Abstract:
A camera provides an autofocus function by capturing a series of images when a lens is at different positions, analyzing the images to identify a best-focused image, and outputting the best-focused image. Unselected images can be discarded. The camera can include a FIFO buffer that receives image data from an image sensor as the image sensor captures images. When a photographer activates a shutter button, an analysis system, which may be implemented in software, analyzes the images in the FIFO buffer and selects the best-focused image for output. In alternative embodiments, the movement of the lens and the capture of the images can be before or in response to activation of the shutter button.

Description:
BACKGROUND 
     Taking clear and sharp photographs generally requires focusing the lens of a camera on the object being photographed. Traditionally, the photographer manually focuses the camera lens for the photograph being taken, so that an image of the object lies on the plane of the film or image sensor that captures the image. This manual focusing can take some time and skill. Accordingly, cameras with autofocusing have been developed to simplify photography. 
       FIG. 1  schematically illustrates a digital camera  100  containing a known autofocus system. Camera  100  includes a lens  110  that is mounted on a drive mechanism  115  capable of changing the distance between lens  110  and an image sensor  120 , and a motor  130  under the control of a controller  140  moves lens  110  for focusing on a target object. A pulse generator  135  measures the rotation of motor  130  and generates a pulse signal that controller  140  can use for tracking movement of lens  110 . For autofocusing, a range finder  150  that receives light through lens  110  determines a distance between an image plane of lens  110  and the plane of image sensor  120 . Range finding may, for example, be implemented using known phase difference detection techniques. Controller  140  receives the distance determined in range finder  150  and activates motor  130  to move lens  110  the distance required to place the image plane of lens  110  on image sensor  120 . When lens  110  is properly focused and the photographer operates the shutter button of camera  100 , image sensor  120  captures the image, which can be digitally stored in a memory  160 . 
     The autofocus system described above generally adds to the cost of camera  100 . In particular, camera  100  requires specific components such as range finder  150  with associated optics (not shown) and pulse generator  135  that may only be used for autofocusing. Additionally, motor  130  and drive mechanism  115  must be able to move lens  110  a controlled distance in either direction, and a relatively expensive stepper motor or voice coil motor may be required for adequate control of drive mechanism  115 . 
     An autofocus system is desired that can reduce camera cost and/or size through reduction of the number and cost of autofocusing components. 
     SUMMARY 
     In accordance with an aspect of the invention, an autofocus system in a digital camera captures a series of images corresponding to a series of positions of a camera lens. The series of images can be digitally analyzed to identify which of the images is best focused, and the best-focused image is kept when a photographer takes a photograph. The other images can be discarded. A focusing mechanism of the camera lens can move the lens through the series of positions and no longer requires a range finder, a pulse generator, or a system that can move the lens to a pre-identified target position. 
     In one mode of operation, a camera continuously captures images and stores image data temporarily in a FIFO buffer. When a photographer snaps a shutter button for the camera, a processor in the camera analyzes the images in the FIFO buffer and stores the best-focused image in an image memory. Otherwise, digital images in the FIFO buffer are sequentially discarded as new images are captured. 
     One specific embodiment of the invention is a photography process. The photography process includes: moving the focus of a lens to a series of positions; operating an image sensor to capture images respectively at the positions; analyzing the images to identify a best-focused image; and saving the best-focused image as a captured photograph. Moving the lens can be in response to activation of a shutter button or can be continuous when a camera is in a mode ready for capturing a photograph. With continuous lens motion, the image sensor can repeatedly capture new images and store image data in a FIFO buffer. The image data in the FIFO buffer can be analyzed when a photographer activates a shutter button or discarded to make room for new image data when the FIFO buffer is full. 
     Another specific embodiment of the invention is a camera including an image sensor; a lens; a lens drive system, and a selection system. The lens drive system is capable of moving focus of the lens relative to the image sensor and is operable while the image sensor captures a series of images. When a photograph is taken, the selection system, which can be implemented in software, analyzes image data from the image sensor, selects a best focused image from among the series of images, and outputs the best-focused image as an output photograph. A buffer such as a FIFO buffer can receive image data from the image sensor and provide image data to the selection system for analysis and selection when capture of a photograph is desired. The lens drive system for the camera can employ an inexpensive DC motor and does not require the ability to seek to a specific lens position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a camera containing a known auto-focus system. 
         FIG. 2  is a block diagram of a camera in accordance with an embodiment of the invention having an auto-focus system with a motor drive. 
         FIG. 3  shows a camera in accordance with an embodiment of the invention having a lens drive system with a stepped cam. 
         FIG. 4  is a block diagram of a camera in accordance with an embodiment of the invention using a shutter button to drive an auto-focus system. 
     
    
    
     Use of the same reference symbols in different figures indicates similar or identical items. 
     DETAILED DESCRIPTION 
     In accordance with an aspect of the invention, a camera captures a series of images corresponding to a series of lens positions and then selects the best of the images to be kept when a photographer captures an image. A lens positioning system for the camera moves the lenses through the series of positions but does not need the capability to seek a specific lens position or to move a specific distance in response to a focus or range measurement, and the camera does not require specialized range finding hardware. Further, the autofocusing process can reduce delays in picture taking since a picture can be captured even before the focused location of the lens is known. 
       FIG. 2  illustrates a digital camera  200  in accordance with an embodiment of the invention. Camera  200  includes a lens  210  mounted in a drive mechanism  220  that moves the focus of lens  210  relative to an image sensor  230 . Lens  210  can be any type of conventional camera lens capable of forming an image on image sensor  230 , and drive mechanism  220  preferably provides a range of motion for lens  210  that permits lens  210  to form an image on image sensor of an object at any distance greater than about 50 mm. 
     In an auto-focusing process for camera  200 , drive mechanism  220  moves lens  210  through the range of motion of drive mechanism  222  while image sensor  230  captures a series of images. The movement of lens  210  can be continuous while the series of images are captured. Generally, continuous motion of the lens will not produce noticeable effects on the image if lens  210  moves only a negligible distance during the exposure time. However, lens  210  can alternatively be move in short steps if movement of the lens must be avoided during image exposure. 
     A first-in first-out (FIFO) buffer  240  temporarily stores the image data from image sensor  230 . Generally, most of the images in FIFO buffer  240  may be out of focus because lens  210  not at the ideal position when the image is taken. However, images taken will be sharp when lens  210  is at or near the ideal position for focusing on the object being photographed. The sharp images will generally show details such as well-defined edges and textures of the object being photographed. In images that are out of focus, the details of the object will be blurred. A processor  250  in camera  200  can analyze the images in FIFO buffer  240  to identify an image having the sharpest contrast, indicating the best focused image. For example, processor  250  can evaluated the standard deviation of all pixel values for each image and select the image having the largest standard deviation as the sharpest image. The image with the sharpest focus can then be stored in image memory  260 . 
     In an exemplary embodiment of the invention, drive mechanism  220  is able to move lens  210  once through its full range of motion (either away from or toward image sensor  240 ) in about 100 to 500 ms, and image sensor  240  captures images at a frame rate of about 5 to 30 frames (or images) per second. One traversal of the range of motion of lens  210  thus provides about 5 to 20 images at different focus adjustments, and FIFO buffer  240  has sufficient capacity to store all the images. Processor  250  can then select the best focused of the images in FIFO buffer  240 . 
     Drive mechanism  220  in the embodiment illustrated in  FIG. 2  includes a drive motor  222  that rotates a cam  223 . Motor  222  is preferably an inexpensive DC motor that continuously rotates cam  223  when activated, but motor can alternatively be a stepper motor that move CAM  223  in steps and stops when each image is captured. A spring mechanism  224  pushes an extending arm of lens barrel  225  against cam  223 . Cam  223  in  FIG. 2  is elliptical but alternatively can have any radially asymmetric shape about its rotation axis. For example, in one alternative embodiment, a portion of the perimeter of cam  223  can form a spiral or a series of steps each having a different radius. 
     Lens barrel  225  contains lens  210  and rides on rails  226  that permit lens barrel  225  to slide along a direction parallel to the optical axis of lens  210 . The position of lens  210  along rails  226  depends on the radius of cam  223  at the point of cam  223  in contact with lens barrel  225 . Since cam  223  is radially asymmetric, continuous rotation of motor  222  thus causes lens  210  to move back and forth through the available range of motion of lens  210 . 
     Motor  222  would generally only be operated when taking of a photograph is anticipated. For example, motor  222  may be activated when a shutter button is depressed to a halfway position. At that point, motor  222  spins, and lens  210  moves while image sensor  230  captures and stores image data for a series of images. The series of images stored in FIFO buffer  240  preferably corresponds to at least one full traversal of the movement range of lens  210  in drive mechanism  220 . When the shutter button is fully depressed and FIFO buffer  240  contains a full series of images, processor  250  analyzes the image data in FIFO buffer  240  and selects a best-focused image. The best image is then stored in image memory  260 . 
     Instead of operating motor  220  for a short time when triggered using the shutter button, camera  200  can be operated in a mode where motor  222  spins continuously and image sensor  230  continuously captures images and transfers image data to FIFO buffer  240 . Accordingly, the control system for motor  220  can simply be a switch that turns motor  220  when camera enters a ready mode and switches off motor  220  when exiting the ready mode. In the ready mode, when the shutter button is pressed to take a picture, FIFO buffer  240  already contains a full series of images for analysis. Processor  250  can then select an image that was actually captured before activation of the shutter button. No delay for lens movement or other autofocusing processes is required. 
       FIG. 3  shows a camera  300  that is the same as camera  200  except that camera  300  has a drive system  320  with a stepped cam  323 . In particular, cam  323  has a set of steps, where each step has a different radius. As cam  323  rotates, lens  210  remains stationary while slide mount  225  stays on one of the steps, and lens  210  moves when slide mount  225  transitions from resting on one step to resting on the next step. Image capture operations of image sensor  230  can be synchronized with rotation of cam  323 , so that each image is captured when lens  210  is stationary. 
       FIG. 4  shows a camera  400  in accordance with another embodiment of the invention. Camera  400  of  FIG. 4  differs from camera  200  of  FIG. 2  primarily in employing yet another alternative drive mechanism  420 . Drive mechanism  420  does not require a motor and is manually operated, for example, through the operation of a shutter button  410 . In particular, as shutter button  410  is depressed, a mechanical system (e.g., a lever) pushes sliding mount  225  along rails  226  so that lens  210  moves from the beginning to the end of its movement range. Image sensor  230  captures a series of images during the movement of lens  210  and stores image data in FIFO buffer  240 . Processor  250  can then analyze the images as described above and store the best-focused image in image memory  260 . 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.