JPH0642719B2 - Autofocus video camera - Google Patents

Description

TECHNICAL FIELD The present invention relates to an autofocus system of a video camera that automatically adjusts a focus based on a video signal obtained from an image pickup device.

(B) Prior art The method of using the video signal itself from the image sensor in the autofocus device of the video camera for the evaluation of the focus control state is essentially parallax-free and has a shallow depth of field. There are many advantages such as being able to focus accurately on a distant subject. Moreover, a special sensor for autofocus is not required, and the mechanism is extremely simple. Conventionally, an example of this autofocus method is "NHK".
The technical report, "S40, Vol. 17, No. 1, Vol. 86, page 21", states "Automatic focus adjustment of TV camera by mountain climbing servo system" by Ishida et al. So-called hill climbing servo control is known. This hill-climbing servo control rotates the lens focus ring so that the amount of high-frequency components of the video signal is always maximized, so the lens does not remain stopped in the out-of-focus state, and it follows very well. It is a good method.

However, this method has a major drawback in that the lens is constantly moved.

One of the drawbacks is that the lens does not stop even if the subject is in focus, so that the shooting screen constantly shakes even after focusing on a stationary object. The lens currently used in TV cameras changes the focal length by rotating the focus ring,
Therefore, the angle of view of the shooting screen changes. For this reason, in this method, in which the focus ring continues to vibrate even after focusing, the image on the screen becomes large or small at a certain cycle, making the screen very difficult to see.

The second drawback is power consumption. Currently, home video cameras often use a battery as a power source because of their portability. When the reverse rotation is repeated, more power is consumed than when the motor is rotated in a certain direction due to the inrush current, and the possible shooting time becomes shorter. In addition, since the focus ring is always rotated, problems such as wear of gears occur.

The applicant has proposed in Japanese Patent Application No. 61-273212 (H04N 5/232) a new autofocus circuit system for eliminating these drawbacks. The outline of the contents will be described below as a conventional example. In the above application, an extremely fine control method is used to improve the focusing accuracy, but here, only the portions related to the present invention will be described.

FIG. 4 is a block diagram of the autofocus circuit according to the above application.

The image formed by the lens (1) becomes a video signal by the image pickup circuit (4) including an image pickup element and is input to the focus evaluation value generation circuit (5). The focus evaluation value generation circuit (5) is constructed, for example, as shown in FIG. The vertical synchronizing signal (VD) and the horizontal synchronizing signal (HD) separated from the video signal by the cycle separating 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 sync signal (VD), horizontal sync signal (HD), and fixed oscillator output, and the luminance signal only within this sampling area. Gate circuit with a gate opening / closing signal that allows the passage of
Supply to (5c).

Only the luminance signal corresponding to the range of the sampling area is high pass filter (HPF) by the gate circuit (5c).
Only the high frequency component is separated after passing through (5d), and amplitude detection is performed by the detection circuit (5e) at the next stage. This detection output is integrated circuit (5f)
After being integrated for each field at, it is converted to a digital value by the A / D converter (5g) to obtain the focus evaluation value of the current field. The focus evaluation value generation circuit (5) configured as described above always outputs the focus evaluation value for one field.

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). After that, the focus motor control circuit (focus motor control means) (10) rotates the focus motor (3) in a predetermined direction and monitors the output of the second comparator (9). 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),
The size is output.

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. , If the output is larger than the preset fluctuation range, the rotation direction is kept 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 and the output of the first comparator (8) is compared.

The first comparator (8) compares the current maximum focus evaluation value stored 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). Of the two comparison signals (S1) and (S2) that are larger than the first threshold value and that are smaller than the preset first threshold value (second mode). 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 evaluation value up to is held.

(13) is a motor position memory that stores the focus ring position by receiving the focus ring position signal that indicates the position of the focus ring (2) that supports the lens (1), and is the same as the maximum value memory (6). Based on the output of the first comparator (8), the focus ring position when the maximum evaluation value is reached is updated so as to always be 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), In order to prevent malfunction due to noise, the second mode in which the current evaluation value at the output of the first comparator (8) is smaller than the preset first threshold value (Δx) as compared with the maximum evaluation value. At the same time, the focus motor (3) is rotated in reverse. After this reverse rotation, the contents of the motor 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 ring (2) has the maximum focus evaluation value. Focus motor when returning to the position
The focus motor control circuit (10) functions to stop (3). At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS).

(11) is the fourth memory that holds the focus evaluation value at that point when the lens stop signal (LS) is issued after the autofocus operation by the focus motor control circuit (10) is finished. This fourth memory (1
The content held in 1) is compared with the current focus evaluation value, and if the value becomes larger than the second threshold value for restart,
Focus motor control circuit assuming that the subject has changed (10)
A subject change signal is output to. Upon receiving this signal, the focus motor control circuit (10) restarts the autofocus operation again and follows the change of the subject.

This method has extremely high followability and high focusing accuracy, but has the following major drawbacks.

Generally, a lens of a video camera is provided with a stopper so that the focus ring does not rotate beyond an infinity point or a near point. As a result, for example, if the subject is closer to the subject distance position where the lens near-point stopper is located, there is a position where the evaluation value becomes maximum at a position beyond the near-point stopper as shown in FIG. The focus motor control circuit (10) continues to rotate the focus motor (3). As a result, the focus motor (3) rotates under a heavy load without stopping and the autofocus operation continues.

The same applies to the point at infinity, where the focus evaluation value is maximum at an infinite position for a subject at infinity, but as a characteristic of hill climbing servo control, that is the peak at the stage when the peak is first reached. Since I cannot recognize that there is something, the autofocus operation can continue without interruption, as above. As a result, the advantage of stopping the lens described in the conventional example is lost.

As one means for solving this drawback, there is a method of detecting that the lens has reached a near point or an infinite point, reversing the focus motor at this end point, and continuing the autofocus operation as it is.

As described in the above-mentioned application, this method is very excellent as an end point processing method, but there is some problem in the operation at the near point. That is, the image greatly fluctuates with a slight lens movement at the near point, so if the focus motor is reversed at the near point and the autofocus operation is performed, the focus evaluation value decreases from the evaluation value at the near point by a certain level. Since the focus motor stops at the position you set, the image that was about to be in focus will be out of focus and focused again.
The operation during this period becomes very difficult to see. This problem is particularly noticeable when the zoom lens is used at the Tele (telephoto) end.

(C) Problems to be Solved by the Invention In the autofocus method using the hill-climbing servo control, there is a malfunction because the apex of the focus evaluation value cannot be determined for the near and far objects. As a countermeasure against this, a method has been proposed in which the focus motor is rotated in the reverse direction when the focus ring reaches the near point and the far point, and the autofocus operation is continued. Occurred and it became a very difficult screen.

(D) Means for Solving the Problems The present invention provides means for detecting that the focus lens has reached the near point, and when the near point detection output is generated, reverses the rotation direction of the focus motor, In addition, the focus evaluation value at that time is recognized as the maximum evaluation value, so that the focus motor is immediately stopped at the near point position.

(E) Action Since the present invention is configured as described above, the out-of-focus image does not occur due to the focus ring reversing at the near point.

(F) Embodiment One embodiment of the present invention will be described below with reference to the drawings. The same parts as those of the conventional example (FIGS. 4 and 5) are designated by the same reference numerals and the description thereof will be omitted.

FIG. 1 is a block diagram of an autofocus circuit which is an embodiment of the present invention. The image formed by the lens (1) becomes a luminance signal by the image pickup circuit (4) having the image pickup element, and the focus evaluation value generation circuit (focus evaluation value detection means)
It is input in (5). The focus evaluation value generation circuit (5) is the fifth one described above.
The focus evaluation value for one field is output, and the focusing operation or restart is performed in the same procedure as described in the conventional example.

(15) is a near-point signal generation circuit, focus ring (2)
Emits an H level signal when reaches a near point. Specific examples of the near-point signal generating circuit (15) are shown in FIGS. 2 and 3. Second
FIG. 3 is an explanatory diagram of a geometrical relationship between the near point signal generating circuit (15) and the lens barrel, and FIG. 3 is an operation explanatory diagram of the near point signal generating circuit (15). In FIG. 3, the focus ring (2) is shown expanded in a plane for convenience of understanding. The near point detection switch (15a) is attached to the non-movable part (18) of the lens (1), and as the focus ring (2) rotates, the near point stopper (17) provided on the focus ring (2) closes. Move the operating part (15b) of the point detection switch (15a) to change the position from (a) to (b). When the operating part (15b) is in the (a) position, the terminals (15c) and (15d) of the near-point detection switch (15a) are short-circuited, and (b)
It is released in position. H signal voltage V is applied to the terminal (15d)
_{Since DD} is supplied, the output terminal (15e)
When b) is in the (a) position, an L signal is output, and when it is in the (b) position, an H signal is output.

The output of the near-point signal generation circuit (15) is the output of the first comparator (8).
It is input to the OR circuit (16) together with (S2). First comparator (8)
As described above, when the current focus evaluation value is smaller than the maximum focus evaluation value up to that point, the L level signal is generated, and when the current focus evaluation value is large, the H level signal is generated.

As a result, as shown in FIG. 6, when there is a point where the focus evaluation value becomes maximum at a short distance from the near point, when the focus ring (2) reaches the near point while performing mountain climbing servo control, However, since the output (S1) of the first comparator (8) is emitted,
The maximum value memory (6) and the motor position memory (13) are updated, and the focus evaluation value at this near point and the near point itself are stored. On the other hand, the output (S2) is maintained at the L level, but the output of the near-point signal generation circuit (15) is at the H level, so the OR circuit (1
6) The output goes high. Focus motor control circuit (1
0) is the focus motor that receives the output of this OR circuit (16)
(3) is reversed to try to rotate the focus ring (2) to the lens position stored in the motor position memory (13), but this motor position memory (13) stores the near point itself. Therefore, as a result, the focus motor (3) does not reversely rotate, but immediately stops when the near point is reached.

Further, as shown in FIG. 7, the lens position which is the maximum evaluation value is slightly farther than the near point, but the difference between the focus evaluation value and the maximum evaluation value at the near point is the first comparator (8 ) Is smaller than the first threshold (Δx) that is the reference of the output (S2), the focus ring (2) passes the apex and reaches the near point, but the output of the OR circuit (16) becomes H. Since the level is reached, it immediately reverses and returns to the lens position where the maximum evaluation value is obtained. (in this case,
In the conventional example, the lens passes through the apex again and rotates until the focus evaluation value decreases by Δ ×, and then it reverses again and returns to the lens position where the maximum evaluation value is obtained, resulting in significant time loss. (G) Effect of the Invention As described above, according to the present invention, even when the apex is closer to the near point, the focus ring can be stopped at the near point without reversing the focus ring while performing the mountain climbing servo. The image that had been used will not be out of focus and the screen will not shake, and stable operation can be performed.

[Brief description of drawings]

1 to 3 relate to an embodiment of the present invention, FIG. 1 is a circuit block diagram, and FIGS. 2 and 3 are explanatory diagrams of a near-point signal generating circuit. Further, FIGS. 4 and 5 are circuit block diagrams of a conventional example, and FIGS. 6 and 7 are characteristic diagrams of the lens position and the focus evaluation value when the apex is near the near point. (1) …… Lens, (2) …… Focus ring, (3) …… Focus motor, (4) …… Imaging circuit, (5) …… Focus evaluation value generation circuit (evaluation value detection means), (6 ) …… Maximum value memory,
(8) …… First comparator, (10) …… Focus motor control circuit, (13) …… Motor position memory, (15) …… Near point signal generation circuit, (17) …… Near point stopper.

Claims (1)

[Claims] 1. A focus evaluation value detecting means for detecting a high frequency component level of a luminance signal obtained from an image pickup device and outputting it as a focus evaluation value, and a focus motor for moving a focus lens between a far point and a near point. A memory unit for updating and storing the maximum value of the focus evaluation value for each predetermined period with the movement of the focus lens in one direction, and the evaluation value once having the maximum value with the movement of the focus lens. Comparing means for detecting that the focus lens has decreased beyond a predetermined threshold value, near point detecting means for detecting that the focus lens has reached a near point, and responding to each output of the comparing means and the near point detecting means. Focus motor control means for controlling the focus motor, and the focus motor control means, when the output of the comparison means or the near point detection means is obtained, After forcibly temporarily reverse the direction of rotation of Sumota autofocus video camera is characterized in that so as to stop the focus motor at the focus lens position corresponding to the maximum value stored in said memory means.