Abstract:
An autofocus system is provided that includes a focus-verification image-recapturing processor and a focus-verification image display processor. The focus-verification image-recapturing processor sets ISO sensitivity to a value lower than a predetermined value and increases an exposure value by adjusting at least one of an aperture value and an exposure time to carry out a focus-verification image-recapturing operation under focusing conditions applied in an antecedent autofocus operation when ISO sensitivity that is set in said auto focus operation exceeds the predetermined value. The focus-verification image display processor displays a focus-verification image obtained by the focus-verification image-recapturing operation when the ISO sensitivity that is set in the auto focus operation exceeds the predetermined value.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a device that displays a focusing image for verification during an autofocus operation performed by a digital camera. 
         [0003]    2. Description of the Related Art 
         [0004]    A contrast-detect autofocus (AF) function has been employed by various types of digital cameras where, in general, a focusing image is temporarily displayed on a monitor of the camera for a photographer to check whether or not the image is adequately in focus. However, the size of the monitor provided on the camera is usually not sufficient for a photographer to verify the quality or conditions of the focusing image when the entire image is displayed on the display. In order to overcome such problems, a camera disclosed in KOKAI 2004-242010 enlarges an area with the highest degree of focusing and further indicates a subarea within the enlarged image having the highest focusing degree by framing the subarea. 
       SUMMARY OF THE INVENTION 
       [0005]    However, when the focusing image is dark, visibility of the focusing image is not sufficient even though a part of the focusing image is enlarged and displayed on the monitor. On the other hand, when ISO sensitivity is increased to make the image brighter, noise also increases and thus visibility deteriorates. 
         [0006]    Accordingly one aspect of the present invention is to provide a system that enables a focusing image with high visibility to always be displayed on the monitor of a digital camera. 
         [0007]    According to the present invention, an autofocus system or a camera that includes a focus-verification image-recapturing processor and a focus-verification image-display processor is provided. The focus-verification image-recapturing processor sets ISO sensitivity to a value lower than a predetermined value and increases the exposure value by adjusting at least one of an aperture value and an exposure time to carry out a focus-verification image-recapturing operation under focusing conditions established in an earlier autofocus operation when ISO sensitivity, which is set in said auto focus operation, is higher than the predetermined value. The focus-verification image display processor displays a focus-verification image obtained by the focus-verification image-recapturing operation when the ISO sensitivity that is set in the autofocus operation is higher than the predetermined value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
           [0009]      FIG. 1  is a block diagram schematically illustrating the general structure of a digital camera to which an autofocusing operation of an embodiment of the present invention is applied; 
           [0010]      FIG. 2  is an example of a live view image displayed on the monitor before the contrast-detect autofocusing operation (CAF operation) is carried out; 
           [0011]      FIG. 3  is a schematic of an enlarged focus-verification image captured at high ISO sensitivity; 
           [0012]      FIG. 4  is a schematic of an enlarged focus-verification image captured by a focus-verification image-recapturing operation of the present embodiment at low ISO sensitivity; 
           [0013]      FIG. 5  is a flowchart illustrating an autofocus operation and a focusing image verification operation carried out in the present embodiment; and 
           [0014]      FIG. 6  is a timing chart illustrating the relationship between vertical sync signals VD and rolling shutter during the CAF operation and the focus-verification image-recapturing operation. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0015]    The present invention is described below with reference to the embodiments shown in the drawings. 
         [0016]      FIG. 1  is a block diagram schematically illustrating the general structure of a digital camera to which an autofocusing operation of an embodiment of the present invention is applied. 
         [0017]    The digital camera  10  may be a digital single-lens reflex camera, but it is not limited to this type. Light made incident to a lens system  11  forms an image on an imaging surface of an image sensor  14 , for example, through an aperture  12  and a color filter array  13 . The image sensor  14  may be controlled by drive signals from an image sensor driver  15 . Image signals obtained by the image sensor  14  may be fed to an image signal processor  16  to be subjected to various types of image signal processing that are well known in the art, and in turn, the image may be displayed on a monitor  17 . At this time, an ISO controller  18  may control the output gain (ISO sensitivity) of the image signals from the image sensor  14 . 
         [0018]    An AF controller  19  may control the positions of the lenses in the lens system  11  to carry out the autofocus process. Further, an aperture controller  20  may control the size of the aperture  12 . Note that the ISO controller  18 , the AF controller  19 , and the aperture controller  20  are controlled by instructions from a controller  21 , and the controller  21  may perform various types of processes based on the manipulation of switches in a switch group  22 . The switch group  22  may include a release switch, an AF button, dial switches, a touch panel, etc. 
         [0019]    In the present embodiment, a contrast-detect method is applied in the autofocus operation. With reference to  FIGS. 1 to 3 , an outline of the autofocus operation and focusing image verification operation of the present embodiment are explained. 
         [0020]      FIG. 2  is an example of a live view image displayed on the monitor  17  before the contrast-detect autofocusing operation (CAF operation) is carried out. Pixel signals within an actual pixel area (or an effective pixel area) A 1  may be read from the image sensor  14 . Four brackets M 1 -M 4  are marks for indicating an AF point. In the present embodiment, area A 2  is the part of area A 1  that is used as the autofocus area and it includes the center area surrounded by the brackets M 1 -M 4 . The autofocus operation is carried out with reference to the contrast between images within the partial area A 2 . 
         [0021]    Namely, in the CAF operation of the present embodiment the area A 2 , which is a portion of the actual pixel area (or the effective pixel area) that is normally sourced for image data, is preset as the focus area and a high frame-rate mode is activated so that the image sensor  14  is driven at a high frame-rate to capture data from the focus area A 2 . And in the CAF operation of the present embodiment, only the pixel signals within the focus area A 2  may be captured repeatedly to carry out the autofocus operation, which uses contrast-detection in association with an operation of the lens system  11  that uses the AF controller  19 . Further, in the present embodiment, a focusing still image (may be an in-focus image) of the area A 2  may be enlarged and displayed on the monitor  17  for a predetermined time (e.g., about 2 seconds) as a focus-verification image when the CAF operation finishes. 
         [0022]    However, in the CAF operation of the present embodiment, high ISO sensitivity (gain) is required to display a bright focus-verification image when a dark object is photographed. Moreover, determination of the focus quality may be difficult when an enlarged image of the area A 2  is output to the monitor under a gain corresponding to high ISO sensitivity, because high ISO sensitivity also increases the noise level, as illustrated in  FIG. 3 . 
         [0023]    Therefore, in the present embodiment, the image of the object is recaptured under the same focusing conditions with relatively low ISO sensitivity when the image-capturing and autofocusing procedures of the CAF operation are performed with ISO sensitivity higher than the predetermined sensitivity. At the same time, the controller  21  instructs the AF controller  19  and the aperture controller  20  to adjust an exposure time and/or aperture value (f number) to achieve an appropriate exposure value that compensates for reduced ISO sensitivity from the ISO controller  18 . 
         [0024]    As illustrated in  FIG. 4 , a focusing image of the focusing area A 2  captured at low ISO sensitivity is enlarged and displayed on the monitor  17 . Thereby, the focusing image with reduced noise and high S/N ratio is displayed on the monitor  17  for a photographer to verify the focusing quality. 
         [0025]    With reference to  FIGS. 1 ,  2 , and  5 , the autofocus operation and the focusing image verification operation of the present embodiment will be explained. 
         [0026]    While live view is displayed on the monitor  17 , the controller  21  may execute the procedures illustrated in the flowchart of  FIG. 5  in parallel with the live view operation. In Step S 100 , whether or not the autofocus (AF) switch (not shown) of the switch group  22  has been activated is determined, and this determination is repeated until the AF switch is turned on. 
         [0027]    Once it is determined in Step S 100  that the AF switch (not shown) is in the ON position, the CAF operation starts at Step S 102 . In the CAF operation, the ISO sensitivity is set based on a photometric value, and in turn pixel signals of focusing area A 2  of  FIG. 2  are sequentially read from the image sensor  14 . Further, the AF operation that refers to the contrast of the sequentially output images of the focusing area A 2  is carried out. 
         [0028]    When the CAF operation finishes at Step S 104 , the focus conditions of the lens system  11  are retained and the last focusing image obtained within the focusing area A 2  in the CAF operation is stored in an image memory as a focus-verification image. At Step S 106 , it is determined whether or not the ISO sensitivity (gain) that is set during the autofocus operation is lower than a predetermined value. When the ISO sensitivity is lower than the predetermined value, such that when it is determined that the ISO sensitivity is sufficiently low, the focus-verification image (the image within area A 2 ), which is stored in the image memory, is enlarged and displayed on the monitor  17  at Step S 108 . 
         [0029]    On the other hand, when it is determined that the ISO sensitivity is higher than the predetermined value at Step S 106 , such that when it is determined that the ISO sensitivity is too high from the aspect of an acceptable noise level, the ISO sensitivity is reset to a low value (a value lower than the above predetermined value) at Step S 112 . Further, the aperture value Av and the shutter speed Tv are adjusted under the retained focus conditions of the CAF operation and a focus-verification image corresponding to area A 2  is captured under the appropriate exposure value Ev. Subsequently, the focus-verification image stored at Step S 104  in the image memory is replaced by the new focus-verification image, which is captured at Step S 112 . 
         [0030]    Note that if the aperture value Av corresponds to the full aperture when recapturing the new focus-verification image, a shallow focus is obtained, which allows for easy verification and enables a high shutter speed Tv. However, the aperture value Av is not restricted to only the full aperture and may be set to any value. Further, it may be configured so that a user sets the aperture value Av. When the recapturing process for the focus-verification image is completed in Step S 112 , the focus-verification image corresponding to the focus area A 2  is enlarged and displayed on the monitor  17 . 
         [0031]    In Step S 110 , it is determined whether the focus-verification image displayed on the monitor  17  has been canceled or whether it has been displayed for a predetermined time. The cancellation of the focus-verification image display may be carried out by operating a predetermined button (not shown) of the switch group  22 , and the predetermined time may be set at about two seconds. When it is determined that either cancellation of the focus-verification image or its display for the predetermined time has taken place, the focusing image verification process in the CAF operation is terminated, and in turn, the normal live view image is displayed on the monitor  17  and the camera stands by in readiness for shooting. On the other hand, when neither cancellation of the focus-verification image nor its display in excess of the predetermined time has been detected, the focusing image verification operation returns to Step S 108  and the enlarged display of the focus-verification image continues. 
         [0032]    With reference to the timing chart of  FIG. 6 , the difference between the image-capturing operations in the CAF operation and the focus-verification image-recapturing operation is explained.  FIG. 6  is a timing chart illustrating the relationship between vertical sync signals VD and a rolling shutter during the CAF operation and the focus-verification image-recapturing operation. 
         [0033]    Since the image sensor  14  is driven at a high frame-rate mode during the CAF operation, the period of the vertical sync signals VD is decreased and the available maximum exposure time is therefore shortened. In  FIG. 6 , the exposure time Tv 1  for the CAF operation is set to the maximum value, which corresponds to the period of the vertical sync signals VD in the CAF operation. 
         [0034]    On the other hand, in the focus-verification image-recapturing operation of Step S 112  ( FIG. 5 ), a relatively low ISO sensitivity is set and a relatively small aperture value (smaller f number or larger lens opening) Av and/or a relatively slow shutter speed (longer exposure time) Tv 2  are selected compared to those in the CAF operation. In  FIG. 6 , the shutter speed Tv 2  (exposure time) of the recapturing operation is approximately doubled with respect to Tv 1  while the aperture value Av is set to the minimum or full aperture. Note that in the example illustrated in  FIG. 6 , an extended period of the vertical sync signal VD is required in order to slow down the shutter speed Tv 2  or extend the exposure time. Therefore, in  FIG. 6 , the period of the vertical sync signals VD is also doubled from that of the CAF operation, and the frame rate is replaced. 
         [0035]    As described above, according to the present embodiment, a focus-verification image of high visibility is always available on the monitor of the digital camera. Namely, in the present embodiment, an image used in focusing verification is recaptured under low ISO sensitivity when the CAF operation is carried out under high ISO sensitivity to capture the image of a dark object. Thereby, a focus-verification image with less noise can be displayed on the monitor and the visibility of the focus-verification image on the monitor is improved. This is especially advantageous when employing a high frame rate to reduce the time of the CAF operation. Further, in the present embodiment, since an enlarged focus-verification image is displayed on the monitor, verification of the focusing image can be more easily facilitated. 
         [0036]    Although the embodiment of the present invention has been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention. 
         [0037]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2010-184386 (filed on Aug. 19, 2010), which is expressly incorporated herein, by reference, in its entirety.