JPH0736054B2 - Automatic focus adjustment device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an autofocus device used for an image pickup device such as a video camera, and more particularly to an autofocus device for performing focus adjustment using a video signal.

[Prior Art] Conventional automatic focusing devices can be roughly classified into active type and passive type. Among them, there is an ultrasonic type as an active type, but in recent years, infrared light is emitted from the camera side to form a spot image on the subject, and the reflected light is detected by a light receiving element to focus. An infrared type for performing alignment is generally used because it has relatively high distance measuring accuracy. The disadvantage, however, in this infrared ray system requires mechanical interlocking mechanism for moving the light receiving element or the like according to the object distance, therefore inevitably structure becomes complicated, and that the poor work efficiency, etc. Also a need complicated adjustments was there.

On the other hand, with regard to the passive type, there are those that use the image signal of the photographing system and those that do not. Among these, the ones that do not use video signals include the Honeywell method using so-called misalignment detection and blur detection, and Canon SST (Solid State Trial).
However, there is a problem in that they also require optical components that are not required in the imaging system, and require advanced adjustment technology.

On the other hand, the autofocus device using the video signal has an advantage that no optical component which is unnecessary for the photographing system is required.
A conventional automatic focus adjusting device using a video signal will be described with reference to FIG.

As shown in the figure, the image formed by the photographing lens is converted into an electric signal by the photographing element 2, and this signal is amplified by the preamplifier (preamplifier) 3. The signal amplified by the preamplifier 3 passes through the process circuit 4 and is input as a video signal to a monitor (not shown) or a video tape recorder. At the same time, the signal amplified by the preamplifier 3 is passed through a bandpass filter (hereinafter referred to as BPF) 5
Is limited to only high frequency components, and a distance is measured through a gate circuit 6 that extracts only a signal corresponding to a predetermined screen range for focusing when displayed on a monitor or the like, for example, a range of substantially the center of a captured image. After the field of view is limited, it is converted by the detection circuit 7 and the integration circuit 8 into an amount indicating the focus state. Amount indicating this focus state (hereinafter referred to as focus amount)
Has a relationship with the defocus amount (defocus amount) of the taking lens 1 as shown in FIG. 4, and according to this relationship,
The motor drive circuit 9 drives the focusing lens in the taking lens 1 to a position where the amount indicating the focus state is maximized via the motor.

Next, the operation of the motor drive circuit 9 shown in FIG. 3 will be described with reference to FIG. Focus (focus) at the start of distance measurement
It is assumed that the state is at the position A in this figure. Further, the focus position in the state where the focusing lens in the photographing lens 1 is moved by a predetermined pitch by a slight amount is designated as B in this figure. The motor drive circuit 9 compares the focus amount f _(A) at the position A obtained from the integrating circuit 8 with the focus amount f _(B) at the position _B , and then f _(B)
If> f _(A) , rotate the motor in the same direction as when moving from position A to position B, and if f _(B) <f _(A) , reverse the direction of rotation of the motor. The focusing lens in the taking lens 1 is moved in the direction of the focal point x _o by.

[Problems to be Solved by the Invention] However, in the conventional automatic focus adjusting device using the video signal as described above, since the determination of the in-focus point cannot be made until the position of the in-focus point is passed once, the captured image Is unnatural, and when the focus lens is largely out of focus (referred to as a large blur state), there is no difference in the focus amount f _n −f _n−1 with a slight movement of the focusing lens.
There is also a drawback that the large blur state is erroneously determined to be the focused state.

Therefore, the present invention eliminates the above-mentioned drawbacks of the automatic focusing apparatus using a video signal, does not give unnaturalness to a shooting screen, and does not erroneously determine a large blur state as a focused state. An object is to provide a focus adjustment device.

[Means for Solving the Problems] In order to achieve the above object, the present invention provides a focus detection unit that determines the focus state of a subject being photographed, a plurality of positions of the moving photographing lens, and Calculating means for predicting the in-focus position of the photographing lens by substituting the value of the focus signal indicating the focus state at the position into a predetermined function indicating the change of the photographing lens position and the focus signal; and the calculating means. Each time the above operation is performed by
And a control unit that drives the photographing lens by an amount smaller than the amount of deviation to the in-focus position predicted by the calculation unit and operates the calculation unit to predict the next photographing lens drive position. It is characterized by

Furthermore, the present invention is characterized in that, when the deviation amount is larger than a predetermined value, the photographing lens is driven based on the value obtained by correcting the deviation amount.

[Operation] The present invention has the following operations.

While moving the taking lens, the focus position is predicted by substituting a plurality of lens positions of the taking lens and the value of the focus signal at each position into a predetermined function indicating the relationship between the taking lens position and the change of the focus signal. Therefore, the focus adjustment operation can be performed quickly and the focusing accuracy can be improved.

Since the shooting lens is driven by an amount smaller than the amount of deviation to the predicted in-focus position, it is possible to prevent overshooting the in-focus point and drive multiple times to the first predicted in-focus position. Since it is driven by calculation, even if the in-focus position changes during the movement of the shooting lens, it is possible to drive the shooting lens while correcting this, and the influence of the error in the predicted value can be reduced. Focusing accuracy can be improved and smooth automatic focus adjustment operation can be performed.

Embodiment An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows the configuration of an automatic focus adjusting device according to an embodiment of the present invention. In the figure, 10 is a control unit composed of a storage device, a microcomputer having an arithmetic function, etc., and 11 is a motor drive circuit for driving the motor of the taking lens 1. The control unit 10 uses the focus amount (amount indicating the focus state) f _(x) supplied from the integration circuit 8 and the focusing lens position produced based on the motor drive output pulse P of the motor drive circuit 11 to produce the position shown in FIG. According to the control procedure as shown, the focus prediction and the focusing lens movement amount are calculated, and a motor drive control signal is output to the motor drive circuit 11. The motor drive circuit 11 rotationally drives a motor (not shown) in the forward or reverse direction in response to a control signal from the control unit 10, thereby moving the focusing lens of the taking lens 1 in the focusing direction. Other configurations are the same as those of the conventional autofocus device shown in FIG. 3 described above, and thus detailed description thereof will be omitted.

The control unit 10 receives the output (focus amount) f _(x) indicating the focus state of the integrating circuit 8 and at the same time takes in the absolute address x of the focusing lens in the taking lens 1 by the output pulse P of the motor drive circuit 11, The address (hereinafter, referred to as a focusing lens position) x is stored in an internal memory (not shown ₎ together with the quantity f _(x) indicating the focus state of the.

The quantity f _(x) indicating the focus state can be approximated by a Gauss distribution with respect to the focusing lens position x, and can be expressed by the following equation (1) if the in-focus lens position is x _o .

f _(x) = a · exp {−b (xx _o ) ² } (1) where a is the coefficient b of the maximum value of the Gaussian distribution.

Therefore, the quantities indicating the focus states at three points of different focusing lens positions are (x ₁ , f ₁ ), (x ₂ , f ₂ ),
If (x ₃ , f ₃ ), the in-focus lens position x _o can be obtained by the following equation (2).

FIG. 2 shows a flowchart for realizing the algorithms of the above equations (1) and (2). Next, referring to the flow chart of FIG. 2, the control unit 10 of FIG.
The control operation of will be described. It should be noted that S1 to S15 indicate step numbers of the control procedure, and are assumed to be stored in advance in the internal program memory of the control unit 10.

First, the position x ₁ of the focusing lens and the focus amount f _(x1) at the start of the operation of the automatic focusing device are input to the internal memory (S
1). Next, the focusing lens in the taking lens 1 is moved in a predetermined direction by a predetermined pitch amount Smm (S2), and the lens position x ₂ and the focus amount f are set in the same manner as S1.
Input _(x2) to the internal memory (S3). Then f _(x1) and f
The size of _(x2) is compared (S4), and if f _(x1) <f _{(x2), the} focusing lens is further moved by Smm in the same direction (S
5) If f _(x1) > f _(x2) , move the focusing lens by 2Smm in the opposite direction (S6), and input the moving position x ₃ and f _(x3) to the internal memory (S7).

Next, S1, S3, and S7 are added to the above-mentioned focus calculation formula (2).
Three sets of values (x ₁ , f _(x1) ), (x ₂ , f _(x2) ),
Substituting (x ₃ , f _(x3) ), we obtain the expected focal point x _o (S
8). Next, in order to determine the moving position of the focusing lens for obtaining the next focus amount f, the value of the moving amount S is calculated by the following equation (3) (S9).

However, n is an integer of 2 or more, for example, 3 Further, if the absolute value of S is found and this absolute value of S | S | becomes smaller than a predetermined minute dead zone width (S10), the in-focus state is obtained. Then, the motor for moving the focusing lens in the taking lens 1 is stopped (S11).

On the other hand, when it is determined that the absolute value | S | of S is larger than ε (S10), it is further determined whether or not the absolute value | S | of S is larger than the predetermined maximum value S '(S12 ), | S | is greater than S ', the movement amount S is limited to S' (S1
3).

When | S | is not larger than S ', the value of S is directly used as the movement amount S (S14).

Then _replace (x ₂ , f _(x2) ) with the value of (x ₁ , f _(x1) ) and replace the value of (x ₃ , f _(x3) ) with the value of (x ₂ , f _(x2) ). After the replacement, the absolute address of the focusing lens in the taking lens 1 is moved by the moving amount S (S15), and the process returns to S7 again,
The value of f _(x3) is newly measured, and the above-described processing procedure is repeated to perform in-focus prediction.

As described above, in the present embodiment, the movement amount S of the focusing lens is calculated every time based on the in-focus predicted value, and the calculated movement amount S is compared with the predetermined dead zone width ε. Therefore, the focusing lens in the photographing lens 1 can be stopped before passing the in-focus position, and the distance measuring operation can be performed without giving unnaturalness to the photographed screen.

Further, if the maximum value a of the Gaussian distribution is calculated by using the above equation (1) at the same time as the above-mentioned prediction of the in-focus point, the gain (gain) of the amplifier in the detection circuit 7 in FIG. Can always be used in a range where is not saturated, and distance measurement accuracy can be improved.

[Effects of the Invention] As described above, according to the present invention, the following effects can be obtained.

While moving the taking lens, the focus position is predicted by substituting a plurality of lens positions of the taking lens and the value of the focus signal at each position into a predetermined function indicating the relationship between the taking lens position and the change of the focus signal. Therefore, the focus adjustment operation can be performed quickly and the focusing accuracy can be improved.

Since the shooting lens is driven by an amount smaller than the amount of deviation to the predicted in-focus position, it is possible to prevent overshooting the in-focus point and drive multiple times to the first predicted in-focus position. Since it is driven by calculation, even if the in-focus position changes during the movement of the shooting lens, it is possible to drive the shooting lens while correcting this, and the influence of the error in the predicted value can be reduced. Focusing accuracy can be improved and smooth automatic focus adjustment operation can be performed.

[Brief description of drawings]

1 is a block diagram showing a circuit configuration of an embodiment of the present invention, FIG. 2 is a flow chart showing a control operation of the embodiment of FIG. 1, FIG. 3 is a block diagram showing a circuit configuration of a conventional apparatus, FIG. 4 is a characteristic diagram showing the relationship between the position x of the focusing lens and the focus amount f _(x) . 1 …… Shooting lens (built-in focusing lens), 2
...... Imaging element, 3 preamplifier, 5 bandpass filter, 6 gate circuit, 7 detection circuit, 8 integration circuit, 10 control unit, 11 motor drive circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Naoya Kaneda Naoya Kaneda 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Canon Inc. Tamagawa Plant (72) Yoichi Iwasaki 770 Shimonoge, Takatsu-ku, Kawasaki-shi, Kanagawa Canon Inc. Tamagawa Works (56) References Japanese Patent Laid-Open No. 59-64816 (JP, A)

Claims (2)

[Claims] 1. A focus detecting means for determining a focus state of a subject being photographed, a plurality of positions of the moving photographing lens, and a focus signal value indicating a focus state at each position as a photographing lens position. A calculation unit that predicts the in-focus position of the photographing lens by performing a calculation by substituting it into a predetermined function that indicates a change in the focus signal; and, each time the calculation is performed by the calculation unit, it is predicted by the calculation unit. The automatic focus is characterized by comprising: a control unit that drives the photographing lens by an amount smaller than the shift amount to the in-focus position and operates the calculation unit to predict the next photographing lens driving position. Adjustment device. 2. The apparatus according to claim 1, wherein when the deviation amount is larger than a predetermined value, the photographing lens is driven based on a value obtained by correcting the deviation amount. Automatic focus adjustment device.