JP4159557B2 - Electronic still camera - Google Patents

Description

  The present invention relates to an electronic still camera using a solid-state imaging device such as a CCD or CMOS, and more particularly to its automatic focusing (autofocus).

  An electronic still camera using a solid-state image sensor is equipped with a focusing mechanism that adjusts the focal distance between the imaging lens and the image sensor and automatically adjusts to eliminate blurring of the subject image formed by the image sensor. ing. As a focusing method, an infrared method, a phase difference detection method, and the like are mainly used in compact cameras, single-lens reflex cameras, and the like, but there are mechanical constraints. Therefore, a method called a contrast method is mainly used in an electronic still camera. This is because the imaging element is used as a distance measuring sensor, and the movable range of the imaging lens is moved from the infinity shooting side to the close-up shooting side, or vice versa, at a position where the imaging lens is moved at a certain interval and the imaging lens is stopped. This is a method that processes the video signal acquired by the image sensor and moves the imaging lens to the position where the high-frequency component is maximized, and there is no need for a separate distance measuring mechanism other than the image sensor and imaging lens. It is widely used because it exists. An example of such a technique is described in Patent Document 1 below.

Japanese Patent Laid-Open No. 9-43507

  However, in the conventional method, it is necessary to alternately repeat the movement of the imaging lens and the time required for charge accumulation for generating a video signal by the imaging device, but two internal vertical periods are required for movement of the imaging lens and charge accumulation. There is a problem that it takes a long time to focus.

The present invention
An image sensor that photoelectrically converts subject light incident through the imaging lens and generates a video signal by performing charge accumulation;
Drive means for moving the imaging lens parallel to the optical axis;
A focus control means for detecting the focus position of the imaging lens by repeatedly analyzing the video signal and moving the imaging lens;
If the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is not more than n times the internal vertical period (n is an integer), the movement of the imaging lens For every n times the internal vertical period,
If the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is greater than n times the internal vertical period (n is an integer), the movement of the imaging lens There is provided an electronic still camera comprising: a control unit that performs the operation every 2n times the internal vertical period.

  According to the present invention, the time required for focusing can be reduced.

Embodiments of the present invention will be described below with reference to the drawings. In the embodiment described below, the electronic camera is a still camera.
Embodiment 1 FIG.
FIG. 1 is a block diagram showing a configuration of an electronic still camera according to Embodiment 1 of the present invention. The electronic still camera shown in FIG. 1 includes an imaging lens 12, a lens driving unit 13, a shutter 14, an imaging unit 16, an image signal processing unit 18, a primary storage unit 20, an interface unit 22, and a display unit. 26, an operation unit 28, a control unit 30, and a lens driving unit 13.

The imaging lens 12 is used for determining the subject light on the imaging unit 16, and a shutter for turning on / off the incidence of the subject light to the imaging unit 16 between the imaging lens 12 and the imaging unit 16. 14 is provided.
The lens driving unit 13 is for moving the imaging lens 12 on the optical axis in parallel with the optical axis, and includes a linear actuator using a voice coil or the like.

The shutter 14 is opened during charge accumulation for imaging and closed at other times.
The imaging unit 16 photoelectrically converts subject light, accumulates charges, generates a video signal, and outputs it. A two-dimensional solid-state imaging device such as a CCD or CMOS sensor, and a circuit for driving the imaging device Etc.
The image signal processing unit 18 performs various digital signal processing on the video signal output from the imaging unit 16 by the main imaging to generate video data, and at the time of focusing processing, imaging for focusing is performed. Analysis of the video signal obtained by (test imaging), detection of the magnitude of the high frequency component, etc. are performed.

The temporary storage unit 20 is configured by a DRAM or the like, and stores the video signal processed by the image signal processing unit 18.
The interface unit 22 includes a slot, and includes a circuit that writes video data to and reads video data from the recording medium 24 that includes an external flash memory that can be inserted into and removed from the slot.
The display unit 26 includes a display element such as an LCD, and is used as a view finder or used to reproduce recorded video.

The operation unit 28 includes a push button for a user to input an operation, a touch panel, and the like. The operation unit 28 includes a camera activation button 28a, an autofocus (AF) button 28b, and a shutter button 28c. When automatic focusing is performed when the shutter button 28c is half-pressed, a switch that responds to the half-pressed state corresponds to the automatic focusing button.
The control unit 30 controls the lens driving unit 13 for moving the imaging lens 12 based on the operation signal from the operation unit 28 and the digital signal processing result in the image processing unit 18, the charge accumulation in the imaging unit 16 and the video signal. Output (reading) control, video data writing to and reading from the temporary storage unit 20, video data writing to the recording medium 24, reading control, display on the display unit 26 (display start, display switching, display stop) Control.

  The control unit 30 includes a focus control unit 34, an exposure amount control 32, and an overall control unit 36. Note that both can be configured by software, i.e., a programmed computer.

Hereinafter, the operation of the control unit 30 during the focus control will be described with reference to FIG.
First, when the camera activation button 28a of the operation unit 28 is pressed (102), the overall control unit 36, the imaging unit 16 and the image signal processing unit 18 are activated, and the display unit 26 is switched to display the exposure amount. The control unit 32 controls the exposure amount (104).
As a result of the display switching, the video signal that is incident through the imaging lens 12 and is generated by the imaging unit 16 is processed by the image signal processing unit 18, is sequentially updated, and is displayed on the display unit 26. At this time, the display unit 26 is used as an electronic viewfinder.
On the other hand, the exposure control unit 32 controls the charge accumulation time based on the level of the video signal output from the imaging unit 16 detected by the image signal processing unit 18. For example, the charge accumulation time is lengthened when the level of the video signal output from the imaging unit 16 is low, and the charge accumulation time is shortened when the level of the video signal output from the imaging unit 16 is high.

Thereafter, when the AF button 28b is pressed on the operation unit 28 (106), the overall control unit 36 requests the focusing control unit 34 to start focusing control (108).
That is, when the overall control unit 36 detects the pressing of the AF button 28b, the overall control unit 36 performs control (AE lock control) to prohibit the change of the exposure amount for the image signal processing unit 18 and the imaging unit 16, and sets the charge accumulation time t. Fix it.
Next, the charge accumulation time t is notified to the focusing control unit 34.

The focusing control unit 34 controls the lens driving unit 13 to move the imaging lens 12 to one end (the infinity shooting side or the close-up shooting side) of the imaging lens movable range, and the AF button 28b is The number L of test imaging is calculated from the charge accumulation time when the button is pressed (110).
The number of test imaging L means the number of charge accumulations and imaging lens movements that can be repeated within the time allowed for focusing (focusing request time) (T). The procedure for obtaining the test imaging number L will be described in detail later with reference to FIG.

After obtaining the test imaging number L, AF test imaging is performed (112). That is, the shutter 14 is opened and the image capturing unit 16 performs charge accumulation for a charge accumulation time determined by the exposure amount determined in step 104, and outputs a video signal.
Next, an image (AF image) obtained by test imaging is signal-processed to obtain an AF evaluation value (114). That is, the video signal output from the imaging unit 16 is processed by the image signal processing unit 18 to obtain an AF evaluation value. As the AF evaluation value, the maximum value of the high frequency component is used. In this case, the maximum value of the high frequency component in a specific region in the imaging screen, for example, the center of the screen may be used as the AF evaluation value.
FIG. 2 shows an example of a change in AF evaluation value when the test imaging positions P1 to P11 are taken on the horizontal axis.

Next, the obtained AF evaluation value is stored in association with the position of the imaging lens at that time (test imaging position) (116).
Next, it is determined whether or not the processing of steps 112 to 116 has been performed at all test imaging positions within the lens movable range, that is, whether or not AF evaluation values have been obtained for all test imaging positions (118). If so (NO in step 118), the lens is moved to the test imaging position by the predetermined distance d from the one end to the other end (120), and the process returns to step 112.
If it is determined in step 118 that AF evaluation values have been obtained for all test imaging positions (YES in step 118), in-focus positions are specified based on all the obtained AF evaluation values (122). For example, the test imaging position (position P6 in the example shown in FIG. 2) giving the largest AF evaluation value is determined as the in-focus position.
Next, the imaging lens is moved to the in-focus position (124).
As a result, the processing is terminated as the completion of focusing (126).
Thereafter, actual imaging is performed.

The details of the process of calculating the test imaging number L in step 110 of FIG. 3 are shown in FIG.
First, the focus control unit 34 acquires the charge accumulation time t (data representing the charge accumulation time t) from the exposure amount control unit 32 (202),

Next, a minimum integer N is calculated such that N × internal vertical period (VD period) v does not fall below the charge accumulation time t (204).
That is,
N × v is greater than or equal to t,
Find an integer N such that (N−1) × v is less than t.
In other words,
When t ÷ v is divisible, N = t ÷ v
When t ÷ v is not divisible, N = [t ÷ v] +1
(However, [t ÷ v] is an integer part of t ÷ v)
Means to perform the operation.
The internal vertical period (VD period) referred to here is a period inside the camera, and is different from the output period (general vertical period) of one frame of video signal from the inside of the camera to the outside. That is, the internal vertical period is a synchronization signal for image data that enters the image signal processing unit 18 from the imaging unit 16 and is a signal with a fixed period generated by a timing generator (not shown), and is temporarily stored from the image signal processing unit 18. Unlike the vertical sync signal of the image output to the unit 20, the display unit 26, and the control unit 30, the cycle of the vertical sync signal is an integral multiple of the internal vertical period.

Next, the number L of test imaging that can be repeated within the allowable time (focusing request time) T for focusing is obtained (206).
L = T ÷ (N × v)
The in-focus request time T may be a fixed value set at the time of system design, and may be variable (for example, selected from a plurality of values) depending on the situation at the time of photographing (subject brightness, frame rate, etc.). It may be.

Next, with respect to the time (lens moving time) m required for the imaging lens 12 to move from the end point to the end point (for example, from the infinity shooting side to the close-up shooting side) of the movable range of the imaging lens, the following determination formula (1)
m ≦ N × v−t (1)
Whether or not is satisfied is determined (208).
Since the movable range of the imaging lens is moved in L times, the movement distance per time is 1 / (L-1) of the movable range, but will be described later when the calculation of the equation (1) is performed. Thus, since it is unclear whether it will be divided into L times or (L / 2) times, instead of the lens movement time per time, the imaging lens Using the movement time m from the end point to the end point of the movable range, the calculation of equation (1) is performed.

When judgment formula (1) is satisfied,
The movement of the imaging lens is performed every N × v period, and the number C of repetitions of charge accumulation and the movement of the imaging lens is set to C = L (210).
On the other hand, if the judgment formula (1) is not satisfied,
The imaging lens is moved every 2 × N × v period, and the number C of repetitions of charge accumulation and imaging lens movement is set to C = L / 2 (212).

FIG. 5 is a diagram showing the timing of charge accumulation and movement of the imaging lens when the number C of charge accumulation and movement of the imaging lens is equal to L.
In the figure, 301 indicates an internal vertical synchronizing signal, and VD indicates one internal vertical period. When the entire imaging system is operating at 30 fps (frames per second), 1 VD = 33.3 ms. Reference numeral 302 denotes a charge accumulation period, and 303 denotes a period during which the imaging lens is moving.

  In this figure, the charge accumulation period 302 extends over two internal vertical periods VD (that is, N is 2). When the system is operating at 30 fps, the screen of the display unit 26 is 15 fps. It is possible to update at a speed.

In the timing chart of FIG. 5, the lens movement period 303 is within the difference between the double internal vertical period and the charge accumulation period 302, and control is performed so that the imaging lens does not move when charge accumulation is being performed. It has been shown that lens movement does not affect charge accumulation. That is, in the above-described determination formula (1), an expression in which N = 2 is substituted,
m ≦ 2 × v−t
Are satisfied, and test imaging is performed every (2 × v) period.

FIG. 6 is a diagram showing the timing of charge accumulation and movement of the imaging lens when the determination formula (1) is not satisfied.
As illustrated, the imaging lens is moved every 2 × 2 × v (N = 2) periods. The charge accumulation is performed every 2 × v as indicated by reference numerals 302a and 302b, but the signal obtained as a result of the charge accumulation 302a overlapping the movement 303 of the imaging lens is not used as an AF test captured image. A signal obtained as a result of the charge accumulation 302b that does not overlap with the movement 303 of the imaging lens is used as an AF test captured image.

  If the charge accumulation and the movement of the imaging lens are performed at the same time, the lens moves during the charge accumulation (that is, the lens moves with the shutter opened), so noise is added to the video signal obtained as a result of the charge accumulation (302a). As shown in FIG. 6, the accurate focus determination is performed by using the video signal obtained as a result of the charge storage 302b in which the charge storage and the movement of the imaging lens do not overlap as shown in FIG. be able to.

  On the other hand, if the imaging lens is moved at a sufficiently long interval (for example, at a cycle four times the internal vertical period as shown in FIG. 6) regardless of the charge accumulation time in each imaging condition, While it is possible to prevent noise from being added to the signal, there is an inconvenience that the time required for focusing becomes long regardless of the photographing conditions.

  According to the present embodiment, the interval of movement of the imaging lens is switched as shown in FIG. 5 or as shown in FIG. The number of charge accumulations and image signal acquisitions that can be repeated within the required focusing time T can be maximized, and focusing can be performed more accurately according to the shooting conditions.

Embodiment 2. FIG.
In the first embodiment, when the determination formula (1) is not satisfied, the movement of the imaging lens is performed every 2 × N × v, for example, 2 × 2 × v periods, as shown in FIG. However, instead of moving the imaging lens every N × v, for example, 2 × v, as shown in FIG. 7, the result of charge accumulation in the charge accumulation time 302 a that overlaps with the imaging lens movement 303 is performed. The obtained video signal is used for processing (coarse adjustment) for specifying a range including the in-focus point. After specifying the range, the image pickup lens is moved within the range specified as described above. A video signal obtained as a result of non-overlapping charge accumulation (302b) may be used, and thereby processing (fine adjustment) for specifying a focal point may be performed.

  As described above, since the imaging lens is moved every N × v period in the coarse adjustment stage, it is possible to shorten the time until the focal point is specified as a whole.

In this case, for example, a part αT (0 <α <1) of the time allowed for focusing (focusing request time) T is used for coarse adjustment, and the remaining time (1-α) T is finely adjusted. Use for.
For example, when the charge accumulation time is equal to or shorter than one internal vertical period, but the sum of the lens movement time and the charge accumulation time exceeds 1 v and is equal to or less than 2 v, the coarse adjustment is performed so as to overlap the lens movement. Is the same as the number A of VD periods within the focusing request time T. On the other hand, in the fine adjustment stage in which lens driving and charge accumulation are not performed simultaneously, it is necessary to move the lens at least three times, and two VD periods are required at one time.
(A-3 × 2) = (A-6)
Each VD period is a period for coarse adjustment, and during this period, (A-6) times of coarse adjustment by lens movement and charge accumulation are performed. FIG. 8 shows a case where A is 14 and (A-6) is 8.

In the figure, P1 and P8 are the first and second ends of the lens movable range, dp is the amount of lens movement per time, and P2 to P7 are one lens in the coarse adjustment stage. This is the lens stop position between the movement and the next lens movement. P2 to P7 and P1 and P8 are test imaging positions in the coarse adjustment stage.
P9, P10, and P11 are test imaging positions in the fine adjustment stage.
The lens movement distance dq in each fine adjustment stage is ½ of the lens movement distance dp in the coarse adjustment stage.

In the illustrated example, in the coarse adjustment stage, it is determined that the position P6 is closest to the in-focus point, and the position P10 is the same as the position P6 (the same position, but different signs are used in the coarse adjustment stage and the fine adjustment stage. And the positions P9 and P11 before and after that are imaged for fine adjustment.
Although the position P10 and the position P6 are the same, in the coarse adjustment stage, the video signal obtained by the charge accumulation partially overlapping with the lens movement is also used in the position P6. It is necessary to accumulate.

It is a block diagram which shows the signal processing and control system of the electronic still camera in Embodiment 1 of this invention. It is a figure which shows an example of the relationship between a test imaging position and AF evaluation value. It is a flowchart which shows the focusing control in Embodiment 1 of this invention. 3 is a flowchart illustrating a process for determining an interval of movement of the imaging lens according to the first embodiment. 3 is a timing chart illustrating the timing of charge accumulation and imaging lens movement in the first embodiment. 3 is a timing chart illustrating the timing of charge accumulation and imaging lens movement in the first embodiment. 10 is a timing chart showing the timing of charge accumulation and imaging lens movement in the coarse adjustment stage of Embodiment 2 of the present invention. FIG. 10 is a diagram illustrating test imaging positions in a coarse adjustment stage and a fine adjustment stage in the second embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 12 Image pickup lens, 13 Lens drive part, 14 Shutter, 16 Image pickup part, Image signal processing part, 18 Image signal processing part, 20 Temporary memory part, 22 Interface part 26 Display part, 28 Operation part, 30 Control part, 32 Exposure amount Control unit, 34 focusing control unit, 36 overall control unit.

Claims (4)

An image sensor that photoelectrically converts subject light incident through the imaging lens and generates a video signal by performing charge accumulation;
Drive means for moving the imaging lens parallel to the optical axis;
A focus control means for detecting the focus position of the imaging lens by repeatedly analyzing the video signal and moving the imaging lens;
If the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is not more than n times the internal vertical period (n is an integer), the movement of the imaging lens For every n times the internal vertical period,
If the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is greater than n times the internal vertical period (n is an integer), the movement of the imaging lens An electronic still camera comprising: control means for performing the operation every 2n times the internal vertical period.   If the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is greater than n times the internal vertical period (n is an integer), the movement of the imaging lens 2. The electronic still camera according to claim 1, wherein the charge accumulation is performed in a state where the imaging lens is not moved, at every 2n times the internal vertical period. When the sum of the time required for charge accumulation for generating a video signal by photoelectric conversion and the time required for moving the imaging lens is larger than n times (n is an integer) the internal vertical period,
The movement of the imaging lens is performed every n times the internal vertical period, and rough adjustment is performed to specify the range including the in-focus position.
The focus position is specified by performing the movement of the imaging lens after the rough adjustment so that the movement of the imaging lens does not overlap with the charge accumulation and within the range specified by the ancestor adjustment. Item 4. The electronic still camera according to Item 1.   4. The electronic still camera according to claim 1, wherein the control unit determines a time for the charge accumulation based on a video signal output from the image sensor. 5.

Images