circuit
9
are held.
In a case where a difference in luminance between the main object and peripheral objects are too much to obtain an optimum state of exposure for the main object by the automatic exposure control means, like in shooting a person under a back-light condition or like in shooting a person illuminated with an excessive front spot light, the exposure state for the person, i.e., the main object, becomes too dark due to an adverse effect of a bright background to result in so-called “black blanking” or too bright to result in so-called “white blanking”. Under such shooting conditions, the AE lock means is sometimes used to eliminate the adverse effect of the luminance of the peripheral objects by varying the focal length of the zoom lens
1
to zoom up the main object, p
Arai Hideyuki
Tamura Kyoji
Canon Kabushiki Kaisha
Robin Blecker & Daley
Vu Ngoc-Yen
Wilson Jacqueline
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