Abstract:
A digital camera maintains a consistent effective focal length upon switching from a first focus mode to a second focus mode, the focus modes having different maximum focal lengths. Maintaining a consistent effective focal length across focus mode changes obviates the need for a user to reframe a scene upon changing focus modes.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to digital cameras and more specifically to digital cameras with multiple focus modes having different focusing ranges. 
     BACKGROUND OF THE INVENTION 
     Some digital cameras, like their film counterparts, include a zoom lens with multiple focus modes, each focus mode having a different focusing range. For example, a zoom lens may have a macro focus mode and a normal focus mode. A macro focus mode allows the lens to focus on objects at a shorter minimum distance for a given focal length than the normal focus mode. This mode is convenient for taking extreme close ups. 
     Depending on the lens design, the range of available focal lengths in each focus mode may be different. In particular, the maximum focal length may be smaller in one focus mode than in the other. For example, a zoom lens may be designed to provide focal lengths of 35 mm to 105 mm in normal focus mode and 35 mm to 75 mm in macro focus mode. A user may zoom out to 105 mm while framing a subject in normal focus mode, only to decide that macro focus mode is the better choice. Upon switching to macro focus mode, however, the digital camera necessarily drops to a smaller focal length within the available range (e.g., 75 mm) for macro focus mode, forcing the user to reframe the picture. 
     It is thus apparent that there is a need in the art for an improved method and apparatus for maintaining a consistent effective focal length in a digital camera. 
     SUMMARY OF THE INVENTION 
     A method for maintaining a consistent effective focal length in a digital camera is provided. An apparatus for carrying out the method is also provided. 
     Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are functional block diagrams of a digital camera in accordance with an illustrative embodiment of the invention. 
         FIGS. 2A and 2B  are a flowchart of the operation of the digital camera shown in  FIGS. 1A and 1B  in accordance with an illustrative embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In a digital camera having multiple focus modes, each focus mode having a different maximum optical focal length, a consistent effective focal length may be maintained across focus-mode changes by combining optical and digital zoom. Digital zoom is typically accomplished by cropping a digital image and, optionally, applying pixel interpolation techniques well known in the image processing art to rescale the image to its original size. In some implementations, interpolation is applied to the previewed version of a digital image shown on the display of the digital camera but not on the final image that is captured. As those skilled in the art will recognize, digital zoom results in some loss of image quality. Therefore, maximum optical zoom may be employed, whenever possible, to achieve the highest possible image quality. Two introductory examples will serve to illustrate the principles of the invention. In both examples, the digital camera is assumed to have a normal focus mode with a focal length range of 35 mm to 105 mm and a macro focus mode with a focal length range of 35 mm to 75 mm. The macro focus mode may focus at distances ranging from, for example, 0.1 m to 0.7 m. The normal focus mode may focus at distances ranging from, for example, 0.5 m to infinity. 
     In the first example, a user attempts to frame a subject at an optical focal length of 105 mm in normal focus mode. The user then decides to switch to macro focus mode to focus at a shorter distance. Consequently, the zoom lens must drop to a maximum focal length of 75 mm. By applying digital zoom at 1.4×, however, the same effective focal length of 105 mm may be maintained. This allows the user to continue composing the shot without having to reframe the subject. 
     In the second example, a user attempts to frame a subject using a combination of digital zoom at 1.33× and optical zoom at 75 mm for an effective focal length of 100 mm while the digital camera is in macro focus mode. The user then decides to switch to normal focus mode. In this case, as much optical zoom as possible may be applied to maintain the same effective focal length of 100 mm. In fact, since 100 mm is less than the maximum focal length of 105 mm in normal focus mode, the effective focal length may be maintained entirely optically. Using as much optical zoom as possible in this situation maximizes the quality of the captured image. 
     Some digital cameras may have a zoom lens in which the normal focus mode has a smaller maximum focal length than the macro focus mode. The same techniques illustrated in the above examples still apply, however, depending on the effective focal length prior to a focus mode change and whether the focus mode change is to a focus mode having a smaller or larger maximum optical focal length. 
     An illustrative embodiment of the invention may be summarized in general terms as follows. A digital camera has at least a first focus mode and a second focus mode, each of which is designed to focus within a predetermined distance range. In some implementations, the digital camera may have more than two focus modes. For example, the first focus mode may be a normal focus mode, and the second focus mode may be a macro focus mode, or vice versa. The first focus mode has a first maximum focal length (“MAX. FL1”). The second focus mode has a second maximum focal length (“MAX. FL2”) that is different from MAX. FL1. The “effective focal length” (“EFL”) of the digital camera throughout this description is the overall magnification factor at which a digital image is captured, whether produced entirely by an optical zoom lens or by a combination of an optical zoom lens and digital zoom. 
     Table 1 summarizes the operation of this illustrative embodiment of the invention, in each of two situations, when the focus mode is switched from the first focus mode to the second focus mode. 
     
       
         
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 EFL &gt; MAX. FL2, 
                 EFL &gt; MAX. FL1, 
               
               
                 MAX. FL1 &gt; MAX. FL2 
                 MAX. FL1 &lt; MAX. FL2 
               
               
                   
               
             
             
               
                 Apply sufficient digital zoom to 
                 Apply as much optical zoom as possible 
               
               
                 maintain effective focal length. 
                 to maintain effective focal length. 
               
               
                   
               
             
          
         
       
     
     As shown in Table 1, digital zoom may be applied to maintain the EFL in the second focus mode whenever the EFL in the first focus mode exceeds MAX. FL2 and MAX. FL1 is greater than MAX. FL2. Additionally, the maximum amount of available optical zoom may be applied to maintain the EFL in the second focus mode whenever the EFL in the first focus mode exceeds MAX. FL1 and MAX. FL1 is less than MAX. FL2. 
       FIGS. 1A and 1B  are functional block diagrams of a digital camera  100  in accordance with an illustrative embodiment of the invention. Digital camera  100  may be a digital still camera, a digital video camera, a combination digital still/video camera, or any other device capable of focusing and digitizing an optical image. In  FIG. 1A , controller  105  communicates over data bus  110  with imaging module  115 , memory  120 , display  125 , focus-mode selection function  130 , and focal length compensation module  135 . Lens system  140  produces optical images that are converted to digital images by imaging module  115 . In a typical implementation, lens system  140  may comprise a zoom lens having multiple focus modes, each focus mode having a different focusing range (e.g., normal mode and macro mode). Focus-mode selection function  130  selects among the different focus modes. Focus-mode selection function  130  may be actuated by a user input device such as a button or menu option. Imaging module  115 , in a typical implementation, may comprise an array of photosensors based on charge-coupled-device (CCD) or CMOS technology, an analog-to-digital converter (A/D), a gain control, and a digital signal processor (DSP) (not shown in  FIG. 1A ). Memory  120  further comprises random access memory (RAM)  145  and non-volatile memory  150 . 
     Referring to  FIG. 1B , Focal Length Compensation Module  135  may further comprise Modules Focal Length Compensation Control  155 , Determine Effective Focal Length  160 , Digital Zoom  165 , and Select Optical Focal Length  170 . These modules may, for example, be executed by controller  105 . Module Focal Length Compensation Control  155  manages the process of maintaining a consistent effective focal length across focus mode changes by invoking the other modules as needed. Module Determine Effective Focal Length  160  retrieves, measures, or computes the effective focal length achieved by lens system  140  or by lens system  140  in combination with digital zoom. Since lens system  140  is typically controlled electromechanically with high precision, the optical focal length of lens system  140  may be set to a specific value by controller  105  (executing module Select Optical Focal Length  170 ) and tracked in, for example, a register or other memory location. Therefore, Module Determine Effective Focal Length  160  may determine the effective focal length by simply reading the current optical focal length of lens system  140  and, if applicable, multiplying the optical focal length by the digital zoom factor. Module Digital Zoom  165  crops digital images and optionally rescales them to their original size as explained earlier in this description. Module Select Optical Focal Length  170  sets lens system  140  to a particular optical focal length in accordance with the allowable focal length range of the applicable focus mode. The functional boundaries characterizing Modules Focal Length Compensation Control  155 , Determine Effective Focal Length  160 , Digital Zoom  165 , and Select Optical Focal Length  170  are purely conceptual. For example, these functions may be implemented as four separate software or firmware modules, as fewer than four software or firmware modules, or in any other fashion that is advantageous, including custom hardware or a combination of hardware and software/firmware. 
       FIGS. 2A and 2B  are a flowchart of the operation of digital camera  100  shown in  FIGS. 1A and 1B  in accordance with an illustrative embodiment of the invention. At  205  in  FIG. 2A , lens system  140  of digital camera  100  is in the first focus mode. As explained above, whether the first focus mode is a macro focus mode or a normal focus mode is arbitrary. The same principles apply in either case. If a request to switch to the second focus mode is received from focus-mode selection function  130  at  210 , Module Determine Effective Focal Length  160  acquires the current effective focal length (EFL) at  215 . At  220 , Module Focal Length Compensation Control  155  determines whether MAX. FL1 is greater than MAX. FL2. If so, control proceeds to  225 . Otherwise, control proceeds to  245  in  FIG. 2B . (Note that, since it is assumed that MAX. FL1 and MAX. FL2 are not equal, MAX. FL1 must be less than MAX. FL2 if the test at  220  is not true.) At  225 , Module Focal Length Compensation Control  155  determines whether the EFL exceeds MAX. FL2. If not, lens system  140  is switched to the second focus mode at  230 , and the process terminates at  240 . Otherwise, control proceeds to  235 , where Module Focal Length Compensation Control  155  determines what digital zoom factor is needed to maintain the EFL, Module Select Optical Focal Length  170  sets the focal length of lens system  140  to a focal length less than or equal to MAX. FL2, Module Digital Zoom  165  applies sufficient digital zoom to maintain the EFL, and the lens system  140  is switched to the second focus mode. 
     If control proceeds to  245  in  FIG. 2B  after the test at  220 , Module Focal Length Compensation Control  155  determines whether the EFL exceeds MAX. FL1. If not, control proceeds to  250 , where lens system  140  is switched to the second focus mode, and the process terminates at  240 . Otherwise, control proceeds to  255 , where Module Focal Length Compensation Control  155  determines whether the EFL exceeds MAX. FL2. If not, the focal length of lens system  140  may be set to the EFL to maximize image quality. If the EFL exceeds MAX. FL2, the EFL may be maintained by setting the focal length of lens system  140  to MAX. FL2 using Module Select Optical Focal Length  170  and adding digital zoom using Module Digital Zoom  165 . 
     The foregoing description of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and other modifications and variations may be possible in light of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and various modifications as are suited to the particular use contemplated. It is intended that the appended claims be construed to include other alternative embodiments of the invention except insofar as limited by the prior art.