JP2517436B2 - Imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to the configuration of a focus or exposure control device for a video camera.

(B) Conventional technology In an autofocus device for a video camera, the method of using the video signal itself from the image sensor for the evaluation of the focus control state has essentially no parallax and has a depth of field. It has many advantages such as being able to focus accurately even for shallow subjects and distant subjects. Moreover, no special sensor for autofocus is required, and the mechanism is extremely simple.

Japanese Patent Application Laid-Open No. 63-215268 (H04N5 / 232) discloses an example of the autofocus device as described above.

The essence of this prior art will be described below with reference to FIGS. 2 and 3.

FIG. 2 is a circuit block diagram of the entire autofocus circuit. The image formed by the lens (1) becomes a video signal by the image pickup circuit (4) including an image pickup element,
It is input to the focus evaluation value generation circuit (5).

The focus evaluation value generating circuit (5) is configured as shown in FIG. 3, for example.

The luminance signal in the video signal is a high pass filter (HPF)
Only the high frequency component is separated after passing through (5f), and amplitude detection is performed by the detection circuit (5d) at the next stage. This detection output is converted to a digital value by the A / D converter (5e), and the gate circuit (5e
In f), only the information in the focus area (FA), which is the sampling area set in the center of the screen as shown in Fig. 6, is extracted and integrated by the integrating circuit (5g) for each field, that is, for one field. Is digitally integrated with respect to to obtain the focus evaluation value of the current field. At this time,
The vertical and horizontal sync signals separated from the luminance signal by the sync separation circuit (5a) are input to the gate control circuit (5b) to set the sampling area. The gate control circuit (5b) sets a rectangular sampling area in the center of the screen based on the vertical and horizontal sync signals and the fixed amplitude output, and allows the passage of luminance signals only within this sampling area. The open / close signal is supplied to the gate circuit (5c). The focus evaluation value generation circuit (5) configured as described above always outputs the focus evaluation value for one field for each field.

Immediately after starting the focusing operation, the first focus evaluation value is held in the maximum value memory (6) and the initial value memory (7). afterwards,
The focus motor control circuit (10) rotates the focus motor (3) in a predetermined direction to rotate the focus ring (2) supporting the light receiving lens (1),
The light receiving lens (1) is displaced in the optical axis direction and the output of the second comparator (9) is monitored. The second comparator (9) compares the focus evaluation value after driving the focus motor with the initial evaluation value stored in the initial value memory (7), and outputs the magnitude.

The focus motor control circuit (10) rotates the focus motor (3) in the first direction until the second comparator (9) outputs a large or small output, and the current focus evaluation value is compared with the initial evaluation value. When the output indicating that the value is large is maintained, the rotation direction is maintained as it is, and when it is determined that the current evaluation value is smaller than the initial evaluation value, the rotation direction of the focus motor is reversed and the first rotation is performed. Comparator (8)
Monitor the output.

The first comparator (8) compares the current maximum focus evaluation value held in the maximum value memory (6) with the current evaluation value, and the current focus evaluation value is stored in the maximum value memory (6). Two comparison signals (S1) that are larger than the contents (first mode) and have decreased below a preset first threshold (second mode)
Output (S2). Here, the maximum value memory (6) is updated based on the output of the first comparator (8) when the current evaluation value is larger than the content of the maximum value memory (6), and the value is always updated. The maximum focus evaluation value up to is retained.

(13) is a position memory that receives a focus ring position signal indicating the position of the focus ring (2) supporting the light receiving lens (1) as the lens position at that time, and stores the focus ring position. Similar to the memory (6), the focus ring position when the maximum evaluation value is reached is updated based on the output of the first comparator (8) so that it is always held.

The focus motor control circuit (10) monitors the output of the first comparator (8) while rotating the focus motor (3) in the direction determined based on the output of the second comparator (9), and the focus evaluation value The focus motor (3) is reversely rotated at the same time when the second mode is instructed such that is smaller than the first evaluation threshold value (M) set in advance as compared with the maximum evaluation value.

Due to the reverse rotation of the focus motor (3), the moving direction of the light receiving lens (1) is changed, for example, to a direction away from the image pickup device or vice versa.

After this reverse rotation, the contents of the position memory (13) and the current focus ring position signal are compared by the third comparator (14) and when they match, that is, the light receiving lens (1) has the maximum focus evaluation value. The focus motor control circuit (10) functions to stop the focus motor (3) when returning to the lens position. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS). Incidentally, FIG. 4 is a diagram showing changes in the lens position and the focus evaluation value in the above-mentioned hill climbing focusing operation. (11) is a fourth memory which holds the focus evaluation value at that time at the same time as the lens stop signal (LS) is issued after the focusing operation by the focus motor control circuit (10) is finished. The fourth comparator (12) compares the content held in the fourth memory (11) with the current focus evaluation value, and the current focus evaluation value is preset in comparison with the content of the fourth memory (11). Second fluctuation range
When the value of the subject changes, the subject is determined to have changed, and a subject change signal is output. Upon receiving this signal, the focus motor control circuit (10) performs the focusing operation again and follows the change of the subject.

On the other hand, similarly, the use of the video signal itself for the evaluation of the exposure state is also excellent in that it does not require a sensor, and is generally widely used. For example, in Japanese Patent Laid-Open No. 1-49484, the luminance signal is digitally integrated for a certain period without passing through the HPF to obtain an exposure evaluation value, and the exposure evaluation value is made to match a reference value obtained in the optimum exposure state. A technique for adjusting the exposure is disclosed.

(C) Problems to be solved by the invention In the autofocus system described above, if the number of samples in the sampling area is 2000, the DC offset voltage of the A / D converter (5e) is ± 1 / 2LS
The focus evaluation value has an error of 2000 only when there is an error of B (LSB: amount corresponding to the least significant bit of A / D conversion),
When the contrast of the subject is low, this error causes the focusing operation to malfunction. Also, A / D
Similar symptoms occur due to variations in the DC potential of the input signal to the converter.

Similarly, in the exposure adjustment, when the subject is extremely dark, the ratio of the error in the A / D converter to the entire exposure evaluation value becomes large, which causes a malfunction in the exposure adjustment.

(D) Means for Solving the Problems The present invention is
The focus or exposure evaluation value is calculated with the outside light blocked, this evaluation value is used as an offset value, and this offset value is subtracted from the focus or exposure evaluation value obtained after this, and this subtracted value is used as the true focus. Alternatively, it is characterized in that an exposure evaluation value is used, and the focusing or exposure adjustment operation is performed based on the exposure evaluation value.

(E) Action Since the present invention is configured as described above, the influence of the DC offset voltage error of the A / D converter and the variation of the DC potential of the A / D converter input from the focusing or exposure adjustment operation. Can be removed.

(F) Embodiment One embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a circuit block diagram of an embodiment of the present invention. The same parts as those of the conventional technique are designated by the same reference numerals and the description thereof will be omitted.

(19) is an aperture mechanism that mechanically adjusts the amount of light incident on the image sensor. The aperture amount is controlled by the iris motor (18) driven by the exposure control signal from the iris control circuit (41). It

(42) is an image pickup circuit (4) and a focus evaluation value generation circuit (5)
Is an AGC amplifier inserted between the two, and has the function of amplifying the luminance signal with a gain determined by the gain control signal from the iris control circuit (41), and performs the exposure adjustment operation together with the diaphragm mechanism (19). Become.

Next, a method of setting the offset value of the focus evaluation value will be described. First, when power is turned on to the entire camera circuit shown in FIG. 1 at the start of shooting, the iris control circuit (41) drives the iris motor (18) so that the diaphragm mechanism is fully closed and the external light to the image sensor is set. Cut off the input. At this time, the iris control circuit (41) is in an offset mode different from the normal exposure adjustment operation. That is, the level of the luminance signal obtained from the image pickup circuit (4) becomes substantially zero, but the iris control circuit (41) performs control to maximize the gain of the AGC amplifier (42), and as a result, the AGC amplifier (41) 42) A certain level of noise is generated at the output, and this is input to the focus evaluation value generation circuit (5) at the subsequent stage.

A / D converter (5
In e), the reference potential is set so that a digital value of about 1 or 2 is output when a no-signal input is given in advance, whereby the focus evaluation value generating circuit (5) reduces the noise to 1 By integrating over the field,
It is possible to obtain a digital integrated value that is a convection current to the DC offset voltage of the A / D converter (5e), that is, an integrated value.

When a certain period of time has passed after the power is turned on and the circuit operation is stabilized, the integrated value output from the focus evaluation value generation circuit (5) is stored in the offset memory (44) as an offset value.

When the storage of the offset value is completed, the iris control circuit (41) shifts from the offset mode to the normal mode, drives the iris motor (18) so that an appropriate light amount can be obtained, and the diaphragm mechanism ( The fully closed state of 19) is released, and the maximum gain state of the AGC amplifier (42) is released.

 After releasing the fully closed state, the focusing operation is started.

That is, after the fully closed state is released, the focus evaluation value generation circuit (5)
The focus evaluation value output from is input to the subtractor (16), the offset value stored in the offset memory (44) is subtracted from the focus evaluation value, and this subtracted value is used as the true focus evaluation value in the subsequent stage. It is supplied to each circuit for the focusing operation of, and executes the focusing operation similar to the conventional example. Therefore, since the true focus evaluation value is a focus evaluation value that is not affected by the DC offset voltage of the A / D converter (5e), the focusing operation itself is one in which the effect of the DC offset voltage is eliminated.

The offset value described above is also used for exposure control. That is, the brightness signal that has passed through the exposure adjustment AGC amplifier (42) is also supplied to the exposure evaluation value generation circuit (43).

The exposure evaluation value generation circuit (43) is composed of a detection circuit (51), an A / D converter (52) and an integration circuit (53) as shown in FIG. 5, and the input luminance signal is a detection circuit. After amplitude detection at (51), A / D conversion at A / D converter (52) and integration at integrator (53), digital integration value of the brightness signal obtained in this way is evaluated for exposure. The value is output to the subtractor (45).

On the other hand, the offset value of the offset memory (44) is converted into the offset value of the entire screen by the arithmetic circuit (48).
That is, the offset value of the offset memory (44) corresponds to the amount in the focus area (FA), but it is necessary to use the offset value for the entire screen in order to use the luminance signal for the entire screen during exposure adjustment. . Therefore, the arithmetic circuit (48) multiplies the reciprocal of the area ratio of the focus area (FA) to the entire screen to calculate the offset value for the entire screen. For example, if the focus area occupies 1/4 of the screen area, the offset memory (4
The offset value of 4) is multiplied by 4 and output to the subtracter (45) as an exposure offset value.

The subtractor (45) subtracts the offset value calculated by the arithmetic circuit (48) from the exposure evaluation value obtained from the generation circuit (43), and uses this subtracted value as the true exposure evaluation value, the comparator (46).
Output to.

The comparator (46) stores the true exposure evaluation value in the reference value memory (47).
Then, the difference and the magnitude relation between the two are output to the iris control circuit (41) by comparing with a reference value held in advance. Here, the reference value is a value taken by the true exposure evaluation value when the optimum exposure state is obtained, and is set based on the actual measurement value obtained in advance by experiments.

The iris control circuit (41) issues an exposure control signal to the iris motor (18) and a gain control signal to the AGC amplifier (42) based on the comparison result of the comparison circuit (46), and the true exposure evaluation value is the reference value. The exposure is controlled to match the reference value in the memory (47). In this exposure control, the control of the incident light amount by the diaphragm mechanism (19) in consideration of the S / N ratio is always prioritized over the electrical gain control of the luminance signal by the AGC amplifier (42). That is, when the subject has normal brightness and the true exposure evaluation value matches the reference value only by adjusting the diaphragm amount of the diaphragm mechanism, the gain of the AGC amplifier (42) is fixed to a constant value. . The true exposure evaluation value reaches the reference value only when the brightness signal level is low and the true exposure evaluation value does not reach the reference value even when the subject is extremely dark and the diaphragm mechanism (19) is fully open. It is controlled so that the gain becomes large.

As described above, in the offset mode immediately after the power is turned on to the camera circuit, the diaphragm mechanism (19) is forcibly closed and the signal from the image pickup circuit (4) becomes substantially zero.
The gain of the amplifier (42) is maximum.

In the above-described embodiment, the common offset value is used for focusing and exposure. However, instead of using the output of the focus evaluation value generating circuit (5) as the exposure offset value, the diaphragm mechanism immediately after the power is turned on. The integrated value of the exposure evaluation value generation circuit (43) obtained when (19) is fully closed is held in the offset memory (49) for exposure different from the offset memory (44) as shown in FIG. After the circuit (41) returns to the normal mode, the subtractor (45 ') subtracts the offset value of the offset memory (49) from the exposure evaluation value of the generation circuit (43) to obtain the true exposure evaluation value. It is also possible.

As described above, in the true focus and exposure evaluation value, the components due to the error of the DC offset voltage of the A / D converter (5e) (52) and the variation of the DC potential of the A / D converter input are removed. With respect to the focusing operation and the exposure adjusting operation based on these evaluation values, malfunctions due to the effects of the errors and variations are prevented.

In this embodiment, the offset value is obtained when the power is turned on, but it is also possible to record the offset value at the time of manufacturing by using the offset value memory (15) as a non-volatile memory.

Needless to say, these processes can be executed by software using a microcomputer.

It is also possible to fix the light receiving lens (1) and move the image sensor back and forth in the optical axis direction by the output of the focus control circuit (10) to change the relative positional relationship between the lens and the image sensor to perform the focusing operation. Is.

(G) Effect of the Invention According to the present invention as described above, the influence of the error of the DC offset voltage of the A / D converter on the focus or the exposure evaluation value is eliminated, and for a subject with low contrast or low illumination, Also enables stable focusing operation or exposure adjustment operation.

[Brief description of drawings]

1 and 5 are circuit block diagrams of one embodiment of the present invention, FIG. 7 is a circuit block diagram of another embodiment, FIGS. 2 and 3 are circuit block diagrams of a conventional example, and FIG. Is a diagram for explaining the hill climbing focusing operation, and FIG. 6 is a diagram for explaining the focus area. (5) ... focus evaluation value generation circuit, (43) ... exposure evaluation value generation circuit, (16) (45) ... subtractor, (44) ... offset memory, (10) ... focus control circuit, (41) ...
… Iris control circuit.

Claims (2)

(57) [Claims] 1. An integrating means for integrating a digital value indicating a high frequency component of a luminance signal corresponding to a specific area of an image pickup screen over a certain period, and an integration result of the integrating means being offset with a disturbance input blocked. An offset value holding means for holding the value as a value, a subtraction means for outputting a subtraction value obtained by subtracting the offset value from the integration result of the integration means at the time of photographing as a true focus evaluation value, and the true focus evaluation value being a maximum. An image pickup apparatus comprising a focusing unit that controls a relative positional relationship between a lens and an image pickup element so as to take a value. 2. An integrating means for integrating a digital value of a luminance signal over a certain period, an offset value holding means for holding an integration result of the integrating means as an offset value in a state where a disturbance input is cut off, and the above-mentioned at the time of photographing. A subtraction unit that outputs a subtraction value obtained by subtracting the offset value from the integration result of the integration unit as a true exposure evaluation value, and exposure adjustment is performed so that the true exposure evaluation value matches a preset reference value. An image pickup apparatus including an exposure adjustment unit.