Focus adjusting apparatus and imaging apparatus

A focus adjusting apparatus includes an optical system including a focus lens, a driver configured to move the focus lens along an optical axis of the optical system, an obtaining unit configured to periodically obtain an evaluation value of a subject image formed via the focus lens, a determining unit configured to determine a target position to which the focus lens is moved based on the evaluation value, and a controller configured to control an operation of the driver based on the determined target position. The controller controls the driver to move the focus lens at a first speed for a first period including a backlash period which is defined from start of the operation of the driver to start of an actual movement of the focus lens, and controls the driver to move the focus lens at a second speed faster than the first speed for a second period after the end of the first period.

BACKGROUND

1. Technical Field

The technical field relates to a focus adjusting apparatus for automatically adjusting a focus, and more particularly to a focus adjusting apparatus having a function for correcting backlash of a lens driver. Further, the technical field relates to an imaging apparatus mounted with such a focus adjusting apparatus.

2. Related Art

In general, it is known that a lens driver for driving a focus lens and the like has backlash in digital cameras. The backlash causes an error at the time of driving a lens to a target position in a focusing operation. For this reason, the backlash should be taken into consideration to make accurate control, and various techniques that cope with the backlash are developed (for example, see JP04-042674A and JP05-060968A).

For example, before a movement of the lens for focusing on a subject is started, backlash should be corrected. For this reason, before normal driving of the lens, it may be considered to move the lens by a moving amount corresponding to the backlash with a lens driver. However, when the lens is moved in order to eliminate the backlash, a contact sound of the lens and the lens driver or a driving sound of the lens driver during the driving of the lens may occasionally occur. In recent years, a digital camera that can capture not only a still image but also a moving image has made an appearance (for example, see JP2008-42404A). When such sounds are loud at the time when such a digital camera captures a moving image, the sounds are disadvantageously recorded as noises.

Further, for a contrast detection-type autofocus (AF) operation for determining a focus position based on an evaluation value of an image captured by an imaging device such as a CMOS sensor, an accurate focus lens position is important. However, when backlash occurs, an accurate focus lens position cannot be obtained, so that a malfunction disadvantageously occurs in the focusing operation.

SUMMARY

To solve the above problem, a focus adjusting apparatus is provided which can eliminate inconvenience caused by the backlash.

In a first aspect, A focus adjusting apparatus is provided, which includes an optical system including a focus lens, a driver configured to move the focus lens along an optical axis of the optical system, an obtaining unit configured to periodically obtain an evaluation value of a subject image formed via the focus lens, a determining unit configured to determine a target position to which the focus lens is moved based on the evaluation value, and a controller configured to control an operation of the driver based on the determined target position. The controller controls the driver to move the focus lens at a first speed for a first period including a backlash period which is defined from start of the operation of the driver to start of an actual movement of the focus lens, and controls the driver to move the focus lens at a second speed faster than the first speed for a second period after the end of the first period.

With this configuration, a driving speed of the focus lens can be switched so that an optimum driving seed is achieved for each of the first period including the backlash period and the second period after the end of the first period. That is, the focus lens is driven at a low speed for the first period including the backlash period, so that the contact sound of the focus lens and the lens driver can be reduced. At the same time, the focus lens is driven at a higher speed for the second period, so that the focus lens can be moved to the target position quickly.

In a second aspect, a focus adjusting apparatus is provided, which includes an optical system including a focus lens, a driver configured to move the focus lens along an optical axis of the optical system, an obtaining unit configured to periodically obtain an evaluation value of a subject image formed via the focus lens, a determining unit configured to determine a target position to which the focus lens is moved based on the evaluation value, a controller configured to control an operation of the driver based on the determined target position, and a setting unit configured to set a reliability flag indicating a backlash period which is defined from start of the operation of the driver to start of an actual movement of the focus lens. The determining unit stops operation of determining the target position based on the evaluation value, while the reliability flag indicates the backlash period.

With this configuration, a malfunction of the focus lens driver based on an inaccurate evaluation value for the backlash period can be prevented.

In a third aspect, an imaging apparatus having the focus adjusting apparatus according to the first or second aspect is provided.

According to the above aspects, the focus adjusting apparatus is provided, which has good usability and prevents inconvenience caused by backlash correction from occurring.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Preferred embodiments are described in detail below with reference to the drawings. The embodiments are described below by using a digital camera as an example.

First Embodiment

1. Configuration of Digital Camera

A configuration of a digital camera according to the first embodiment is described with reference to the drawings.

FIG. 1is a block diagram illustrating an electric configuration of a digital camera100according to the first embodiment. The digital camera100includes a camera body102and an interchangeable lens101mountable to the camera body102. The digital camera100can perform a contrast-type autofocus operation based on image data (contrast value) generated by a CMOS image sensor150.

1-2. Configuration of Camera Body

The camera body102includes the CMOS image sensor150, a liquid crystal display (LCD) monitor163, an electronic viewfinder (EVF)162, a camera controller153, a body mount140, a release button160, a moving image recording button161, a power supply154, and card slot165.

The camera controller153controls respective sections such as the CMOS image sensor150according to an instruction from an operating member such as the release button160or the moving image recording button161, so as to control the entire operation of the digital camera100. The camera controller153transmits a vertical synchronizing signal to a timing generator (TG)151. Collaterally, the camera controller153generates an exposure synchronizing signal based on the vertical synchronizing signal. The camera controller153transmits periodically the generated exposure synchronizing signal to the lens controller120(described later) via the body mount140and a lens mount130. The camera controller153uses a DRAM155as a work memory at the time of the control operation and the image processing operation. The camera controller153may be composed of a hard-wired electronic circuit, or may be composed of a microcomputer using a program. Further, the camera controller153may be formed in a semiconductor chip with or separately from an image processor and/or the work memory.

The CMOS image sensor150is an imaging device for capturing a subject image incident via the interchangeable lens101to generate image data. The generated image data is converted from analog-format data to digital-format data by an AD converter152. The image data digitalized by the AD converter152is subjected to various image processes by the camera controller153. The various image processes include, for example, a gamma correction, a white balance adjustment, a flaw correction, a YC conversion, an electronic zoom process, and a JPEG compression, but the processes are not limited to them. Instead of the CMOS image sensor150, for example, another imaging device such as an NMOS image sensor or a CCD image sensor may be used. The camera controller153obtains a contrast value as an evaluation value for each of frames from the image data generated by the CMOS image sensor150. The camera controller153executes the autofocus operation based on the respective contrast values (evaluation values) for the respective obtained frames. The autofocus operation is detailed later.

The CMOS image sensor150operates at timing controlled by the timing generator151(TG). The operation of the CMOS image sensor150controlled by the timing generator151includes a still image recording operation, an operation of capturing a through image, a data transfer operation, an electronic shutter operation, and the like. The through image is mainly a moving image, and is displayed on the LCD monitor153and the electronic viewfinder162(EVF) in order that a user determines a composition for taking a still image.

The LCD monitor163is arranged on a rear surface of the camera body102and displays an image represented by image data for display processed by the camera controller153. The LCD monitor163can display selectively the moving image and the still image. Besides images, the LCD monitor163can display entire setting conditions of the digital camera100and the like. The first embodiment describes the LCD monitor163as one example of a display unit, but the display unit is not limited to this. For example, an organic EL display may be used as a display unit.

The electronic viewfinder162(EVF) is arranged on an upper rear surface of the camera body102, and displays an image represented by image data for display processed by the camera controller153similarly to the LCD monitor163.

Display on the LCD monitor153and the electronic viewfinder162(EVF) may be carried out simultaneously, or be switched to either one of them. When the simultaneous display is carried out, an image to be displayed on the LCD monitor163and an image to be displayed on the electronic viewfinder162may be same or different.

The flash memory156functions as an internal memory for storing image data and the like. The flash memory156stores programs and parameters to be used for control of the camera controller153.

The card slot165is a connecting unit to which the memory card164is attachable. The card slot165can be electrically or mechanically connected to the memory card164. Further, the card slot165may have a function for controlling the memory card164.

The memory card164is an external memory containing a storage element such as a flash memory. The memory card164can store data such as image data processed by the camera controller153. Further, the memory card164can output the data such as image data to be stored therein. The image data outputted from the memory card164is processed by the camera controller153, and is displayed (reproduced) on the LCD monitor163or the like. The first embodiment describes the memory card164as one example of an external recording medium, but the external recording medium is not limited to this. For example, a recording medium such as an optical disc may be used as the external recording medium.

The body mount140can be mechanically or electrically connected to the lens mount130of the interchangeable lens101. The body mount140enables transmission/reception of data between the camera body102and the interchangeable lens101via the lens mount130. The body mount140transmits an exposure synchronizing signal and another control signal received form the camera controller153to the lens controller120via the lens mount130. Further, the body mount140transmits a signal received from the lens controller120via the lens mount130to the camera controller153.

The power supply154supplies power for driving the digital camera100. The power supply154may be, for example, a dry battery or a rechargeable battery. As the power supply154, power supplied from the outside via a power cord may be supplied to the digital camera100. When the power is turned ON, the camera controller153supplies power to the entire camera body102. The camera controller153supplies power to the interchangeable lens101via the body mount140and the lens mount130. The lens controller120supplies the power received via the lens mount130to the entire interchangeable lens101.

The release button160receives a user's operation. The release button160is operated at two steps including a half pressing and full pressing. When the half-pressing of the release button160is made by the user, the camera controller153executes the autofocus operation. Further, when the full-pressing of the release button160is made, the camera controller153records image data generated at the full-press operation to the memory card164.

The moving image recording button161receives a user's operation. While viewing a through image displayed on the LCD monitor163, the user can press down the moving image recording button161at any timing. When the moving image recording button161is pressed down, the camera controller153starts recording of the moving image data to the memory card164at the timing of pressing-down the button161. While recording the moving image, the camera controller153continuously performs the autofocus operation. When the moving image recording button161is again pressed down during the recording the moving image, the camera controller153ends the storage of the moving image data to the memory card164.

1-3. Configuration of Interchangeable Lens

The interchangeable lens101includes an optical system, a lens controller120, and a lens mount130. The optical system includes a focus lens110and a zoom lens112. Although not shown inFIG. 1, a camera shake correction lens (image stabilizer) may be provided.

The lens controller120controls the entire operation of the interchangeable lens101. The lens controller120may be composed of a hard-wired electronic circuit, a microcomputer using a program, or the like.

A DRAM121functions as a work memory to be used for controlling the lens controller120. A flash memory122stores programs, parameters, and lens authentication information (described later) to be used for the control of the lens controller120.

The zoom lens112is a lens for changing magnification of a subject image to be formed by the optical system of the interchangeable lens101. A lens structure of the zoom lens112may be of any number of lenses or of any groups of lenses. A zoom ring115is provided to an outer package of the interchangeable lens101, and transmits a rotating operation by the user to the zoom lens112. With the rotating operation, the zoom lens112moves along the optical axis of the optical system. A zoom lens position detector113detects a position of the zoom lens112. The lens controller120acquires the position of the zoom lens112from the zoom lens position detector113so as to be capable of obtain zoom magnification of the optical system. The information about the zoom magnification obtained by the lens controller120is transmitted to the camera controller153via the lens mount130.

The focus lens110is a lens for changing a focus state of a subject image incident from the optical system and formed on the CMOS image sensor150. A lens structure of the focus lens110may be of any number of lenses or of any groups of lenses. A focus lens driver111drives the focus lens110to cause the focus lens to advance and retreat along the optical axis of the optical system under the control of the lens controller120. A focus ring114is provided to the outer package of the interchangeable lens101. When the focus ring114is rotated by a user, information about an operating amount (rotation amount) of the focus ring114is transmitted to the lens controller120. The lens controller120controls the focus lens driver111based on the information about the received operating amount of the focus ring114to drive the focus lens110. The focus lens driver111can be realized by, for example, a stepping motor, a DC motor, or an ultrasonic motor.

1-4. Correspondence of Terms

The camera controller153is an example of an obtaining unit or a determining unit. The focus lens driver111is one example of a driver. The lens controller120is one example of a controller. The configuration including the focus lens110, the camera controller153, the lens controller120and the focus lens driver111is one example of the focus adjusting apparatus. The digital camera100is one example of the imaging apparatus.

2. Operation of Digital Camera

The operation of the digital camera100having the above constitution is described below.

2-1. Capturing Preparation Operation

At first, an operation for preparing for image capturing of the digital camera100is described.FIG. 2is a diagram for describing the capturing preparation operation of the digital camera100.

When the user turns on the power supply154of the camera body102with the interchangeable lens101mounted to the camera body102, the power supply154supplies power to the interchangeable lens101via the body mount140and the lens mount130(S400). The camera controller153requests the authentication information about the interchangeable lens101from the lens controller120(S401). The authentication information about the interchangeable lens101includes information about whether the interchangeable lens101is attached and information about whether an accessory is attached. The lens controller120responds to a lens authentication request form the camera controller153(S102).

The camera controller153requests the lens controller120to perform an initializing operation (S403). In response to this, the lens controller120performs the initializing operation such as resetting of a diaphragm (not shown) or resetting of an OIS lens (not shown). The lens controller120returns information about the completion of the lens initializing operation to the camera controller153(S404).

The camera controller153requests lens data from the lens controller120(S405). The lens data is stored in the flash memory122. The lens controller120reads the lens data from the flash memory122, and transmits it to the camera controller153(S406). The lens data is a characteristic value specific to the interchangeable lens101such as a lens name, an F number and a focus distance.

When the camera controller153recognizes the lens data about the interchangeable lens101mounted to the camera body102, the digital camera100becomes in a state capable of capturing an image. In this state, the camera controller153periodically requests lens state data representing the state of the interchangeable lens101from the lens controller120(S407). The lens state data includes, for example, zoom magnification information about the zoom lens112, position information about the focus lens110and aperture value information. In response to this request, the lens controller120transmits the requested lens state data to the camera controller153(S408).

In this state, the digital camera100can operate in a control mode for displaying an image represented by image data generated by the CMOS image sensor150as a through image on the LCD monitor163. This control mode is called a “live view mode”. In the live view mode, since the through image is displayed as a moving image on the LCD monitor163, the user can determine a composition for taking a still image while viewing the LCD monitor163. The user can select whether the live view mode is set. A control mode other than the live view mode to be selected by the user includes a control mode for guiding a subject image from the interchangeable lens101to the electronic viewfinder (EVF). As a method of the autofocus operation in the live view mode, a contrast detection method is suitable. This is because the CMOS image sensor150continuously generates image data in the live view mode so that the autofocus operation of a contrast detection method using the image data is made easy.

When the autofocus operation in the contrast detection method is performed, the camera controller153requests data for contrast AF from the lens controller120(S409). The data for contrast AF is data necessary for the autofocus operation in the contrast detection method, and includes, for example, a focus lens driving speed, a focus shift amount, an image magnification, and a contrast AF availability information. In response to this request, the lens controller120returns the requested data for contrast AF to the camera controller153(S410).

The digital camera100performs the capturing preparation operation in the above manner. The digital camera100performs the autofocus operation according to the contrast detection method. A so-called hill-climbing autofocus operation is described later.

2-2. Backlash Correction Operation

In a driving system mechanism to be driven by a motor, a mechanical gap having directionality called “backlash” frequently occurs. When such backlash occurs in the driving system of the focus lens110, it is difficult to recognize an accurate stop position of the focus lens110to prevent high-accuracy focusing. For this reason, the lens controller120should correct the backlash between the focus lens110and the focus lens driver111when driving the focus lens110.

The backlash correction operation differs between a case where the focus lens110starts to move from the stop position to a direction same as the moving direction before the stop (hereinafter, referred to as a “forward direction”) and a case where it starts to move from the stop position to a direction opposite to the moving direction before the stop (hereinafter, referred to as a “reverse direction”).

The backlash occurs on both sides of contact points between the focus lens110and the focus lens driver111. When the focus lens110moves from the stop position in the forward direction, the backlash at the contact point between the focus lens110and the focus lens driver111at the time of movement before the stop becomes a problem. Further, when the focus lens110moves in the reverse direction, the backlash at the contact point opposite to the contact point between the focus lens110and the focus lens driver111at the time of movement before the reversal becomes a problem.

The backlash correction operations for the movement of the focus lens110in the forward and reverse directions are described below with reference toFIGS. 3A to 4C.

FIGS. 3A to 3Care diagrams for describing the backlash correction at the time of the forward direction movement.FIGS. 3A to 3Cschematically illustrate a position relationship between the focus lens110and the focus lens driver111. InFIGS. 3A to 3C, a position A is a temporary stop position of the focus lens110. Further, direction B is a driving direction of the focus lens110.

FIG. 3Ais a diagram showing a state when the focus lens110moving to the direction B by the focus lens driver111reaches the position A. At this time, backlash is not present at the contact point between the focus lens110and the focus lens driver111. After the focus lens110stops at the position A, backlash (gap) C may occasionally occur between the focus lens110and the focus lens driver111as shown inFIG. 3Bdue to a weight of the focus lens110, external impact, or the like. With the backlash C, even though the focus lens driver111starts an operation to move the focus lens110to the direction B, the focus lens110does not start to move until the focus lens driver111moves by the backlash C. That is, after the focus lens driver111starts the operation and moves by the backlash C, the focus lens110starts to move. Such a period, caused by the backlash, from start of the operation of the focus lens driver111until the focus lens110actually moves is called “a backlash period”.

In the first embodiment, as shown inFIG. 3C, before normal lens driving, the focus lens110is moved in the forward direction (direction B), so that the backlash C is corrected (eliminated). The backlash C generally varies according the individual digital cameras100. For this reason, a value relating to the backlash C is measured at the time of manufacturing the digital camera100and is stored in the flash memory122of the interchangeable lens101.

The lens controller120transmits a driving control signal for correcting the backlash C to the focus lens driver111based on a value relating to the backlash C stored in the flash memory122. For example, when the focus driver111is composed of a stepping motor and 300 steps are required for correcting the backlash C, the lens controller120transmits a control signal for instructing the driving for 300 steps to the focus driver111. The same holds true for a case where another motor such as a DC motor or an ultrasonic motor is applied to the focus lens driver111. After the backlash C is corrected, the focus lens driver111drives the focus lens110to a next target position.

The backlash correction for the reverse direction movement is described below.FIGS. 4A to 4Care diagrams for describing the backlash correction for the reverse direction movement.FIGS. 4A to 4Cschematically illustrate the position relationship between the focus lens110and the focus lens driver111. InFIGS. 4A to 4C, the position A is a temporary stop position of the focus lens110. Further, the direction B is a driving direction of the focus lens110before the temporary stop, and a direction B′ is a driving direction of the focus lens110after the temporary stop.

FIG. 4Ais a diagram showing a state when the focus lens110moving in the direction B by the focus lens driver111reaches the position A. At this time, no backlash is present at the contact point between the focus lens110and the focus lens driver111. After the focus lens110stops at the position A, the backlash C may occasionally occur between the focus lens110and the focus lens driver111as shown inFIG. 4Bdue to the weight of the focus lens110, external impact, or the like. When the focus lens111reverses from the state of stopping at the position A, not the backlash C but backlash D present in the reverse direction becomes a problem (seeFIG. 4B).

In this embodiment, as shown inFIG. 4C, the focus lens110is moved in the reverse direction to correct the backlash D. Similarly to the backlash C, the backlash D generally varies according to the individual digital cameras100. Hence, the value relating to the backlash D is measured at the time of manufacturing the digital camera100, and is stored in the flash memory122of the interchangeable lens101. The lens controller120transmits a driving control signal for correcting the backlash D to the focus lens driver111based on the value relating to the backlash D stored in the flash memory122. For example, when the focus driver111is composed of a stepping motor and 500 steps are required for correcting the backlash D, the lens controller120transmits a control signal for instructing the driving for 500 steps to the focus driver111. The same holds true for a case where another motor such as a DC motor or an ultrasonic motor is applied to the focus lens driver111. After the correction of the backlash D, the focus lens driver111drives the focus lens110to a next target position.

The digital camera100performs the autofocus operation according to the contrast detection method. The so-called hill-climbing autofocus operation of the digital camera100, and a case where the hill-climbing autofocus operation malfunctions are described below.

FIG. 5is a diagram for describing the hill-climbing autofocus operation. The hill-climbing autofocus operation includes detecting a position of the focus lens110at which a maximum contrast value can be obtained as a focus position from image data generated by the CMOS image sensor150based on contrast values obtained for respective frames, and moving the focus lens to the detected position. InFIG. 5, a vertical axis represents the contrast value obtained for each frame and a horizontal axis represents the position of the focus lens110.

The user sets a composition, an exposure condition, and the like while viewing a through image displayed on the LCD monitor163, and then half-presses the release button160. When the camera controller153receives the half-press operation of the release button160, the focus lens110is moved from the current position to a side of the CMOS image sensor (Near side) by a predetermined distance. Thereafter, the camera controller153drives the focus lens110to a subject side (Far side). When a frame interval by which the CMOS image sensor150generates image data is, for example, 33 msec (1 sec/30 frames), the camera controller153obtains a contrast value at each 33 msec. The camera controller153, while obtaining the contrast value from the image data generated by the CMOS image sensor150, drives the focus lens110.

As shown inFIG. 5, as the position of the focus lens110is closer from the Near end to the focus position, the obtained contrast value becomes larger. When the position of the focus lens110passes the position of the focus lens110at which the contrast value becomes a peak (position “a”), the contrast value reduces. The camera controller153monitors whether the obtained contrast value becomes larger, passes the peak, and reduces. When the focus lens110passes the focus position “a”, the obtained contrast value reduces. When the obtained contrast value passes the peak and reduces, the camera controller153moves the focus lens110to return to the position “a” at which the contrast value becomes the peak.

As described above, when receiving the half-press operation of the release button160, the camera controller153performs the auto focus operation.

2-3-2. Focus Lens Driving Speed in Recording Still Images and Moving Images.

The focus lens driving speeds in recording still images and moving images are described with reference toFIGS. 6A and 6B.FIG. 6Aillustrates a relationship among the focus lens driving speed (motor pulse count), a contact sound, a driving sound, and a contrast evaluation value noise (hereinafter, referred to as a “evaluation value noise”), in the still image recording mode.FIG. 6Billustrates a relationship among the focus lens driving speed (motor pulse count), a contact sound, a driving sound, and an evaluation value noise, in the moving image recording mode.

InFIGS. 6A and 6B, the “focus lens driving speed” is represented by a pulse count per second as an example where the focus lens driver111is a stepping motor. “The contact sound” represents a level of a sound generated by contact of the focus lens110and the focus lens driver111when the focus lens111is driven at the corresponding driving speed. “The driving sound” represents a level of a sound generated by the focus lens driver111when the focus lens111is driven at the corresponding driving speed. “The evaluation value noise” represents a level of a noise included in the obtained contrast evaluation value (namely, a level of an influence on the autofocus operation).

At first, the focus lens driving speed the still image recording mode is described with reference toFIG. 6A. In the still image recording mode, different focus lens driving speeds are set between the normal driving and the backlash correction driving of the focus lens110.

At the time of the normal driving of the focus lens110, the lens controller120controls the focus lens driver111so that it is driven at a high speed, namely, 1200 pulse/sec. The focus lens110is driven at such a high speed, and thus the contact sound and the driving sound become loud. However an attention does not have to be paid to the levels of the contact sound and the driving sound, since a still image is captured and a sound is not recorded. At this time, the evaluation value noise is low, and thus the camera controller153can perform the accurate autofocus operation.

At the time of the backlash correction driving of the focus lens110, the lens controller120controls the focus lens driver111so that it is driven at 300 to 1200 pulse/sec. When the target position (focus point position) is known, the focus lens driving speed at the time of the backlash correction driving in the still image recording mode is set to the highest speed (1200 pulse/sec). When the target position (focus point position) is searched, the focus lens driving speed may occasionally be set to a lower speed. At this time, the contact sound becomes loud, but an attention does not have to be paid to the level of the contact sound because the still image is recorded. Further, the driving sound changes according to the driving speed, but since a still image is recorded and the sound is not recorded, the level of the driving sound does not have to be taken into consideration. At this time, the evaluation value noise is low, so that the accurate autofocus operation can be performed.

The focus lens driving speed in the moving image recording mode is described below with reference toFIG. 6B. In the moving image recording mode, different focus lens driving speeds are set between the normal driving and the backlash correction driving of the focus lens110.

At the time of the normal driving of the focus lens110, the lens controller120controls the focus lens driver111so that it is driven at a low speed, namely, 300 pulse/sec. When the focus lens driver111is driven at the low speed, an influence of the contact sound becomes louder. But after the backlash correction, the focus lens110already contacts the focus lens driver111, and thus another contact does not occur during the driving. As a result, the contact sound does not have to be taken into consideration. On the other hand, the driving sound and the evaluation value noise are low. Therefore, the digital camera100can perform the accurate autofocus operation while realizing silent sound driving.

At the time of the backlash correction driving in the moving image recording mode, the lens controller120controls the focus lens driver111so that it is driven at an ultralow speed, namely, 150 pulse/sec. Since the focus lens110is driven at the ultralow speed, although the contact sound and the driving sound are low, the evaluation value noise becomes loud.

As described above, the digital camera100changes the driving speed of the focus lens110between the still image recording mode and the moving image recording mode. In the still image recording mode, it is desirable that the focus lens110reaches the focus position as fast as possible. Further, a still image is recorded, and thus the contact sound and the driving sound at the time of driving the focus lens110does not have to be taken into consideration. For this reason, in the still image recording mode, the lens controller120controls the focus lens driver111to move the focus lens110at a high speed. On the other hand, in the moving image recording mode, since a sound is also recorded, it is desirable that the sound is as low as possible. Further, it is not preferable that a change in image magnification during the recording of moving images is large. For this reason, in the moving image recording mode, the lens controller120controls the focus lens driver111to move the focus lens110at a low speed.

In the digital camera100, the driving speed in the normal driving (namely, the moving speed of the focus lens110after the backlash correction) is set to be faster than the driving speed in the backlash correction in the still and moving image recording modes. The reason for this control is described below.

A relationship between the evaluation value noise and the focus lens driving speed is described with reference toFIGS. 7A and 7B.FIG. 7Ais a diagram describing the evaluation value noise when the driving speed of the focus lens110is low.FIG. 7Bis a diagram describing the evaluation value noise when the driving speed of the focus lens110is high.

As shown inFIG. 7A, when the focus lens110is driven at a low speed, a gradient of the contrast value per unit time becomes gentle. For this reason, in the autofocus operation of the contrast detection method, an influence of a change in the evaluation value noise to be superimposed on the contrast value becomes larger than a substantial change in the contrast value to be obtained. In the case where the influence of the evaluation value noise is large, as shown inFIG. 8, although the obtained contrast evaluation value does not substantially change, the camera controller153happens to determine the peak of the contrast evaluation value. That is, the camera controller153detects that the contrast value at a position that is not actually the focus position is a peak value to erroneously determine the focus position. As a result, the malfunction of the autofocus operation occurs. In order to avoid this problem, the digital camera100according to the first embodiment uses a reliability flag. Details of the reliability flag are described later.

On the other hand, as shown inFIG. 7B, when the focus lens110is driven at a high speed, the gradient of the contrast value per unit time becomes large. For this reason, in the autofocus operation of the contrast detection method, a substantial change in the obtained contrast value becomes larger than the evaluation value noise. For this reason, the above erroneous determination of the focus position hardly occurs.

In the first embodiment, as described above, the driving speeds of the focus lens110are different between the normal driving and the backlash correction of the focus lens110. Since the backlash between the focus lens110and the focus lens driver111is not necessary, it is desired to correct the backlash as fast as possible. However, since the silent driving is desired at the recording of moving images, it is desired that the focus lens driving speed is controlled to a low speed at the normal driving. On the other hand, since the contrast value is not necessary at the backlash correction driving, the increase in the evaluation value noise due to the reduction in the focus lens driving speed is not a problem. Therefore, according to the present embodiment, in the moving image recording mode, the focus lens110is driven at a low speed such that the evaluation value noise does not become loud while the silent driving is being realized, during the normal driving. On the other hand, during the backlash correction driving, in order to securely realize the silent driving, the focus lens110is driven at a speed lower than that in the normal driving (ultralow speed). For this reason, in the backlash correction driving, the obtained contrast value is prevented from being evaluated, with reference to the reliability flag (details are described later).

As described above, in the digital camera100according to the first embodiment, optimum driving speeds of the focus lens are set respectively for the backlash correction driving and the normal driving. As a result, while the silent driving is being realized, the influence of the evaluation value noise can be reduced, and thus the accurate autofocus operation can be performed.

2-3-3. Reliability Flag

The reliability flag is used for the camera controller153to determine whether the obtained contrast value can be trusted. For example, when the reliability flag shows “0”, the determination is made that the obtained contrast value cannot be trusted, and when “1”, the determination is made that the obtained contrast value can be trusted. In the following description, transmitting the reliability flag “0” refers to setting the reliability flag to OFF, and transmitting the reliability flag “1” refers to setting the reliability flag to ON. The reliability flag is set by the lens controller120, and is transmitted to the camera controller153. Before the backlash correction operation is started in the moving image recording mode, the lens controller120sets the reliability flag to “0”, and after the backlash correction operation is ended in the moving image recording node, the reliability flag is set to “1”.

2-3-4. Process Using Reliability Flag

A process for avoiding a malfunction of the autofocus operation while realizing the low-speed driving at the backlash correction in the recording of moving images using the reliability flag is described, with reference toFIG. 9.

First, the digital camera100is set to the still image recording mode. When the still image recording mode is set in the camera body102(S500), the camera controller153informs the lens controller120of the setting, and the still image recording mode is set also on the side of the interchangeable lens101(S501). At this time, the driving speed of the focus lens110and the driving speed of the backlash correction are set with values for the still image recording mode.

The camera controller153monitors whether the moving image recording button161is pressed down by the user (S502). When the moving image recording button161is pressed down in the still image recording mode, the camera controller153sets the camera body102into the moving image recording mode (S503). The camera controller153informs the lens controller120that the moving image recording mode is set (S504).

In response to this information, the lens controller120sets the interchangeable lens101into the moving image recording mode (S505). At this time, the driving speed of the focus lens110and the driving speed of the backlash correction are set with values for the moving image recording mode (S506). As described above, the driving speed of the backlash correction at the recording of moving images is set to be slower than the driving speed at the recording of still images or a frame rate of image data.

Subsequently, the camera controller153monitors whether the autofocus operation is instructed (S507). The instruction of the autofocus operation includes an instruction based on a user's operation of the release button160and an instruction based on a change in the focus state of a subject. When the autofocus operation is instructed, the camera controller153transmits the instruction for driving the focus lens to the lens controller120(S508).

When the lens controller120receives the instruction for driving the focus lens, it transmits the reliability flag to the camera controller153(S509). That is, the lens controller120turns off the reliability flag at step S509(the contrast value is not reliable).

After the lens controller120turns off the reliability flag, it performs the backlash correction operation on the focus lens110and the focus lens driver111(S511). The camera controller153refers to the reliability flag. When the reliability flag is OFF, the camera controller153does not use the contrast value obtained during the backlash correction as an evaluation value for determining the focus position (S510).

As described above, in the moving image recording mode, the backlash correction is made for a longer period than the period for the backlash correction and the frame interval of the image data in the still image recording mode. Therefore, there is a possibility that the camera controller153may erroneously detect the peak of the contrast value. In the present embodiment, the camera controller153does not use the contrast value obtained during the backlash correction for the determination of the focus position (S510). As a result, the peak of the contrast value, namely, the erroneous determination of the focus position can be avoided.

When the backlash correction is ended, the lens controller120sets the reliability flag on (S512). The lens controller120starts to move the focus lens110with the focus lens driver111in order to determine the focus position (S513). That is, the above-described hill-climbing autofocus operation is started. At this time, the camera controller153evaluates the contrast value to be obtained (S514). When detecting the position to be the peak of the contrast value, the camera controller153determines the detected position as the focus position (S515).

As described above, the digital camera100sets the reliability flag off during the backlash correction in the moving image recording mode, and does not use the obtained contrast value for the determination of the focus position. As a result, particularly in the recording of moving images, the digital camera100can realize the low-speed driving of the focus lens for silent driving without causing the problem due to the backlash correction.

In particular, an idea of the present embodiment is effective when a heavy-weighted lens is driven. In order to drive the heavy-weighted lens, a plurality of gears are used for low-speed driving so that a torque is raised. Backlash is stored by the increase in the number of gears to be used, and thus influence of the backlash becomes large. Even in such a case, the digital camera100sets the reliability flag off during the backlash correction, and does not use the obtained contrast value for the determination of the focus position. Hence, the digital camera100can perform the accurate autofocus operation.

In recording of the moving images and still images, regarding setting of the driving speed of the focus lens driver111, the driving speed is set so that the focus lens driver111corrects backlash, for a time longer than a cycle with which the contrast value is obtained at recording of moving images, and for a time shorter than the cycle at recording of still images. As a result, the digital camera100drives the focus lens110at a high speed at recording still images, and drives the focus lens110at a low speed at recording moving images.

The digital camera100according to the first embodiment includes the optical system containing the focus lens110, the focus lens driver111for moving the focus lens110along the optical axis of the optical system, the camera controller153periodically obtains an evaluation value of a subject image formed via the focus lens110, the camera controller153for determining a target position to which the focus lens110moves based on the evaluation value, and the lens controller120for controlling the operation of the focus lens driver111based on the determined target position. The lens controller120controls the focus lens driver111to move the focus lens110at a first speed for a first period including the backlash period which is defined from the start of the operation of the focus lens driver111to the start of the actual movement of the focus lens110. The lens controller120controls the focus lens driver111to move the focus lens110at a second speed faster than the first speed for a second period after the end of the first period. With such a constitution, the digital camera100can switch the driving speed of the focus lens110so that the driving speed becomes optimum respectively for the first period including the backlash period and the second period after the end of the first period.

The digital camera100according to the present embodiment includes the optical system including the focus lens110, the focus lens driver111for moving the focus lens110along the optical axis of the optical system, the camera controller153for periodically obtaining an evaluation value of a subject image formed via the focus lens110, the camera controller153for determining a target position to which the focus lens110moves based on the evaluation value, the lens controller120for controlling the operation of the focus lens driver111based on the determined target position, the lens controller120for setting the reliability flag indicating the backlash period defined from the start of the operation of the focus lens driver111to the start of the actual movement of the focus lens110. The lens controller120stops the operation for determining the target position based on the evaluation value while the reliability flag indicates the backlash period. With such a constitution, the digital camera100can prevent a malfunction of the focus lens driver111based on an inaccurate evaluation value at the backlash period.

4. Other Embodiments

The above example describes the case where the hill-climbing autofocus operation is performed in both the recording of still images and the recording of moving images, but the autofocus operation is not limited to this. That is, the hill-climbing autofocus operation may be performed at the recording of still images according to pressing of the release button, and a continuous autofocus operation may be performed at the recording of moving images. Also in this case, it goes without saying that the speed of the backlash correction at the recording of moving images is set to be lower than that in the still image recording mode or the frame rate of the image data.

The above example describes the case where the contrast value obtained when the reliability flag is off is not evaluated for the determination of the focus position, but the process based on the reliability flag is not limited to this. That is, when the reliability flag is off, the contrast value may not be obtained. In other words, the digital camera100according to the embodiment does not determine a focus position based on the contrast value obtained for the period corresponding to the backlash correction.

In the above example, after the backlash correction is ended, the lens controller120sets the reliability flag on, but the method for changing the reliability flag is not limited to this. That is, the lens controller120may estimate timing at which the backlash correction is ended, and set the reliability flag on earlier by a cycle at which the camera controller153obtains the contrast value. In other words, the digital camera100according to this embodiment does not determine a focus position based on the contrast value obtained for the period corresponding to the backlash correction.

In the above example, the driving speed of the focus lens driver111is heightened when the focus lens110is started to move after the backlash correction is ended, but the method for changing the driving speed is not limited to this. That is, the lens controller120may estimate the timing at which the backlash correction is ended and heighten the driving speed of the focus lens driver111earlier by a cycle at which the camera controller153obtains the contrast value. In other words, during the backlash correction driving, the timing, at which the backlash correction driving is changed to the normal driving for determining the focus position based on the obtained contrast value, may be estimated and the driving speed of the focus lens driver111may be heightened.

The above example describes the case of the digital camera100composed of the interchangeable lens101and the camera body102, but the configuration of the digital camera is not limited to this. That is, the idea of the embodiment can be applied to even a camera in which a lens is integral with a body.

The above example describes the digital camera as one example of an imaging apparatus, but the idea of the embodiment can be applied to a focus adjusting apparatus to be used in an imaging apparatus such as a movie camera or a mobile phone with a camera.

INDUSTRIAL APPLICABILITY

The aforementioned embodiment is useful for a focus adjusting apparatuses to be used in an imaging apparatuses such as a digital camera, a movie camera, or a mobile phone with a camera.