JP3960647B2 - Automatic focusing device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an automatic focusing device applicable to various optical devices.
[0002]
[Prior art]
Conventionally, a subject image is projected onto an image sensor by an imaging lens, a high-frequency component (contrast value) of the subject image indicating the degree of focus is obtained from a video signal output from the image sensor, and a position where the contrast value is maximized is obtained. There is a focusing method in which an imaging lens is driven at the position determined as a focusing position. This focusing method is called a “mountain climbing method” and is described in, for example, NHK technical research report (1965, Vol. 17, No. 1, No. 86, pages 21-37).
[0003]
The focusing operation based on the “mountain climbing method” will be described with reference to FIGS. 6 and 7.
FIG. 6 shows the relationship between each imaging lens position and the contrast value of the subject image when photographing the subject. In the figure, stop positions of the imaging lens that moves stepwise are indicated by lens positions L _{0 to} L ₆ , and contrast values obtained at the respective lens positions are indicated by C _{0 to} C ₆ , respectively. Lens position L ₃ is a focus position.
[0004]
As shown in FIG. 6, the contrast value increases from the infinite position to the in-focus position, and after the maximum value is shown at the in-focus position, it shows a mountain-shaped characteristic that decreases toward the closest position. The in-focus position is detected using such contrast characteristics. The infinite position of the imaging lens is ideally the lens position where the contrast value is 0, but in reality, the moving distance of the imaging lens is a position that can move to the infinity side due to mechanical restrictions. Since the position is limited, the position where the imaging lens is closest to the imaging element is treated as an infinite position.
[0005]
FIG. 7 shows a flowchart of focusing control based on the “mountain climbing method”. After the lens position is initially moved to L ₀ , the lens is moved in the closest direction (forward rotation direction). First, the contrast value C _n (C ₀ in FIG. 6) is input in step S1, the motor is driven forward so as to move the imaging lens by one step in step S2, and then the contrast value C _{n + 1 in} step S3. (C ₁ in FIG. 6) is input. In step S4, C _n and C _{n + 1} are compared, and if C _{n + 1} is larger than C _{n, the} focus position is approached. Therefore, n is incremented in step S5, and the image pickup lens is further brought closer to the camera. The motor is driven to rotate forward so that one step is closer to the side. If it is detected by the size comparison in step S4 that C _n is larger than C _{n + 1} , then the imaging lens passes through the in-focus position and exists at the lens position L ₄ . Therefore the motor reverse rotation to at returning only one step an imaging lens to the lens position L ₀ direction at step S6, the focusing operation is completed by driving stop.
[0006]
As described above, in the “mountain climbing method”, when the imaging lens reaches the lens position L ₄ , it is recognized that the imaging lens has passed the maximum contrast position L ₃ , and the imaging lens is reversely driven to the maximum contrast position to complete the focusing operation. To do.
[0007]
Japanese Patent Laid-Open No. 1-22981 discloses an automatic focusing device based on the “mountain climbing method” as described above and using the chromatic aberration of the imaging lens to increase the focusing speed. This automatic focusing device extracts high-frequency components of each color signal obtained from a color image sensor, compares the integration level of each high-frequency component at a constant period, determines the direction of defocus, and images based on the determination result The lens is driven.
[0008]
The in-focus position is slightly different for each wavelength of the subject light based on the optical chromatic aberration of the imaging lens or the like. If an imaging lens having a predetermined chromatic aberration is used as shown in FIG. 8, the light from the point light source P is imaged at points R, G, and B by red light, green light, and blue light, respectively.
[0009]
Also, assuming that the lens system is designed with green light as a reference, red, green, and red included in the received image signal with the R, G, and B points as imaging surfaces (Q1, Q2, and Q3) The high-frequency component of blue light has color ratios as shown in FIGS. 9 (a), 9 (b) and 9 (c).
[0010]
Therefore, when the result of FIG. 9 (a) is obtained, the imaging surface is at Q1, so the imaging lens is moved to the subject side, and when the result of FIG. 9 (c) is obtained, the imaging surface is at Q3. Therefore, by moving the image pickup lens to the image pickup device side, it is possible to bring it closer to the planned focal plane (Q2), and when the result of FIG.
[0011]
According to the focusing method using the optical chromatic aberration of the image pickup lens, the defocus direction can be recognized without driving the image pickup lens, so that there is an advantage that the focusing speed is increased.
[0012]
[Problems to be solved by the invention]
However, the focusing method using the optical chromatic aberration of the imaging lens described above is not a problem if the color components (red, green, and blue) included in the subject are evenly distributed, but the color components are evenly distributed. In the case of a subject that has not been operated, there is a possibility of causing a malfunction.
[0013]
For example, in the case of a subject with many red components, the high-frequency components of red, green, and blue light included in the received image signals on the imaging surfaces Q1, Q2, and Q3 shown in FIG. The color ratio shown in c) is expected. As shown in FIG. 9, when the result of FIG. 10B, in which the color ratio is approximated to FIG. 9A, is obtained, the imaging lens is moved to the subject side. This means that, despite the in-focus position, a malfunction has occurred that further moves the imaging lens to the imaging element side.
[0014]
The present invention has been made in view of the above circumstances, and can increase the focusing speed by utilizing the chromatic aberration of the lens, and can reliably focus without causing a malfunction. An object is to provide an automatic focusing device.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, the present invention takes the following measures.
The present invention is an automatic focusing device that forms a subject image on an image sensor with an imaging lens and controls the position of the imaging lens based on a contrast value of a video signal read from the image sensor, and is included in the video signal Preferentially using one color signal from the plurality of color signals, and moving the imaging lens so that the in-focus position related to the one color signal appears before the planned focal plane of the imaging lens; In the automatic focusing device, the focusing operation is terminated after the imaging lens is moved in the same direction by a predetermined distance from the imaging lens position where the contrast value of the one color signal shows the maximum value.
[0016]
According to the present invention, the in-focus position of one color signal is shifted from the planned focal plane of the imaging lens due to the chromatic aberration of the imaging lens, and the planned focal plane of the imaging lens is moved closer to the imaging element from a certain direction. The in-focus position related to the color signal is detected before the planned focal plane of the imaging lens. Since the distance from the lens position when the focus position of one color signal is detected to the planned focal plane of the imaging lens can be known in advance, the lens position when the focus position of one color signal is detected When the imaging lens is moved in the same direction by a predetermined distance that is previously known, the planned focal plane of the imaging lens is moved to the imaging plane of the imaging device and is brought into focus. Therefore, the reverse driving of the lens can be eliminated from the focusing operation, and the focusing speed can be increased.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below.
FIG. 1 shows the overall configuration of the automatic focusing apparatus according to the present embodiment.
Light of a subject image captured from an optical system (not shown) is guided to the imaging lens 1. The imaging lens 1 can be driven in the optical axis direction by a lens driving circuit 2 composed of a pulsed stepping motor, and forms a subject image with a degree of focus corresponding to the lens position on the color imaging device 3. . The color image sensor 3 captures a subject image projected by the imaging lens 1 and outputs a video signal. The image pickup device driver 4 drives the color image pickup device 3. The video signal output from the color imaging device 3 is input to an AMP / CDS circuit 5 that performs signal amplification, sample hold, and double correlation sampling. The video signal subjected to signal processing by the AMP / CDS circuit 5 is input to the A / D converter 6 and converted into a digital video signal. The video signal output from the A / D converter 6 is input to the color separation circuit 7. The color separation circuit 7 separates the video signal into R (red), G (green), and B (blue) three primary color signals and a luminance signal. The R, G, and B color signals separated by the color separation circuit 7 are input to correspondingly provided high-pass filters (hereinafter referred to as “HPF”) 8-1, 8-2, and 8-3 for luminance. The signal is input to the HPF 8-4. Each HPF 8-1, 8-2, 8-3, 8-4 extracts a high-frequency component amount (contrast value) indicating the degree of focusing from each signal and inputs it to the arithmetic processing circuit 9. The arithmetic processing circuit 9 is composed of a CPU, a ROM, a RAM, and the like, and a focusing function described later is realized by the CPU executing a program stored in the ROM or the like.
[0018]
The pulse generation circuit 10 generates sampling pulses for the AMP / CDS circuit 5 and the A / D converter 6 and various pulses such as a horizontal synchronization signal and a vertical synchronization signal for the color separation circuit 7.
[0019]
FIG. 2 is a functional block relating to the focusing function provided in the arithmetic processing circuit 9.
The R, G, B contrast values and the luminance signal contrast value input to the arithmetic processing circuit 9 are input to the color signal selection unit 11. The color signal selection unit 11 selects a contrast value of a designated color (including a luminance signal) indicated by a color selection signal set by an operator or the like.
[0020]
The AF evaluation value calculation unit 12 integrates the contrast value selected by the color signal selection unit 11 to calculate an AF evaluation value. An integration area designation signal created using the vertical synchronizing signal (VD) and horizontal synchronizing signal (HD) constituting the input image captured by the color image sensor 3 and the sampling clock of the A / D converter 6 calculates the AF evaluation value. Input to the unit 12. The position and size of the integration area can be arbitrarily set by the integration area designation signal.
[0021]
The single color focus determination unit 13 obtains a focus position for the designated color (hereinafter referred to as “temporary focus position”) based on one of the R, G, and B AF evaluation values, and the temporary focus. A control function is provided that further moves and focuses the lens without reversing the driving direction by the amount of defocus from the position to the original focusing position of the imaging lens (hereinafter referred to as “scheduled focal plane”).
[0022]
Here, the imaging lens 1 has axial chromatic aberration as shown in FIG. 3, and the temporary focus position Fg by the green component and the focus position Fy (scheduled focal plane) by the luminance signal are one as shown in FIG. Suppose you are doing it. The temporary focusing position Fb by the blue component is positioned two steps before the focusing position Fy of the luminance signal from the infinite position toward the closest side, and the temporary focusing position Fr by the red component is infinite from the closest position. It is located two steps before the focus position Fy of the luminance signal toward the large side. One step of lens movement corresponds to a driving amount of one step of the stepping motor of the lens driving circuit 2.
[0023]
The distance from the temporary focusing position of each color to the planned focal plane (the amount of defocus) is determined in advance by the design of the imaging lens 1. The imaging lens 1 used in this embodiment has two steps for each.
[0024]
When using the AF evaluation value of the blue component focusing control toward the closest side from the infinity position in the "hill-climbing method", the provisional in-focus position F b at a position passing through one step from Karigoase position Fb It can be recognized that it has passed, and its position is a place where one step is left to the planned focal plane (Fy). Further, when focusing control is performed from the closest position to the infinity side by the “mountain climbing method” using the AF evaluation value of the red component, the temporary focusing position Fr is set at a position that has passed one step from the temporary focusing position Fr. It can be recognized that it has passed, and its position is a place where one step is left to the planned focal plane (Fy).
[0025]
The number of steps from the position where it can be recognized that each of the temporary in-focus positions Fb and Fr has been passed to the planned focal plane (Fy) when focusing control is performed from each direction using the “hill climbing method” using the AF evaluation value of each color. (Both steps in this example) are stored in the defocus amount correspondence table 14 respectively.
[0026]
In the example shown in FIG. 4, since the in-focus position Fy in the luminance signal and the in-focus position Fg in the green signal coincide with each other, the remaining number of steps = 0 for the green signal. Depending on the design of the lens, the in-focus position may not match, and in such a case, there are remaining steps for green.
[0027]
Since the color signal currently used for the focus control is determined by the color selection signal, the “remaining step number” of the color signal indicated by the color selection signal is read from the defocus amount correspondence table 14 to obtain a single color focus determination unit. 13 is read. Details of focus control by the single color focus determination unit 13 will be described later.
[0028]
The drive control unit 15 is a part that outputs an instruction of a drive direction and a drive timing to the lens drive circuit 2 based on a command value given from the single color focus determination unit 13 or the luminance focus determination unit 16.
[0029]
The brightness focus determination unit 16 is a part that performs “mountain climbing” focus control based on the brightness signal, and outputs a command value for focus control to the drive control unit 15. Use with the single color focus determination unit 13 is instructed by an operator by a color selection signal. When the luminance signal is selected by the color selection signal, focusing control is executed under the operation of the luminance focusing determination unit 16.
[0030]
Next, focusing control of the embodiment configured as described above will be described.
FIG. 5 is a flowchart for focus control by the single color focus determination unit 13.
The flowchart of FIG. 5 shows a case where the blue signal (B) is selected as the designated color and the imaging lens is driven from the infinite position toward the closest side. As an initial setting, the imaging lens 1 is moved to an infinite position farthest from the color imaging device 3.
[0031]
In step T1, the “remaining step number” of the blue signal instructed by the color selection signal is fetched from the defocus amount correspondence table 14, and focusing control is started by selecting the contrast value of the blue component.
[0032]
Note that the parameter used by the single color focus determination unit 13 for focus control is not the contrast value itself but the AF evaluation value. However, in contrast to the flowchart of FIG. The value is shown.
[0033]
In step T2, the AF evaluation value (C _n ) of the blue component at the current imaging lens position (infinity position) is input from the AF evaluation value calculation unit 12 to the single color focus determination unit 13, and in step T3, the imaging lens 1 Is driven forward by one step. When the blue signal is selected, the forward rotation direction is a direction in which the imaging lens 1 is moved to the closest side.
[0034]
In step T4, the AF evaluation value (C _{n + 1} ) of the blue component after one step movement is input, and in step T5, the AF evaluation value (C _n ) before one step movement and the current AF evaluation value (C _{n + 1).} ). If C _{n + 1} is large, it is close to the temporary in-focus position Fb, so that n is incremented in step T6 and the imaging lens 1 is further moved one step closer to the closest direction. While C _n ≦ C _{n + 1} , by repeating the process of incrementing n and moving the imaging lens 1 in the closest direction step by step, the imaging lens 1 passes through the temporary focusing position Fb of the blue signal. .
[0035]
When the AF evaluation value (C _n ) before movement becomes larger than the current AF evaluation value (C _{n + 1} ) in step T5, the imaging lens 1 passes through the temporary in-focus position Fb as shown in FIG. This means that the target focal plane (Fy) has been moved up to one step before. Therefore, when it is detected that C _n is greater than C _{n + 1,} stop the further forward driving by "remaining number of steps" of the blue signal taken from the defocus amount correspondence table 14 in Step T7 In step T1 This completes the focusing operation.
[0036]
Next, a case where the red signal is selected as the designated color will be described.
In the initial setting, the imaging lens 1 is moved to the closest position closest to the color imaging device 3, and a red signal is designated by the color selection signal.
[0037]
If the “remaining step number” of the red signal instructed by the color selection signal is fetched from the defocus amount correspondence table 14 to the single color focus determination unit 13, the AF evaluation value (C _n ) at the closest position is obtained. And the imaging lens 1 is driven forward by one step with the direction toward the infinity as the normal rotation direction. Thereafter, as in the case of the blue signal, _n is repeatedly incremented until C _n becomes larger than C _{n + 1,} and the imaging lens 1 is moved in an infinite direction step by step. If C _n becomes larger than C _{n + 1, the in-} focus operation is terminated by further forward driving after the “remaining step number” of the red signal and then stopping.
[0038]
If it is desired to perform focusing control using a luminance signal, the luminance signal is selected using a color selection signal. When the luminance signal is selected, the contrast value of the luminance signal is input to the AF evaluation value calculation unit 12, and the AF evaluation of the contrast value is given to the luminance focus determination unit 16 to focus on the so-called “mountain climbing method”. Control is applied.
[0039]
According to such an embodiment, when the imaging lens 1 is driven to rotate forward from the infinite position or the closest position in the opposite direction, several steps before the planned focal plane that is the original focusing position of the imaging lens. Since the focus control is performed using the contrast value of each color component in which the temporary focus position exists, and it is only necessary to perform forward rotation for several steps to the planned focal plane after finding the temporary focus position by each color component. It is possible to eliminate the waste of inversion driving of the imaging lens 1, and it is possible to focus several steps faster than the focusing control using the luminance signal.
[0040]
Also, when focus control is performed using a luminance signal, the lens position is returned to the opposite direction after passing the original focus position of the imaging lens, so that the original blurred image changes to a focused image. Then, after changing to a blurred image, the image changes again to a focused image. On the other hand, according to the above-described embodiment, since there is no lens reversal operation, the image stops at the focused image from the blurred image, which gives an uncomfortable feeling to the observer watching the imaging screen. Focus operation is performed.
[0041]
Note that an auxiliary light source may be used in a low illumination environment. By selecting a color component close to the colored light of the auxiliary light source by the color selection signal, the information amount of the color signal used for focusing control is larger than that of the other color components, and the focusing accuracy can be increased. For example, if the color light of the auxiliary light source is red light, the focusing accuracy can be increased by selecting the red component with the color selection signal.
[0042]
Although the case where the imaging lens 1 having the axial chromatic aberration shown in FIG. 4 is used has been described, it is not limited to the case shown in FIG. 4, and focus control is performed with each color component according to the design of the imaging lens 1. The number of remaining steps and the lens moving direction can be determined.
[0043]
As mentioned above, although demonstrated based on embodiment, this specification includes the following invention.
(1) In an automatic focusing device that forms a subject image on an image pickup element by an image pickup lens and controls the position of the image pickup lens based on a contrast value of a video signal read from the image pickup element.
Contrast extraction means for extracting a contrast signal representing the contrast of the subject image from a plurality of color signals included in a video signal read from the image sensor;
Calculation for calculating an evaluation value indicating the degree of focus on the color signal by preferentially using the contrast signal of one color signal corresponding to the moving direction of the imaging lens among the contrast signals extracted by the contrast extracting means. Means,
Control means for ending the focusing operation when the imaging lens is moved a predetermined distance in the same direction from the imaging lens position where the evaluation value of one color signal calculated by the calculation means shows the maximum value. Features an automatic focusing device.
[0044]
According to the present invention, a contrast signal representing the contrast of a subject image is extracted from a plurality of color signals included in a video signal read from an image sensor, and one color signal used preferentially from these extracted contrast signals. An evaluation value of the contrast signal is calculated. The imaging lens is stopped when it is further moved a predetermined distance in the same direction from the imaging lens position where the evaluation value of this one color signal shows the maximum value. Since the distance from the imaging lens position where the evaluation value of the color signal shows the maximum value to the planned focal plane of the imaging lens can be known in advance by the design of the imaging lens, the imaging lens should be focused without moving in the reverse direction. Can do.
(2) In an automatic focusing device that forms a subject image on an image sensor with an imaging lens and controls the position of the imaging lens based on a contrast value of a video signal read from the image sensor,
Color separation means for color-separating a video signal read from the image sensor into a plurality of color signals;
Contrast extraction means for extracting high frequency components from a plurality of color signals separated by the color separation means;
Color selection means for selecting a high-frequency component of one color signal to be used preferentially from high-frequency components of a plurality of color signals extracted by the contrast extraction means;
Evaluation value calculation means for calculating a focus evaluation value from a high-frequency component of the color signal selected by the color selection means;
The lens position at which the focus evaluation value of one color signal calculated by the evaluation value calculation means is maximized in the direction in which the planned focal plane of the imaging lens appears after the focus position of the one color signal appears. A single-color focusing determination unit that detects by moving the imaging lens, and moves the imaging lens from the detected imaging lens position in the same direction by a predetermined distance to end the focusing operation. Automatic focusing device to do. According to the present invention, the video signal read from the image sensor is color-separated into a plurality of color signals, and the high-frequency component of one color signal to be used preferentially is selected from the high-frequency components extracted from these color signals. A focus evaluation value is calculated from the high-frequency component of the selected color signal and is given to the single color focus determination means. When the lens position at which the focus evaluation value of one color signal is maximized is detected, the focus operation is terminated when the lens position is further moved a predetermined distance in the same direction.
(3) In the automatic focusing device described in (1) or (2) above,
An automatic focusing device characterized by preferentially using one color signal close to the emission color of an auxiliary light source used at low illuminance.
[0045]
According to the present invention, since focusing control is performed using a color signal having a larger amount of information than other color signals, focusing accuracy is improved.
(4) In the automatic focusing device described in (1) or (2) above,
The focus control using the focus evaluation value of one color signal and the focus control using the focus evaluation value of the luminance signal can be switched preferentially from among a plurality of color signals. Automatic focusing device to do.
According to the present invention, it is possible to quickly switch between a focusing method using a focusing evaluation value of one color signal and a focusing method using a focusing evaluation value of a luminance signal.
[0046]
【The invention's effect】
As described above in detail, according to the present invention, since one color signal is selected from a plurality of color signals and used preferentially for focusing control, focusing using the chromatic aberration of the lens is performed. It is possible to provide an automatic focusing device that can increase the speed and that can reliably focus without causing a malfunction.
[Brief description of the drawings]
FIG. 1 is an overall configuration diagram of an automatic focusing apparatus according to an embodiment of the present invention.
FIG. 2 is a functional block of a portion related to a focusing function of the arithmetic processing circuit in the embodiment.
FIG. 3 is a diagram illustrating axial chromatic aberration of an imaging lens.
FIG. 4 is a diagram illustrating a relationship between contrast characteristics of each color and luminance signal and a lens step position.
FIG. 5 is a flowchart of focusing control in the embodiment.
FIG. 6 is a diagram illustrating a relationship between a lens position of an imaging lens and a contrast value.
FIG. 7 is a flowchart showing a conventional “mountain climbing” focusing operation.
FIG. 8 is a diagram illustrating an imaging surface of each color according to chromatic aberration.
FIG. 9 is a diagram illustrating a ratio of high-frequency components of each color of a light-receiving image signal on an imaging surface arranged at a focus position of each color component.
FIG. 10 is a diagram illustrating a ratio of high-frequency components of each color of each received light image signal in a subject in which color components are not evenly distributed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Imaging lens 2 ... Lens drive circuit 3 ... Color imaging device 4 ... Imaging device driver 5 ... AMP / CDS circuit 7 ... Color separation circuit 8-1, 8-2, 8-3, 8-4 ... HPF
DESCRIPTION OF SYMBOLS 9 ... Arithmetic processing circuit 11 ... Color signal selection part 12 ... AF evaluation value calculation part 13 ... Single color focus determination part 14 ... Defocus amount correspondence table 15 ... Drive control part 16 ... Luminance focus determination part

Claims (3)

In an automatic focusing apparatus that forms an image of a subject on an image pickup element by an image pickup lens and controls the position of the image pickup lens based on a contrast value of a video signal read from the image pickup element.
Precise use of one color signal from a plurality of color signals included in the video signal so that the in-focus position related to the one color signal appears before the planned focal plane of the imaging lens The focusing operation is terminated after moving the lens, moving the imaging lens in the same direction by a predetermined distance from the imaging lens position where the contrast value of the one color signal shows the maximum value. Charcoal device. The automatic focusing device according to claim 1, wherein
An automatic focusing device characterized in that any one of a green signal, a red signal, and a blue signal is preferentially used from among a plurality of color signals included in the video signal. The automatic focusing device according to claim 2, wherein
Drive means for moving the imaging lens stepwise by pulse driving, and the imaging lens is scheduled from the imaging lens position where the in-focus position of the red signal can be detected or the in-focus position of the blue signal. An automatic focusing device characterized by providing a margin for a predetermined step before the focal plane.

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