JP3093057B2 - Automatic focus detection device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an autofocus device such as a video camera or an electronic still camera, which extracts a high frequency component in a luminance signal taken from an image pickup system, and a lens according to the level of the extracted high frequency component. The present invention relates to an automatic focus detection device that detects a focal point by controlling a position.

[0002]

2. Description of the Related Art A hill-climbing servo system is generally used as an autofocus (hereinafter referred to as "AF") device using a video signal from a video camera or the like. This is a method of extracting a high-frequency component in a video signal and detecting a focal point based on the extracted high-frequency component. There are two types of detection circuits that perform signal processing after extracting the high-frequency component: an integral detection type and a peak detection type. The former integrates the signal within the range-measuring frame set for each screen, and focuses on the lens position with the highest contrast.If the subjects with different focal lengths exist simultaneously within the range-measuring frame, erroneous measurement is performed. There is a problem that a distance is easily generated and a sufficient output cannot be obtained when there are few objects in which a contrast can be obtained within a distance measuring frame. The latter detects the peak of the signal in each screen, and focuses on the lens position that detects one point with the highest contrast.If a high-luminance subject exists in the distance measurement frame, the video signal is saturated. There is a problem that correct distance measurement cannot be performed.

[0003]

Therefore, the present applicant has already proposed a system in which the integral detection type and the peak detection type are used in combination, and focusing control is performed using the steepest output of the peaks. (Japanese Patent Application No. 3-185557). According to this, even if the subject has low illuminance and low contrast, and even if a plurality of subjects coexist in the distance measurement frame, it is possible to quickly and accurately detect the in-focus position. In the above proposal, two filters having different cutoff frequencies are used in a part of the configuration, but an autofocus circuit (Japanese Patent Application Laid-Open No. 3-297282) has been proposed in which a plurality of filters are used. This is achieved by detecting, in a video camera autofocus circuit, middle and high frequency components in a luminance signal from an image sensor with a plurality of filters having different characteristics, and differentiating the evaluation values of the filters having the plurality of different characteristics. Switching is performed according to the sensitivity, and the control range is widened to perform highly accurate focus control.

However, the above-mentioned method is used when a peak cannot be obtained at a correct focus position, for example, when distance measurement is performed on a high-luminance object with a point light source or when distance measurement is performed on a moving object. There was a possibility of erroneous ranging. An object of the present invention is to solve the above-described problems. When focus signals of multiple systems having different characteristics are obtained at the same time, and two or more focus signals of these focus signals have the same peak position, the focus position is determined. Focusing reduces the distance measurement error and increases the possibility of focusing even when the point light source is the subject or when the subject is moving, and focuses on a wide range from low-contrast subjects to high-contrast subjects. An object of the present invention is to provide an automatic focus detection device which enables the above. It is another object of the present invention to provide an automatic focus detection device capable of performing the focus control at high speed.

[0005]

In order to achieve the above object, an automatic focus detection device according to the present invention extracts a high frequency component in a luminance signal taken from an image pickup system, and adjusts a lens position according to the level of the high frequency component. In an automatic focus detection device that controls and detects a focal point, a plurality of high-pass filters having different cutoff frequencies for extracting high-frequency components in a luminance signal and a plurality of distance measurement frames of different sizes are formed on a screen. A gate means for generating a window pulse corresponding to each ranging frame by a synchronization signal, an integration detection type detection circuit and a peak detection type detection circuit, and each of the plurality of filter outputs is provided by the integration detection type detection circuit. And a peak detection type detection circuit, and obtains an integrated output and a peak output for each of the window pulses corresponding to the respective ranging frames, thereby obtaining a multi-system. A focus signal detection unit that outputs a focus signal; and a control unit that controls, when the peak positions of two or more focus signals are equal among the multi-system focus signals, the peak positions as focus positions. I have. In order to achieve another object, the control means according to the present invention, in addition to the above-described configuration, performs each peak determination of the multi-system focus signal by driving a lens in a large step, and When it is determined that the peak positions of two or more focus signals of the signals are equal, the driving direction of the lens is reversed, and the lens is driven in fine steps to move to the peak position.

[0006]

According to the above arrangement, a wide range of focus can be achieved from a low contrast to a high contrast subject, and distance measurement errors with respect to a moving object or a point light source are reduced. Further, high-speed focusing control becomes possible.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an automatic focus detection device according to the present invention. The present invention relates to a case where a peak cannot be obtained at a correct focus position, such as a case where distance measurement is performed on a high-luminance object of a point light source or a case where distance measurement is performed on a moving object. This is based on the fact that the peak positions of four focus signals having different characteristics are different from each other through experiments. CCD by lens unit 1
An image of a subject (not shown) is formed on 2. The signal photoelectrically converted by the CCD 2 is supplied to the camera process circuit 3. The camera process circuit 3 creates a video signal by adding a synchronization signal, a setup level, and the like.

The video signal is sent to another circuit such as a recording circuit (not shown), and a part of the luminance signal is sent to the high-pass filters 4 and 5. The cut-off frequency of the high-pass filter 4 is fc ₁ (for example, 200 KHz), and a high-frequency component equal to or higher than the frequency fc ₁ is extracted from the luminance signal. Also,
The cut-off frequency of the high-pass filter 5 is fc ₂ (for example, 1 MHz) higher than that of the filter 4, and high-frequency components higher than the frequency fc ₂ are extracted from the luminance signal. The high-frequency components of the cut-off frequency fc ₁ or higher extracted by the high-pass filter 4 are amplified by the amplifier 6, and the high-frequency components of the cut-off frequency fc ₂ or higher extracted by the high-pass filter 5 are amplified by the amplifier 7 and sent to the AFIC 8. Can be
The gate IC 9 uses the horizontal synchronizing signal HD and the vertical synchronizing signal VD output from the camera process circuit 3 to generate two types of window pulses P for a large ranging frame and a small ranging frame.
_1, and outputs the P _2.

FIG. 2 is a schematic diagram showing an example of the large and small distance measuring frames. C indicates the entire screen, A indicates the range of the large ranging frame, and B indicates the range of the small ranging frame. The window pulses P ₁ (large ranging frame) and P ₂ (small ranging frame) defining the ranging frame range are sent to the AFIC 8. The AFIC 8 has two types of detection circuits, an integral detection type and a peak detection type. The two high frequency component signals having different cutoff frequencies and two window pulses P are provided.
_1, respectively from P ₂ different characteristics four focus signal S
₀ , S ₁ , S ₂ and S ₃
Output to The focus signal S ₀ blocked a small distance measurement frame range frequency fc ₁ or more high frequency components, a signal outputted by detecting a peak detection type detection circuit (for ultra low contrast). The focus signal S ₁ has a cutoff frequency f
c A signal output by integrating _one or more high-frequency components by an integration detection type detection circuit (for low contrast). The cut-off frequency fc ₂ or more high-frequency component focus signal S ₂ is a small distance measurement frame range, a signal which is integrated to output an integration detection type detection circuit (for high contrast). The focus signal S ₃ cut off by a large distance measurement frame range frequency fc ₂ or more high-frequency components, a signal which is integrated to output an integration detection type detection circuit (motion, for toner portion).

The microcomputer 10 performs a predetermined operation based on these four focus signals, sends a lens drive signal to the lens drive circuit 11, and controls the lens position so that the focus lens of the lens unit 1 is brought to the in-focus position. FIG. 3 is a flowchart showing a control sequence of the microcomputer. Microcomputer 1
0 controls the lens drive circuit 11 to drive the focus lens from the closest position to the infinite position direction, and outputs a focus signal S ₀ ,
Start taking in S ₁ , S ₂ and S ₃ . The lens drive at this time is performed in coarse steps (step “hereinafter, S
It is called T. 1). Then, it is determined whether or not the focusing lens has been driven to the infinite position (ST2). If the camera has reached the infinite position, it is determined that focusing is impossible (ST7). If the position has not reached the infinite position, the focus signal is determined to be increased or decreased, and the peak is determined accordingly (ST).
3).

FIG. 4 is a diagram showing the relationship between the focus signal and the lens position for explaining the above-described increase determination, decrease determination, and peak determination. A thick solid line indicates a focus signal level characteristic, and the focusing lens is moved in a rough step, that is, at a high speed as shown by a dotted line from the closest position to an infinite position. In this process, the microcomputer 10 performs the following calculation on each of the focus signals S ₀ , S ₁ , S ₂ and S ₃ . When the minimum value of the focus signal in the focus signal at the arbitrary time t _n to time t _n and Si _n from the start of the lens drive and Si _min, the difference data ΔS
calculating a i _un = Si _n -Si _min. Then, the difference data DerutaSi _n is determined whether greater than a predetermined threshold value V _A, if _{ΔSi un = Si n -Si min>} V A, the increase determination if there is a significant increase in the signal Then, the increase determination flag FUPi = 1 is set. on the other hand,
Assuming that the maximum value of the focus signal until time t _n is Si _max ,
Performs also the calculation of ΔSi _dn = Si _n -Si _max, the difference data DerutaSi _dn is determined whether constant greater than the threshold V _B, if _{ΔSi dn = Si n -Si max <} -V B, the It is determined that there is a significant decrease in the signal, and the decrease is determined, and a decrease determination flag FDNi = 1 is set.

Then, if there is a signal that is determined to decrease after the increase is determined, it is determined that there is a significant peak, and peak determination is performed, and FUPi ∩FDNi ′ = 1. Note that i
= 0, 1, 2, 3, where _u represents an increase and _d represents a decrease. 'Indicates a change. In this way, peak determination is performed for each of the focus signals S ₀ , S ₁ , S ₂ and S ₃ . Next, peak determination is performed for two or more focus signals, and it is determined whether or not the peak positions match (ST4). If no peak determination has been performed, only one focus signal has been subjected to peak determination, or if two or more focus signals have been subjected to peak determination and their positions do not match, step 2 is performed. Return. When the peak positions of two or more focus signals match, the microcomputer 10 reverses the lens driving direction to move the focusing lens in fine steps (low speed).
The lens drive signal to be moved by the constant pulse is output, and the peak position is searched (ST5). Then, when it exceeds the peak position, it reverses again and stops at the peak position (ST6). As described above, the focusing lens can be quickly moved to the focal point without erroneous distance measurement.

[0013]

As described above, according to the present invention, a plurality of high frequency signals extracted by high-pass filters having different cutoff frequencies are defined by defining a plurality of distance measurement frame ranges having different sizes from each other, and integrating and peak detection. The multi-system focus signals having different characteristics are generated by the shape detection circuit, and focusing is performed only when the peak positions of two or more focus signals among these focus signals coincide. Therefore, erroneous distance measurement is reduced for a high-luminance subject or a moving subject of a point light source, and the focusing probability can be significantly improved. In addition, by using multi-system focus signals having different characteristics, a low-contrast subject to a high-contrast subject can be focused, and the dynamic range is expanded. Since the focus signal of the large ranging frame is included in the focus signals of the multiple systems, it is unlikely that a hollow spot occurs. Further, the focusing lens is driven at a rough step, that is, at a high speed, until the peak is determined, and thereafter, the focusing lens is driven at a fine step, so that high-speed and high-precision focusing control can be performed.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram showing an embodiment of an automatic focus detection device according to the present invention.

FIG. 2 is a diagram illustrating an example of a large and small distance measuring frame in FIG. 1;

FIG. 3 is a flowchart showing a control sequence of the microcomputer.

FIG. 4 is a diagram illustrating a relationship between a focus signal and a position of a focusing lens, and illustrating a locus of a moving focusing lens during focusing control.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 lens unit 2 CCD 3 camera process circuit 4, 5 high-pass filter 6, 7 amplifier 8 AFIC (focus signal detecting means) 9 gate IC (gate means) 10 microcomputer 11 lens drive circuit S ₀ , S ₁ , S ₂ , S ₃ Focus signal A Large ranging frame B Small ranging frame

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/232 G02B 7/28 G02B 7/36

Claims (2)

(57) [Claims] An automatic focus detection device for extracting a high-frequency component in a luminance signal taken from an image pickup system, controlling a lens position according to the level of the high-frequency component, and detecting a focal point, comprising: a high-frequency component in the luminance signal; In order to form a plurality of high-pass filters with different cutoff frequencies and a plurality of ranging frames of different sizes on the screen,
A gate means for generating a window pulse corresponding to each ranging frame by a synchronization signal, an integration detection type detection circuit and a peak detection type detection circuit,
Each of the plurality of high-pass filter outputs is input to the integral detection type detection circuit and the peak detection type detection circuit, and an integrated output and a peak output are obtained for each of the window pulses corresponding to the respective distance measurement frames, thereby obtaining a multi-system focus signal. And a control means for controlling, when the peak positions of two or more focus signals are equal among the multi-system focus signals, the peak position as a focus position. Automatic focus detection device. 2. The multi-system focus signal peak determination of the multi-system focus signal is performed by driving a lens in a large step. 2. The automatic focus detection device according to claim 1, wherein when it is determined that the distances are equal to each other, the driving direction of the lens is reversed and the lens is driven in fine steps to move to the peak position.