Abstract:
A focusing image display device is equipped with an autofocus processor, a spatial frequency detector, a magnification determiner and a focusing image display processor. The autofocus processor performs an autofocus operation using an image within a partial area of an effective pixel area. The spatial frequency detector calculates a spatial frequency of a focusing image within the partial area after the autofocus operation is completed. The magnification determiner determines the magnification of the focusing image in accordance to the spatial frequency. The focusing image display processor modifies the resolution of the focusing image with respect to the magnification.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a device for displaying a focusing image on a monitor of a digital camera and a method thereof. 
         [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 represented 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, since there are many different types of photography, such as portrait photography, landscape photography and so on, a preferred magnification of the focusing image depends on the particular type of the subject being photographed. 
         [0006]    Therefore, one aspect of the present invention is to accommodate the magnification of a focusing image based on the type of the subject being photographed. 
         [0007]    According to the present invention, a focusing image display device is provided that comprises an autofocus processor, a spatial frequency detector, a magnification determiner and a focusing image display processor. 
         [0008]    The autofocus processor performs an autofocus operation using an image within a partial area of an effective pixel area. The spatial frequency detector calculates a spatial frequency of a focusing image within the partial area after the autofocus operation is completed. The magnification determiner determines a magnification of the focusing image in accordance with the spatial frequency. The focusing image display processor modifies the resolution of the focusing image based on its magnification. 
         [0009]    According to another aspect of the present invention, a digital camera is provided that comprises the autofocus processor, the spatial frequency detector, the magnification determiner and the focusing image display processor. 
         [0010]    Further, according to another aspect of the present invention, a method for displaying a focusing image is provided that includes carrying out an autofocus operation using an image within a partial area of an effective pixel area, calculating a spatial frequency of a focusing image within the partial area after the autofocus operation is completed, determining the magnification of the focusing image in accordance to the spatial frequency, and modifying the resolution of the focusing image with respect to its magnification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which: 
           [0012]      FIG. 1  is a block diagram schematically illustrating the general structure of a digital camera of an embodiment of the present invention; 
           [0013]      FIGS. 2A and 2B  show two typical images having different spatial frequencies and their respective areas that are used in the autofocus operation; 
           [0014]      FIG. 3  is a graph representing the relationship between the spatial frequency and a magnification of the focusing image displayed for verification; and 
           [0015]      FIG. 4  is a flowchart showing the processes for the CAF operation and a display operation for the focusing image. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The present invention is described below with reference to the embodiments shown in the drawings. 
         [0017]      FIG. 1  is a block diagram schematically illustrating the general structure of a digital camera to which an embodiment of the present invention is applied. 
         [0018]    The digital camera  10 , for example, is a digital single-lens reflex camera. Light made incident to a lens system  11  forms an image on an imaging surface of an image sensor  14  (e.g., a CMOS image sensor) 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 . 
         [0019]    An AF controller  19  may control the positions of the lenses in the lens system  11  to carry out the autofocus operation. Further, an aperture controller  20  may control the size of the aperture  12 . Note that the image sensor driver  15 , 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, including the autofocus operation (detailed later), based on the manipulation of switches in a switch group  22 . The switch group  22  may include a release switch, AF button, dial switches, a touch panel, etc. 
         [0020]    With references to  FIGS. 2A ,  2 B and  3 , an outline of a focusing-image display verification method for the present embodiment will be explained. Note that in the present embodiment a contrast-detect autofocus (CAF) operation is applied. 
         [0021]      FIGS. 2A and 2B  represent captured images of two dissimilar types of objects having different spatial frequencies. In  FIGS. 2A and 2B , CAF areas A 2  and A 3  are indicated within an actual pixel area (or an effective pixel area) A 1 , and are used in a CAF operation to compare the contrast between them. In  FIG. 2A , a person&#39;s face in large scale is positioned at the center of the image. On the other hand, in  FIG. 2B , three people are imaged with distant background view. In general, images of the type in  FIG. 2B  have higher spatial frequency compared to images of the type in  FIG. 2A , which makes it relatively difficult to verify the focusing degree of an image having a high spatial frequency like the one in  FIG. 2B . 
         [0022]    Therefore, in the present embodiment a focusing image within the CAF area is magnified with the focus point at its center and displayed on the monitor  17  (see  FIG. 1 ) after the CAF operation is completed, with the magnification being controlled in accordance to the spatial frequency within the CAF area. The relationship between the spatial frequency and the magnitude of the present embodiment is represented in the graph of  FIG. 3 . Namely, in the present embodiment, a focusing image is enlarged by a greater magnification as the spatial frequency is increased for display on the monitor  17 . Note that as in the example indicated in  FIG. 3 , in a lower spatial frequency band the magnification linearly increases as the spatial frequency increases until it surpasses a predetermined spatial frequency, at which point the magnification is sustained at a constant (the maximum magnification). However, the relationship between the spatial frequency and the magnification is not restricted to the present example. Further, the focusing image may be larger than the CAF area, such that an area surrounding the CAF area may be included. 
         [0023]    Next with reference to the flowchart of  FIG. 4 , a focusing-image display operation of the present embodiment will be explained. Note that the focusing-image display operation is carried out mainly by the controller  21  (see  FIG. 1 ). 
         [0024]    The operations indicated in  FIG. 4  are carried out when a through-the-lens image (live preview) is displayed on the monitor. When an AF request is detected in Step S 100 , a CAF operation is initiated in Step S 102 . Namely, a CAF operation is carried out in cooperation with the AF controller  19  by comparing the contrast between images captured successively within the CAF area (see  FIG. 1 ). 
         [0025]    When the CAF operation of Step S 102  is completed, whether or not a request for a focus-verification assist has been detected is determined in Step S 104 . The focus-verification assist request designates whether or not to perform the above-mentioned magnifying control while a focusing image is displayed. For example, whether the request exists is determined by whether or not the focus-verification assist mode has been preset by a user. 
         [0026]    When it is determined that the focus-verification assist request does not exist, this process is immediately terminated and the normal through-the-lens operation or live preview starts. However when it is determined that the focus-verification assist request exists, the last image within the CAF area during the CAF operation of Step S 102  is stored in memory (not shown) at Step S 106 . The spatial frequency of the image stored in memory is then calculated and, for example, a magnification to the size of the through-the-lens image is determined with reference to a lookup table that corresponds to the graph in  FIG. 3  in Step S 108 . 
         [0027]    In Step S 110  the resolution of an image of the CAF area, which is stored in the memory, is modified based on the magnification obtained in Step S 108  and, in turn, the image is magnified and displayed on the monitor  17  (see  FIG. 1 ). In Step S 112 , a determination is made as to whether or not either the display of the focusing image on the monitor  17  has been canceled or a predetermined time has elapsed from the beginning of the display of the focusing image. The representation may be cancelled by the manipulation of a predetermined button (not shown) included in the switch group  22  and, for example, the predetermined time may be preset to approximately 2 seconds. 
         [0028]    The determination of Step S 112  is repeated until either the display of the focusing image has been cancelled or the predetermined time has elapsed. When either one of these events occurs the focusing-image display operation is completed and the normal through-the-lens image display or live preview resumes. 
         [0029]    As described above, according to the present embodiment, a focusing image can be enlarged at an appropriate magnification associated with the spatial frequency of an object image and displayed on a monitor. 
         [0030]    Note that only the pixel signals from the CAF area within the effective pixel area may be read out to perform an accelerated CAF operation. Further, this CAF operation and method of displaying the focusing-image in this invention may be combined, such that the size of a focusing-image displayed on the monitor may be determined from the spatial frequency of the CAF area. 
         [0031]    Further, although the present embodiment has been described for a single-lens reflex camera, the present invention is not restricted to a digital camera and may also be applied to a device, including a cell phone and the like, which is provided with a digital camera. 
         [0032]    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. 
         [0033]    The present disclosure relates to subject matter contained in Japanese Patent Application No. 2010-200880 (filed on Sep. 8, 2010), which is expressly incorporated herein, by reference, in its entirety.