JP2877379B2 - Auto focus camera - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a camera autofocus apparatus that performs automatic focus adjustment based on a luminance signal in an imaged video signal obtained from an image sensor.

(B) Conventional technology A method of using a high-frequency component of a video signal itself from an image sensor for evaluation of focus control in a camera autofocus apparatus is essentially a method in which parallax does not exist and the depth of field of copy is shallow. There are many excellent points such as the ability to accurately focus on subjects that are far and far away. In addition, no special sensor for autofocus is required, and the mechanism is extremely simple.

JP-A-63-125910 (G02B7 / 11) discloses an example of the so-called hill-climbing autofocus method. Here, the essence of this prior art will be described with reference to FIGS. 2 and 3. FIG. FIG. 2 is an overall circuit block diagram of the prior art. In this figure, an image formed by a lens (1) becomes a video signal by an imaging circuit (4) including an imaging device, and a focus evaluation value generation circuit ( Input to 5). The focus evaluation value generation circuit (5)
It is configured as shown in the figure. The vertical synchronizing signal (VD) and the horizontal synchronizing signal (HD) separated from the video signal by the sync separation circuit (5a) are input to the gate control circuit (5b) to set a sampling area as a focus area. In the gate control circuit (5b), the vertical synchronization signal (VD),
Based on the horizontal synchronizing signal (HD) and the fixed oscillator output that drives the image sensor, a rectangular sampling area is set at the center of the screen, and a gate opening / closing signal that allows the passage of the luminance signal only in the range of this sampling area is set. Supply to the gate circuit (5c).

Only the luminance signal corresponding to the area within the focus area by the gate circuit (5c) is passed through the high-pass filter (HP
F) After passing through (5d), only high-frequency components are separated, and amplitude detection is performed by the next-stage detection circuit (5e). This detection output is A / D
The data is converted into a digital value by the conversion circuit (5f) at a predetermined sampling cycle, and is sequentially input to the integrator (5g).

Specifically, the integrator (5g) is an adder that adds the A / D conversion data and the latch data of the subsequent latch circuit,
This is a so-called digital integrator composed of a latch circuit that latches the added value and is reset every field.
The sum of all A / D conversion data for the field period is output as a focus evaluation value. Therefore, the focus evaluation value generating circuit extracts the luminance signal in the focus area in a time-division manner, further digitally integrates the high frequency component over one field period, and outputs the integrated value as the focus evaluation value of the current field. become.

Immediately after the start of the autofocus operation, the first focus evaluation value is held in the maximum value memory (6) and the initial value memory (7). Thereafter, the focus motor control circuit (10) rotates the focus motor (3) for moving the lens (1) in the direction of the optical axis via the focus ring (2) in the predetermined direction, and the second comparator (9). Monitor the output. 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 of the comparison.

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 smaller than the initial evaluation value. If the output is larger than the preset fluctuation range, the rotation direction is held as it is, and the current evaluation value is smaller than the fluctuation range as compared with the initial evaluation value. When the output is made, the rotation direction of the focus motor (3) is reversed,
The output of the first comparator (8) is monitored.

The first comparator (8) compares the current maximum focus evaluation value held in the maximum value memory (6) with the current focus evaluation value, and the current focus evaluation value is stored in the maximum value memory (6). Two different H-level comparison signals (P1) and (P2) are output which are larger than the contents (first mode) and decreased to be equal to or larger than the preset first threshold (second mode). Here, the maximum value memory (6) is updated based on the output of the first comparator (8) when the current focus evaluation value is larger than the content of the maximum value memory (6). The maximum value of the focus evaluation value up to the present is held.

(13) is a position memory which receives a focus ring position signal indicating the position of the focus ring (2) supporting the lens (1) and stores the focus ring position as a lens position. Similarly the first
Based on the output of the comparator (8), updating is performed so that the lens position when the maximum evaluation value is reached is always maintained. Here, the focus ring (2) is a focus motor (3)
It is a known technique that the lens (1) advances and retreats in the optical axis direction according to the rotation. The focus ring position signal is output by a potentiometer that detects the focus ring position. The focus motor (3) is a stepping motor, and the amount of rotation of this motor in the directions of the near point and the ∞ point is positive and negative. Step amount
The position of the focus ring or the focus motor can be expressed by this step amount.

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 In order to prevent malfunction due to noise, a second mode in which the current evaluation value at the output of the first comparator (8) is smaller than the preset first threshold value (Δy) as compared with the maximum evaluation value is provided. Instructed (reach Q in Fig. 4)
At the same time, the focus motor (3) is reversed. 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 focus evaluation value of the lens (1) becomes maximum. The focus motor control circuit (10) functions to stop the focus motor (3) when returning to the position (P). At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS) to complete the focusing operation.

(11) a fourth memory in which the focusing operation by the focus motor control circuit (10) is completed and the lens stop signal (LS) is issued, and at the same time the focus evaluation value at that time is held, The fourth comparator (12) uses the fourth memory (1
The content held in 1) is compared with the current focus evaluation value. If the value becomes larger than the second threshold value for restart, it is determined that the subject has changed, and the focus motor control circuit (10) notifies the focus motor control circuit (10) of the subject change. A 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.

(C) Problems to be Solved by the Invention The above-mentioned conventional technique has extremely high followability and high focusing accuracy, but the illuminance of the subject is different from the frequency of one field of the video signal. If the frequency changes at a constant frequency, a malfunction may occur due to the change in the illuminance. This situation will be described in some detail below.

For example, in a video camera of the NTSC system, the frequency of one field is 60 Hz, but such a malfunction occurs when this is used under the illumination of a discharge lamp such as a fluorescent lamp which is lit at 50 Hz. Since the brightness of the discharge lamp lit at 50 Hz fluctuates at a frequency of 100 Hz, the illuminance of the
It changes at a frequency of 00Hz. One field of the video signal is 60
Hz, a ripple of 20 Hz, which is the beat frequency, occurs.

That is, the average luminance level for each field when a predetermined subject is photographed under illumination having a constant illuminance is usually maintained at a constant value as shown in FIG. 5A, but as described above.
Flicker occurs under the illumination of a 50 Hz discharge lamp, and (M1) → (M2) → (M
3) → (M1) → ... The luminance level fluctuates. by the way,
Similar to the focus evaluation value average brightness level in the prior art, the distance to the subject does not change, and even if the subject itself does not change, it has a characteristic that changes in proportion to the amount of light incident on the image sensor. When the flicker occurs as shown in FIG. 5B, the focus evaluation value fluctuates even though the same subject is photographed while maintaining the same subject distance. When the focusing operation is performed based on the affected focus evaluation value as in the above-described conventional technique, the peak detection of the focus position is erroneously performed, or the subject does not change assuming that the maximum value of the focus evaluation value has changed. Nevertheless, there is a fear that the focusing operation may be restarted.

(D) Means for Solving the Problems The present invention provides an integrating means for integrating a high-frequency component of a luminance signal obtained by an image sensor over one field period;
Lens relative position changing means for changing the relative position of the lens with respect to the image sensor; 3n + 1, 3n + 2,
The integration output of the 3n (n is an integer) field is set to the first to third fields.
Separating means for separating as a focus evaluation value;
Focus position detecting means for storing the relative lens positions at which the focus evaluation value becomes maximum as first and third focus positions in first to third position memories, and in the first to third position memories. Focusing position determining means for setting an intermediate position among the stored relative positions of the lenses to be the final focusing position at which the focusing state has been reached, and the lens relative position changing means includes: When the relative position of the lens is changed to the determined final joint position, the operation of changing the relative position of the lens is stopped.

(E) Function Since the present invention is configured as described above, even if flicker occurs due to a 50 Hz discharge lamp, the influence on the focusing operation can be minimized.

(F) Example An example of the present invention will be described below with reference to the drawings.

FIG. 1 is an overall circuit block diagram of the present embodiment. In FIG. 1, the same parts as those in FIG. 2 of the conventional example are denoted by the same reference numerals, and description thereof will be omitted.

The luminance signal in the captured video signal from the imaging circuit (4) is
As in the prior art, the digital integrated value of the high-frequency component of the luminance signal over one field period in the focus area at the center of the screen is input to the focus evaluation value generating circuit (5) serving as the integrating means. It is output as a focus evaluation value.

(20): Fixed contact (20a) → (20b) for each field
→ (20c) → (20a) →… The movable contact piece (20
d) is a switch circuit serving as a separating means, which is located at the fixed contact (20a) side in the initial state, so that the focusing operation is started and the first field of the first field generated from the focus evaluation value generation circuit (5) is started. The focus evaluation value is supplied to the fixed contact (20a), and the focus evaluation value of the second field is set to the fixed contact (20a).
b) In the third field, the focus evaluation value is a fixed contact (20c)
And the same operation is performed thereafter. Therefore, the focus evaluation value of the 3n + 1 (n is an integer) field is supplied to the fixed contact (20a) every three fields, and the fixed contact (20a) is supplied.
The focus evaluation value of the 3n + 2 field is supplied to b), and the focus evaluation value of the 3n field is supplied to the fixed contact (20c) every three fields.

The focus evaluation value supplied to the fixed contact point (20a) is used as a first focus evaluation value (F1) as an initial value memory (7) at a subsequent stage, and a second focus evaluation value (F1).
It is supplied to a comparator (9) and a first focus position detection circuit (21). The focus evaluation values supplied to the fixed contacts (20b) and (20c) are the second and third focus position detection circuits (31) and (41) as the second and third focus evaluation values (F2) and (F3). ).

The initial value memory (7) stores the first value of the first focus evaluation value,
That is, the focus evaluation value immediately after the start of the focusing operation is held, and the second comparator (9) stores the initial focus evaluation value held in the initial value memory (7) and the first focus evaluation value (F1) obtained thereafter. Are compared, and the magnitude is output.

As in the prior art, the focus motor control circuit (50) serving as the lens relative position changing means moves the focus motor (3) in a predetermined initial direction until the second comparator (9) outputs a large or small output. ) To rotate the current first
If the output that the focus evaluation value is larger than the preset focus evaluation value is larger than the preset fluctuation range, the current rotation direction is maintained, and the output that the focus evaluation value is smaller than the fluctuation range is output. Is performed, the rotation direction of the focus motor (3) is determined so that the rotation direction is reversed.

After the rotation direction is determined, the first to third focus position detection circuits (21), (31), and (41) detect the focus ring position at which the first to third focus evaluation values take the maximum value, that is, the lens position. And the first to third in-focus position data (D1) and (D2), respectively.
Output as (D3).

The first in-focus position detection circuit (21) has a maximum value memory (2
6), a comparator (28) and a position memory (23), and the maximum value memory (26) is a comparator (28) as in the conventional example.
, The maximum value of the first focus evaluation value (F1) is held based on the comparison signal (P1), and the position memory (23) is the lens position at which the maximum value held in the maximum value memory (26) is obtained. The comparator (28) always compares the contents of the maximum value memory (26) with the first focus evaluation value (F1), and the latest first focus evaluation value is the maximum value data of the maximum value memory (26). A comparison signal (P1) is issued when it is determined to be greater than the threshold value, and an output (P2) is issued when it is determined that the first focus evaluation value is lower than the maximum value data by a first threshold (△ y). Therefore, H
Position memory (2) when the bell comparison signal (P2) is issued
3) The first lens position where the lens position becomes the maximum value of the first focus evaluation value
The focus position (G1) is reached, and first focus position data (D1) indicating this is output.

The second and third focus position detection circuits (31) and (41) also include maximum value memories (36) and (46), comparators (38) and (48), and position memories (33) and (43). From the comparators (38) and (48), the comparison signal (P1) was recognized each time the second and third focus evaluation values became maximum, and a decrease by a first threshold (△ y) was recognized from each maximum value. Sometimes a comparison signal (P2) is issued.
The maximum value memory and the position memory are updated each time the comparison signal (P1) is issued, and the lens positions of the position memories (33) and (43) when the comparison signal (P2) becomes H level are the second and the second. The second and third in-focus positions (G2) and (G3) that take the maximum value of the three focus evaluation values are obtained, and the second and third in-focus position data (D2) and (D3) indicating this are output. In addition, the first to third
The focus position detection circuits (21), (31), (41) correspond to focus position detection means.

A focus position determination circuit (51) serving as a focus position determination means includes:
Based on the first to third in-focus position data (D1), (D2), and (D3), an intermediate position in the first to third in-focus positions (G1), (G2), and (G3) corresponding to each position data Is selected as the final in-focus position (J). For example, in the example of FIG. 6, since the relationship of G2 <G3 <G1 holds, the third in-focus position (J) is used as the final in-focus position (J). G3) is selected.

The three comparison signals (P) from the comparators (28), (38) and (48)
2) are all input to the AND gate (52), and the output of the AND gate (52) is supplied to the focus control circuit (50). Then, any of the comparison signals (P2) is H
Level, that is, when it is recognized that each of the first to third focus evaluation values has dropped from the respective maximum value by the first threshold value (△ y), the output of the H level from the AND gate (52). Is issued, and upon receiving this, the focus motor control circuit (50) immediately reverses the focus motor (3).

After the reverse rotation of the motor, the third comparator (14) compares the current lens position with the final in-focus position (J), and when they match, that is, after the reverse rotation of the focus motor (3), the third comparator (14) makes the final focus. It emits an output when it is determined that it has returned to position (J).

Upon receiving this output, the focus motor control circuit (50) issues a lens stop signal (LS) assuming that the focus state has been reached, stops the focus motor (50), and completes a series of focus operations.

FIG. 6 is a diagram showing the change of the first to third focus evaluation values with respect to the lens position. Each curve shows the transition of the first to third focus evaluation values (F1), (F2), and (F3) with respect to the lens position. Each point "○" is a focus motor (3)
Shows the first to third focus evaluation values actually obtained for each field in consideration of the change in the lens position in one field period due to the driving of the lens, and the distance (L) between the two points is This corresponds to the amount of movement of the lens (1) caused by driving the focus motor (3) during one field period.
In FIG. 6, in order to make the description easy to understand, the reverse rotation of the focus motor (3) due to the fall of the first threshold value (△ y) is not considered, and the focus evaluation value for the entire region from the near point to the ∞ point is not considered. Shows the change.

After the completion of a series of focusing operations, the change of the subject can be confirmed by the comparators (28), (38), and (48) after the lens stop signal (LS) is generated.
Are used for confirming a change in the subject, the latest first to third focus evaluation values are constantly compared with the contents of the maximum value memories (26), (36), (46), and these comparison outputs are used as the focus motor control circuit (50). ), If any of the first to third focus evaluation values fluctuates above a second threshold value set in advance, it is determined that the subject has moved or changed, and all memories are reset and the focus motor (3) is turned on. Restart and restart the focusing operation.

In the above embodiment, the focus motor control circuit (50)
The drive of the focus motor (3) is controlled based on the instruction of (1), and the lens (1) moves forward and backward in the optical axis direction to change the relative position of the lens with respect to the image sensor, but instead fixes the lens itself. However, it is also possible to use a bimorph or a motor to move the image sensor in the optical axis direction based on a command from the focus motor control circuit (50). In the focus position determination circuit (51),
When the final focus position (J) is determined, the intermediate position between the first to third focus positions (G1), (G2), and (G3) is simply selected. Instead, the average position of the three focus positions is determined. Even if it is calculated, it can be dealt with.

(G) Effects of the Invention As described above, according to the present invention, it is possible to prevent a malfunction of the focusing operation due to the influence of flicker even under illumination of a 50 Hz discharge lamp or the like.

[Brief description of the drawings]

1 is a circuit block diagram of an embodiment of the present invention, FIG. 6 is a diagram showing a change in focus evaluation value of the present embodiment, FIG. 2 is a circuit block diagram of a conventional example, and FIG. Block diagram, 4th
FIG. 5 is a diagram illustrating the principle of hill-climbing autofocus, and FIG. 5 is a diagram illustrating a change in luminance level due to flicker. (5) Focus evaluation value generating circuit (3) Focus motor (20) Switch circuit (23) (33) (43)
...... Position memory.

Claims (1)

(57) [Claims] 1. An integrating means for integrating a high-frequency component of a luminance signal obtained by an image sensor over one field period; a lens relative position changing means for changing a relative position of a lens with respect to the image sensor; 3n + 1 after the start of integration , 3n + 2, 3n (where n is an integer) separating means for separating the integrated outputs of the fields as first to third focus evaluation values, and a lens relative position when the first to third focus evaluation values become maximum, respectively. , As the first and third focusing positions,
A focus position detecting means to be stored in a position memory; and a focus position determination for setting an intermediate position among the relative positions of the respective lenses stored in the first to third position memories to be a final focus position at which a focus state has been reached. Means for changing the relative position of the lens when the relative position of the lens is changed to the final palm position determined by the in-focus position determining means. Auto focus camera.