JP2708904B2 - Auto focus camera - Google Patents

Description

The present invention relates to an auto-focus camera such as an electronic still camera having an auto-focus function.

(B) Conventional technology In a camera autofocus apparatus, a method of using a high-frequency component of a video signal itself from an image sensor for evaluation of focus control essentially has no parallax and has a large depth of field. There are many excellent points, such as being able to focus accurately on a shallow or distant subject. 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. 4 and 5. FIG. FIG. 4 is an overall circuit block diagram of the prior art. In this figure, an image formed by a focus 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). Focus evaluation value generation circuit (5)
Is configured as shown in FIG. A vertical sync signal (VD) separated from the video signal by a sync separation circuit (5a),
The horizontal synchronization signal (HD) is input to the gate control circuit (5b) to set a sampling area as a focus area. The gate control circuit (5b) sets a rectangular sampling area in the center of the screen based on the vertical synchronizing signal (VD), the horizontal synchronizing signal (HD), and the output of a fixed oscillator that drives the image sensor. The gate circuit (5c) supplies a gate opening / closing signal that allows the passage of the luminance signal only in the range.

Only the luminance signal corresponding to the area within the focus area by the gate circuit (5c) is passed through the high-pass
F) After passing through (5), only high-frequency components are separated, and amplitude detection is performed by a detection circuit (5e) at the next stage. 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 comprising a latch circuit which latches the adder 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. Next, the operation of FIG. 4 will be described with reference to the flowchart of FIG. Immediately after the autofocus operation is started, the first focus evaluation value is kept the maximum value memory (6) and an initial value memory (7) (S _1).
Then, focus motor control circuit (10) is a lens (1) is rotated in a predetermined direction the focus motor for advancing and retracting in the optical axis direction (focus control unit) (3) (S ₂₎ 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 that is larger is made, than the preset fluctuation width, retain their direction of rotation (S _5), the current evaluation value is compared to the initial evaluation value, than the variation width If the output of a small has been made in the direction of rotation of the focus motor (3) in the opposite (S _4), monitors the output of the first comparator (8) (S _3).

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 types of comparison signals (P) that are larger than the contents (first mode) and decreased to a predetermined first threshold or more (second mode)
1) (P2) is output. Here, the maximum value memory (6) is
Based on the output of the first comparator (8), if the current focus evaluation value is greater than the content of maximum value memory (6) is that value is updated (S _7), always focus evaluation value to date Is held (S ₆ ).

(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. The position memory is the same as the maximum value memory (6). Based on the output of the one comparator (8), the focus ring position when the maximum evaluation value is reached is updated so as to be always maintained. Here, it is a well-known technique that the focus ring (2) is rotated by the focus motor (3), and the lens (1) moves in the optical axis direction in accordance with 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. As the 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. 7)
At the same time focus motor (3) is reversed (S _8).
After this reversal, the contents of the motor position memory (13), and the current focus ring position signal are compared in the third comparator (14) (S _9), as for rotating the focusing motor (3) until it matches The focus motor is controlled (S ₁₀ ), and the focus motor (3) is stopped (S ₁₁ ) when they match, that is, when the focus ring (2) returns to the position (P) where the focus evaluation value becomes the maximum. The control circuit (10) functions. At the same time, the focus motor control circuit (10) outputs a lens stop signal (LS).

(C) Problems to be Solved by the Invention In the autofocus operation in the above-described conventional technique, a very small amount (every one field) of the focus ring is constantly moved from the closest distance to the infinity of the focusable range of the lens. The focusing operation is performed, and it takes 2 to move the focus lens from the close range to infinity.
It takes seconds, but there is no problem with a video camera whose main purpose is to shoot moving images.

However, in general, an electronic still camera captures an image of a subject during a moment as a still image. In the above-described conventional method, which takes two seconds from the time the shutter button is pressed to the time the autofocus operation is completed, the photographer takes a release time lag. Is not only very difficult to use, but also there is a risk of missing a photo opportunity, which is not practical.

Therefore, the present invention is to provide an autofocus camera which can perform an extremely short time (for example, 0.5 seconds) until a photographing operation including an autofocusing operation is completed when a shutter button is pressed. It is.

(D) Means for Solving the Problems The present invention provides an autofocus operation in which an autofocus operation is performed by detecting a high-frequency component level of an image pickup video signal obtained from an image pickup device as a focus evaluation value every predetermined period. In the camera, in order to solve the above problem, the focus lens is moved in relatively coarse steps from infinity to the closest point of the object distance, and first search means for obtaining a focus evaluation value for each step, After moving the focus lens close to the subject distance corresponding to the first maximum focus evaluation value obtained by the first search means, the focus lens is further moved in small steps near the subject distance. A second search means for obtaining a second maximum focus evaluation value from a focus evaluation value for each minute step It was.

(E) Function According to the above configuration, the autofocusing operation is completed in a very short time.

(F) Embodiment Hereinafter, an autofocus camera according to the present invention will be described with reference to FIGS.

FIG. 1 is a block diagram of a main part of an electronic still camera having an auto-focus function, and the same parts as those in FIG. 4 are denoted by the same reference numerals and description thereof is omitted. The conventional operation of the autofocus lens (1) from the closest point to the point at infinity of the focusable range of the lens requires two seconds. Then, since a video signal of 120 fields can be obtained in 2 seconds, a focus evaluation value can be obtained in 120 steps. Therefore, the focusing accuracy of the focus lens (1) is infinite from the shortest distance of the focusable range of the lens. It can be considered 1/120 of the distance.

Hereinafter, the operation of FIG. 1 will be described with reference to the operation flowchart of FIG. 2 and the characteristic diagram of FIG.

First, the camera when (Ca) is the auto focus mode, moves before the start of the autofocus operation focusing ring (2) the point at infinity (S _1). When the auto focus operation is started, the first focus evaluation value obtained immediately after the start is held in the maximum value memory (100) (S ₂ ),
Then the focus motor control circuit (104) drives the focus motor (3), 10 times the speed of the conventional operation toward the focus ring (2) in the direction of the short distance, i.e. rotated at 0.2 seconds (S ₃₎ . As a result, a video signal of 12 fields can be obtained in 0.2 seconds, so that a focus evaluation value for 12 steps can be obtained. With the rotation of the focus ring (2), the first comparator (101) compares the current maximum focus evaluation value with the current maximum focus evaluation value held in the maximum value memory (100) ( S _4), the current focus evaluation value is a signal is greater than the content of maximum value memory (100). Where the maximum value memory (100) is
If the current evaluation value is larger than the content of the maximum value memory (100) based on the output of the first comparator (101), the value is updated, and the maximum value of the focus evaluation value up to the present is always held. It is (S _5).

A position memory 102 receives a focus ring position signal [or a motor position signal] indicating a position to the focus ring (2) [or a focus motor (3)] and stores the focus ring position [or a motor position]. This position memory (102) is the first position memory (102).
a), second position memory (102b), third position memory (102b)
c), and the first position memory (102a), like the maximum value memory (100), stores focus ring position data [Focus ring position data when the focus evaluation value becomes the maximum value based on the output of the first comparator (101). Or the motor position data] is constantly updated. The second position memory (102b) always holds focus ring position data (point in FIG. 3) one step before the focus ring position data [or motor position data] when the focus evaluation value reaches the maximum value. The data is updated as described above, and the third position memory (102c) stores the focus ring position data [or the motor position data] one step after the focus ring position data [or the motor position data] when the focus evaluation value reaches the maximum value (see FIG. 3). ) Is updated so that the data is always held.

By performing from infinity close distance of the focusing range of the lens over the search operation (S _6), the pseudo-maximum focus evaluation value (first maximum focus evaluation value) evaluation value in FIG. 3 point] finds As a result, the true maximum value of the focus evaluation value (the second maximum focus evaluation value) [the evaluation value at point f ₀ in FIG. 3] is 0.2 seconds after the shutter button is pressed. That is, it is possible to know the distance, that is, about how far the subject exists.

Then the focus motor control circuit (104) is a focus motor (3) is reversely rotated at high speed in accordance with the contents of the data stored in the third position memory (102c) (S _7), thereby the focus ring (2) The position one step before the first maximum focus evaluation value obtained by the search (FIG. 3c)
(S ₈ ) (S ₉ )
First focus evaluating value at that time (focus evaluation value in FIG. 3 c point) is held in the maximum value memory (100) (S _10).
Thereafter, in order to search for the second maximum focus evaluation value, the operation shifts to an operation of rotating the focus ring at a low speed [an accuracy of 1/120 (every field) as in the related art], which will be described below. Firstly a low speed focus motor (3) to infinity direction (S ₁₁₎ [state of the FIG. 3, for monitoring the output of the first comparator (101).

The first comparator (101) compares the current maximum focus evaluation value held in the maximum value memory (100) with the current focus evaluation value, and the current focus evaluation value is stored in the maximum value memory (100). Two comparison signals are output (S ₁₂ ), which are larger than the contents (first mode), and are reduced (second mode) by a preset first threshold value Δy (see FIG. 3) or more. Here, the maximum value memory (100) is updated based on the output of the first comparator (101) when the current evaluation value is larger than the content of the maximum value memory (100). the maximum value of the focus evaluation values up is held (S _13).

The first position memory (102a) always holds focus ring position data when the focus evaluation value reaches the maximum value based on the output of the first comparator (101), similarly to the maximum value memory (100). It is updated to (S _13).

The focus motor control circuit (104) includes a first comparator (10
The output of 1) is monitored, and when the second mode in which the current focus evaluation value is smaller than the maximum focus evaluation value [the state in FIG. 3] is instructed by the output of the first comparator (101), the focus motor ( 3) to reverse the (S ₁₄₎ [state of the Figure 3. After this reversal, the contents of the first position memory (102a), and the current motor position is compared by the second comparator (103) (S _15),
Until it coincides controlled to allowed to slow rotation of the focus motor (3) (S _16), the focus ring (2) stops the focus motor (3) as returned to the local maximum point of the focus evaluation values when a match (S ₁₆ ) As described above, the focus motor control circuit (104) functions. At the same time, the focus motor control circuit (104) outputs a lens stop signal (LS) [state in FIG. 3].

With the above operation, it is possible to perform a high-accuracy and high-speed autofocus operation.

In the above-described embodiment, the focus ring is temporarily moved to the point at infinity and then moved to the closest point before the start of the autofocus operation, but the focus ring is moved to the closest point once. You may make it move to an infinite point.

Further, it is not always necessary to move the focus ring to the point at infinity or the closest point before the start of the autofocus operation.
The focus ring may be moved so as to search for the maximum focus evaluation value, and the initial position of the focus ring is not limited.

Further, the maximum value memory (100), the first comparator (101), the position memory (102), the second comparator (10
3) Needless to say, the focus motor control circuit (104) can be constituted by a one-chip microcomputer.

(G) Effects of the Invention As described above, the autofocus camera of the present invention can obtain a highly accurate focusing state in a short time, and is particularly suitable when used as an electronic still camera.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main part of an embodiment of an autofocus camera according to the present invention, FIG. 2 is a view showing a flowchart for explaining the autofocus operation, and FIG. FIG. 4 is a block diagram showing the main parts of a conventional autofocus camera, FIG. 5 is a block diagram showing details of the main parts, and FIG. 6 is a diagram for explaining the conventional autofocus operation. FIG. 7 shows a flowchart, and FIG. 7 is a characteristic diagram for explaining a conventional focusing operation. (1) Focus lens, (2) Focus ring, (3) Focus motor, (4) Imaging circuit, (5) Focus evaluation value generation, (100) Maximum value memory , (101)... First comparator, (102)... Position memory, (103)... Second comparator, (104)... Focus motor control circuit.

Claims (1)

(57) [Claims] 1. An auto-focus camera in which an auto-focus operation is performed by detecting a high-frequency component level of an imaged video signal obtained from an image sensor as a focus evaluation value every predetermined period. A first search means for moving in a relatively coarse step from infinity to the closest point to obtain a focus evaluation value for each step; and a first maximum focus evaluation value obtained by the first search means. After moving the focus lens to the vicinity of the corresponding subject distance, the focus lens is further moved in fine steps in the vicinity of the subject distance, and the focus evaluation value for each of the fine steps is calculated as a second maximum focus evaluation value. An autofocus camera comprising: a second search unit that obtains a value.