JPH0829667A - Automatic focusing method - Google Patents

Abstract

PURPOSE:To provide an automatic focusing method shortening a focusing time and smoothly performing the operation of the focusing lens by predicting a position on which focusing evaluation value becomes maximum while the focusing lens is moved in the direction in which the evaluation value increases in an automatic focusing method performing focusing by moving the focusing lens to the position on which focusing evaluation value becomes maximum. CONSTITUTION:A focusing lens is moved in the direction in which a focusing evaluation value is increased (S10) and the position of the focusing lens and the focusing evaluation value on the relevant position are detected. While the focusing evaluation value is increased, the position of the focusing lens on which the focusing evaluation value becomes maximum is predicted based on the detected data (S14) and the focusing lens is moved to stopped on the predicted position (S18). When the detection of the focusing evaluation value is continuously performed on this position and the focusing evaluation value is changed, the focusing adjustment is again executed (S20).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus method, and more particularly to an autofocus method for adjusting the position of a focus lens so that a focus evaluation value is maximized.

[0002]

2. Description of the Related Art Conventionally, in an autofocus method in a television camera or the like, a focus evaluation value indicating the sharpness of a subject image is detected based on a high frequency component in a video signal,
It is well known that the focus lens of the photographing optical system is moved to the position where the focus evaluation value is maximum to focus.

FIG. 4 is an explanatory view showing a path along which the focus lens moves until the position of the focus lens at which the focus evaluation value is maximized is detected in the conventional autofocus method. The focus evaluation value with respect to the position of the focus lens is assumed to have a shape like the curve shown in FIG. If the focus lens is initially located at the point in the figure, the focus lens moves to the right in the figure as the focus evaluation value increases. At this time, the focus evaluation value is detected at the same time. Then, when the focus lens reaches the position in the figure, it is detected that the focus evaluation value starts to decrease, and it is recognized that the position in the figure becomes the maximum point of the focus evaluation value. When the position of the maximum point (the same figure) is recognized, the focus lens moves to this position and stops.

[0004]

However, in the conventional autofocus method, in order to detect the maximum point of the focus evaluation value, the maximum focus evaluation value is detected and the focus evaluation value is detected. That is, while moving the focus lens in the direction in which the focus evaluation value increases, the detection is performed up to the position where the focus evaluation value starts to decrease. Therefore, the focus lens has to perform an operation of reversing the moving direction and returning to the in-focus point after passing through the in-focus point once, which has a drawback that it takes time to focus and the operation is not smooth. It was

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an autofocus method capable of shortening the time until focusing in the autofocus method and smoothly operating the focus lens. To aim.

[0006]

In order to achieve the above object, the present invention provides a focus evaluation value indicating the sharpness of a subject image based on a video signal obtained through a photographing optical system including a focus lens and an image sensor. In the autofocus method of adjusting the position of the focus lens so that the focus evaluation value is maximized, the focus lens is moved in a direction in which the focus evaluation value increases, and the focus evaluation value increases during the period. To obtain focus evaluation values at at least three focus lens positions, and predict the focus lens position at which the focus evaluation value is maximum based on the focus lens positions and the focus evaluation values corresponding to them.
It is characterized in that the focus lens is moved to the predicted focus lens position.

[0007]

According to the present invention, at the time of focus adjustment by the autofocus method, the focus lens for adjusting the focal length of the photographing optical system is moved in the direction of increasing the focus evaluation value, and the position of the focus lens and its position The focus evaluation value is detected. Then, while the focus evaluation value is increasing, the position of the focus lens having the maximum focus evaluation value is predicted by using the detected data, and the focus lens is moved to the predicted position and stopped. As a result, the focus lens stops at the in-focus point without passing through the in-focus point.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the autofocus method according to the present invention will be described in detail below with reference to the accompanying drawings. FIG. 1 is a block diagram showing an internal configuration of an autofocus device in a television camera to which the autofocus method according to the present invention is applied.

In FIG. 1, a subject image is formed on a light receiving surface of a solid-state image sensor (CCD) 20 by a photographing optical system 10 including a focus lens 12, a variable power lens 14, a master lens 16 and a diaphragm 18. Then, after being converted into an electric signal here, it is output to the image pickup circuit 22. The image pickup circuit 22 forms a video signal from the signal output from the CCD 20, and outputs this video signal to the external video display device 50 such as the viewfinder 48 or monitor TV. As a result, the viewfinder 48 and the external image display device 50
Displays the video being shot.

Next, the case of the autofocus mode for automatically adjusting the focus will be described. In addition, mode switching between a manual focus mode in which the focus lens 12 and the like are manually operated to move the focus lens 12 to perform focus adjustment and a remote focus mode in which focus adjustment is performed by remote operation are performed by a mode selector switch (not shown). It is supposed to be.

In the autofocus mode, the image pickup circuit 22 filters the luminance signal into a high pass filter (HP).
F) 24, and outputs a sync signal to the focus area selection gate 28, adder 30, and
Output to the CPU 32. The HPF 24 extracts a high frequency component included in the brightness signal. The higher the sharpness of the image, the more the extracted high-frequency components are included. Therefore, by integrating the high-frequency components, it is possible to quantify the average sharpness of the image in the integration range.

The high frequency component passing through the HPF 24 is A /
It is converted into a digital signal by the D converter 26 and input to the focus area selection gate 28. The focus area selection gate 28 is a circuit that extracts only a signal corresponding to the focus area at the center of the imaging screen.
Subject (main subject) imaged in this focus area
Extract only information about.

The digital signal extracted by the focus area selection gate 28 is output to the adder 30, and the digital signals for one field are integrated. This integrated value is used as a focus evaluation value indicating the sharpness of the image by the CPU.
Is output to 32. The CPU 32 is the focus lens 12
The position of the focus lens 12 and the focus evaluation value output from the adder 30 are recorded in a storage unit (not shown) inside the CPU 32 while moving the unit by a unit movement amount in the direction of increasing the focus evaluation value. Then, based on the stored data, the position of the focus lens 12 having the maximum focus evaluation value is predicted, and the focus lens 12 is moved to this position. The details will be described later.

The position of the focus lens 12 is detected by the potentiometer 40 and output to the CPU 32 via the A / D converter 38. Also, the focus lens 1
The movement of 2 is performed based on a position command signal indicating the focus lens position output from the CPU 32. That is,
The position command signal output from the CPU 32 is converted into an analog signal by the D / A converter 33 and applied to the motor drive circuit 34. The other input of the motor drive circuit 34 is from the potentiometer 40 to the focus lens 1
A signal indicating the position 2 is added, the motor drive circuit 34 takes the difference between these two input signals, and outputs a drive signal obtained by amplifying this difference signal to the focus motor 36.
When the focus motor 36 is driven, the focus lens 12 moves in the optical axis direction via a drive mechanism (not shown).

Next, the processing procedure of the autofocus method will be described with reference to the flowchart of FIG. First, the CPU
Reference numeral 32 moves the focus lens 12 along the optical axis in one direction (either forward or backward) by the unit movement amount, and detects an increase or decrease in the focus evaluation value. At this time, if the focus evaluation value increases, the focus lens 12 is moved in the same direction as it is,
If the focus evaluation value decreases, move in the opposite direction. That is, the direction in which the focus evaluation value increases is detected, and the focus lens 12 is moved in that direction (S10).

The focus lens 12 is moved by a unit movement amount, and the adder 3 is added to the storage unit inside the CPU 32.
The focus evaluation value output from 0 and the position of the focus lens 12 are stored for each vertical blanking period. At the same time, the increment of the current focus evaluation value with respect to the focus evaluation value obtained in the previous vertical blanking period is calculated, and this is divided by the unit movement amount to obtain the increase rate. Then, it is determined whether or not the increase rate of the focus evaluation value is lower than the previously calculated increase rate (that is, whether or not the increase rate exceeds the maximum) (S1).
2). Here, the calculation of the increase rate of the focus evaluation value is performed when the focus lens 12 has a value of 2 on the focus evaluation value curve.
This is to determine whether or not the position is near the maximum point of the focus evaluation value for which the following equation approximation holds.

In step S12, after the rate of increase in the focus evaluation value begins to decrease, the focus lens 12 is moved by the unit movement amount and every vertical blanking period.
The position x of the focus lens 12 where the focus evaluation value is maximum
Predict _p (S14). The position x _p of the focus lens 12 where the focus evaluation value becomes maximum is calculated from this approximation formula by approximating the vicinity of the maximum point of the focus evaluation value curve with a quadratic curve. The quadratic approximation of the focus evaluation value curve is obtained from the data of the focus lens position at three points and the focus evaluation value using the Lagrange's interpolation method. The three points of data to be used are selected in order from the newest one among the data recorded in the storage section.

Now, the Lagrange's interpolation method will be described. FIG. 3 shows the position of the focus lens 12 on the x-axis, y
The focus evaluation value is shown on the axis. Focus lens 1 now
2 moves at intervals of the unit movement amount S, and focus evaluation values at three points a, b, and c in the figure are detected (the current position of the focus lens 12 is point c). From the data of these three points, the position x _P of the focus lens 12 where the focus evaluation value becomes maximum is obtained.

Assuming that the x coordinate of point a is 0, the coordinates of each point are: a (0, y _A ) ... (1) b (S, y _B ) ... (2) c (2S, y _C ) ... (3 ).

A maximum point P of a quadratic curve passing through these three points
The x coordinate x _P of (x _P , y _P ) is obtained by using the Lagrange's interpolation polynomial. a (x ₀ , y ₀ ), b (x ₁ ,
The formula P (x) of the focus amount curve passing through the three points of y ₁ ), c (x ₂ , y ₂ ) is expressed as follows. Substituting (1), (2), and (3) into the equation (5), L ₀ (x) = {(x−x ₁ ) (x−x ₂ )} / {(x ₀ −x _{1 ) (x 0 -x 2)}} = {(x-s) (x-2s)} / {(0-s) (0-2s)} = (x 2 -3sx + 2s 2) / (2s 2) ... ( _{6) L 1 (x) =} {(x-x 0) (x-x 2)} / {(x 1 -x 0) (x 1 -x 2)} = {(x-0) (x-2s )} / {(s-0 ) (s-2s)} = (x 2 -2sx) / (- s 2) ... (7) L 2 (x) = {(x-x 0) (x-x 1 _{)} / {(x 2 -x} 0) (x 2 -x 1)} = {(x-0) (x-s)} / {(2s-0) (2s-s)} = (x 2 - sx) / (2s ² ) ... (8) Substituting equations (6), (7), and (8) into equation (4), P (x) = y _A · (x ² -3sx + 2s ² ) / (2s ^{_{2) -y B · (x 2}} -2sx) / (s 2) + y C · (x 2 -sx) / (2s 2) ^{_{= (Y A / 2s 2 -y}} B / s 2 + y C / 2s 2) x 2 - maximum time _{(3y A / 2s-2y B} / s + y C / 2s) x + y A differential equation of P (x) is 0 and takes a value of dP (x) / dx = 2 (y a / 2s 2 -y B / s 2 + y C / 2s 2) x p - (3y a / 2s-2y B / s + y C / 2s) = 0 Therefore, the distance x _p from the point a to the focus position P is given by the equation (9).

Maximum point x obtained as described above_pIs a focus review
The value curve is not actually a quadratic curve, and the data
Considering that there is an error in
There is an error from the maximum of the value curve. Therefore,
The focus lens 12 is moved by a unit movement amount to
Focus evaluation value at point d
Read, and this time using the data of points b, c and d
Maximum point x by Lagrange's interpolation method_pAsk for. This
The position of the focus lens 12 as
Going close to the maximum point of the evaluation value curve, the maximum point x
_pWill be calculated. At this time, x_pIs gradually the actual focus
Since it is thought that it will converge to the maximum point of the evaluation value curve,
x_pWhen the change amount of becomes smaller than a certain threshold, x_p
Is converged, this x_pThe position of is regarded as the focal point (S
16). This threshold is based on aperture value and zoom (magnification)
Set the depth of focus as a guide. (Aperture value
And zoom (magnification) are different from those of the diaphragm 18 in FIG.
By a detection device (not shown) provided on the double lens 14.
It is output to the CPU 32. ) However, for some reason x_pDoes not converge
In some cases, x _pOf the focus evaluation value before
Near 0, which is determined by aperture value and zoom (magnification)
When it becomes a value within a predetermined range beside, this position is regarded as the in-focus point.
None The focus lens may be stopped.

When the focal point x _p is determined in step S16 as described above, the focus lens 12 is moved to this focal point and stopped (S18). As a result, the focus adjustment ends, and the focus lens 12 remains stationary unless the subject or the television camera moves. However, when the subject or the TV camera itself moves and the distance between the subject and the TV camera changes, it is necessary to newly adjust the focus. In order to restart the focus adjustment, it is determined in step S20 whether or not there is a change in the focus evaluation value, and if there is a change, the above step S1.
The focus adjustment from 0 to S18 is performed again.

In the Lagrange's interpolation method, the focus evaluation value curve is approximated by a quadratic curve using three-point data, but if the number of observation data used in the interpolation method is further increased,
It is also possible to approximate the focus evaluation value curve with a higher-order polynomial of degree higher than or equal to the degree. If the calculated x _p is far from the current position, the focus lens is moved at high speed immediately before x _p , and then the focus lens position and the focus evaluation value are detected at low speed with high accuracy to achieve high speed and high speed. Precision focus adjustment can be realized.

Further, in the above embodiment, the autofocus method according to the present invention is used for the television camera, but the invention is not limited to the television camera and can be applied to all optical devices which require focus adjustment.

[0025]

As described above, according to the autofocus method of the present invention, the focus lens for adjusting the focal length of the photographing optical system is moved in the direction in which the focus evaluation value increases and the focus evaluation value increases. While predicting the position where the focus evaluation value is maximum while stopping, and stopping the focus lens without passing to this position, the focus lens moves only in one direction, and the focus lens moves smoothly in a short time. The lens can be guided to the in-focus position.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a television camera to which an autofocus method according to the present invention is applied.

FIG. 2 is a flowchart showing an embodiment of a processing procedure of an autofocus method according to the present invention.

FIG. 3 is a diagram showing the relationship between the focus evaluation value and the position of a focus lens that moves with a unit movement amount s.

FIG. 4 is an explanatory diagram showing a movement locus of a focus lens in a conventional autofocus method.

[Explanation of symbols]

 10 ... Shooting optical system 12 ... Focus lens 14 ... Variable magnification lens 16 ... Master lens 18 ... Aperture 20 ... CCD 22 ... Imaging circuit 24 ... HPF 26 ... A / D converter 28 ... Focus area selection gate 30 ... Adder 32 ... CPU 34 ... Motor drive circuit 36 ... Focus motor

Claims (4)

[Claims] 1. A focus evaluation value indicating the sharpness of a subject image is obtained based on a video signal obtained via a photographing optical system including a focus lens and an image sensor, and the focus evaluation value is maximized so as to maximize the focus evaluation value. In an autofocus method for adjusting the position of a lens, the focus lens is moved in a direction in which the focus evaluation value increases, and focus evaluation values at at least three focus lens positions are obtained during a period in which the focus evaluation value increases. The focus lens position is predicted based on the focus lens position and the focus evaluation value corresponding thereto, and the focus lens position is maximized, and the focus lens is moved to the predicted focus lens position. Autofocus method to do. 2. The focus lens position prediction is repeated a plurality of times, and the focus lens position to which the focus lens should move is determined when the variation in the predicted focus lens position falls within a predetermined range. The autofocus method according to claim 1. 3. The movement of the focus lens is stopped when the increase amount of the focus evaluation value is within a predetermined range near 0 before the predicted variation of the focus lens position is within the predetermined range. The autofocus method according to claim 2, wherein: 4. When the predicted focus lens position is farther than the current position, the focus lens is moved at a high speed until just before the focus lens position, and then the focus lens is moved at a low speed to predict the focus lens position. The autofocus method according to claim 1, wherein the autofocus method is performed.