Abstract:
A focus calibration apparatus allows fine adjustments to be made to the focal distance between a camera lens and sensor, in particular to take into account small changes in the mechanical tolerances such as when changing lenses. The calibration apparatus has a simple mechanism, like a focus ring, that a user can easily manipulate to simplify and speed the lens calibration process.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application is a continuation of U.S. patent application Ser. No. 12/625,451, filed Nov. 24, 2009, which claims priority from U.S. Provisional No. 61/117,850, filed Nov. 25, 2008, which is incorporated by reference herein. 
     
    
     BACKGROUND 
       [0002]    The present disclosure relates to focus adjustment for camera lenses. 
         [0003]    Generally, cameras accept visual light through a camera lens and capture that light on a recording surface—film, or as is now common, a digital sensor. In order to reproduce an object clearly, the object must be in focus. The focus level of the various objects in a camera viewfinder may be adjusted by changing the size of the camera lens aperture or by varying the distance between the camera lens and the recording surface. 
         [0004]    For many premium cameras, such as high-end movie cameras, it is desirable that the focus be as precise as possible. Images from these cameras are often displayed in large formats, in which small errors in focus are readily apparent. In these contexts, even minor factors, like discrepancies in the mechanical tolerances at the lens mount, can throw off the focus of an image. 
         [0005]    It is common practice to calibrate a lens&#39;s focus when it is first placed onto a camera with thin shims placed in a camera&#39;s lens mount at the base of the lens. The camera operator uses these shims, often made of paper, to make precise adjustments to the focal length, between the camera lens and sensor. In the case of a movie camera, this calibration process is laborious and time-consuming. Factors such as heat and repeated use of the shims introduce new variables, as they can change the thickness of the shim or camera components and alter the focal length between camera lens and sensor. 
         [0006]    The time-consuming calibration process delays filming and therefore generates expense. New technology is desirable to simplify the lens calibration process. 
       SUMMARY 
       [0007]    A focus calibration apparatus allows fine adjustments to be made to the focal distance between a camera lens and sensor, in particular to take into account small changes in the mechanical tolerances when changing lenses, or focal length changes due to factors such as temperature changes. The calibration apparatus has a simple control, like a focus ring, that a user can easily manipulate to simplify and speed the lens calibration process. 
         [0008]    In one embodiment, a camera comprises a camera body with an image-receiving surface for receiving an image from a camera lens. The surface may comprise film, or an electronic sensor such as a CCD or CMOS sensor. The camera lens is mounted onto a lens mount carried by the camera body. Between the camera lens and the camera body, or built in as part of the lens mount, is a focus calibration apparatus. The focus calibration apparatus comprises two supports such as rings or discs—a lens-mount disc and a camera-mount disc—and a control such as a rotating ring. 
         [0009]    The camera-mount disc attaches to the camera and the lens-mount disc attaches to the lens. Manipulation of the control makes fine adjustments in distance measured along an as-mounted optical path between the camera mount and the lens mount. 
         [0010]    In one embodiment, each disc threads into the rotating ring. The discs are linked to each other with pins that maintain them in the same rotational relationship, even as a user manipulates the rotating ring, but which permit axial shortening or lengthening along the optical path. The rotating ring has two sets of opposed female threads. One set of threads mates with the camera-mount disc. The second set of threads mates with the lens-mount disc. As a user twists the rotating ring, the opposed threads cause the two discs to move longitudinally, either away or toward one another along the optical path. 
         [0011]    The focus calibration apparatus allows controlled adjustment of the length along the optical path between the sensor and the lens of about 0.002 inches or less, in some embodiments about 0.001 inches or less, and, in some embodiments of about 0.0005 inches or less. Adjustment may be on a continuous basis, or in a stepped function. 
         [0012]    In certain embodiments, a camera is provided including a camera body. The camera may further include an image plane adapted to receive an image from a camera lens. In some embodiments, the camera further includes a camera-mount disc attached to said camera body, having threads on its perimeter. The camera may have a rotating ring threaded onto the threads of said camera-mount disc. In certain embodiments, the camera includes a lens-mount disc having threads on its perimeter. The lens-mount disc is threaded onto said rotating ring in some embodiments. 
         [0013]    A focus calibration system is provided in certain embodiments for mounting between a camera and a lens to permit fine focus adjustment. The system can include a camera mount configured for mounting to a camera. The system may further include a lens mount configured for mounting to a lens. In certain embodiments, a control is provided. In some embodiments, manipulation of the control may change a distance between the camera mount and the lens mount as measured along an optical path extending through the camera mount and the lens mount. 
         [0014]    In accordance with a further aspect of the present invention, there is provided a method of calibrating focus following a lens exchange on a camera. In certain embodiments, the method comprises the steps of providing a camera having an image plane and a calibration system and a first lens mounted thereon. In some embodiments, the first lens is removed from the camera and a second lens is attached to the camera. In certain embodiments, the method includes calibrating the focus by adjusting a distance between the image plane and a second lens through a continuous range of motion without removing the second lens from the camera. In one implementation of the invention, the adjusting step comprises rotating a ring about an optical axis of the lens. 
         [0015]    Further features and advantages of the present invention will become apparent to those of skill in the art in view of the detailed description of preferred embodiments which follows, when considered together with the attached drawings and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  depicts a movie camera with a focus calibration apparatus. 
           [0017]      FIG. 2  depicts a movie camera with an exploded view of a focus calibration apparatus. 
           [0018]      FIG. 3  depicts a focus calibration apparatus in isolation from the movie camera and lens. 
           [0019]      FIG. 4  depicts the operation of a focus calibration ring. 
           [0020]      FIG. 5  depicts an exploded view of a focus calibration apparatus. 
           [0021]      FIGS. 6A and 6B  depict cross sections of a focus calibration apparatus. 
           [0022]      FIG. 7  depicts another embodiment of a focus calibration apparatus, with a clamping mechanism. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The focus calibration embodiments are described herein primarily in the context of movie cameras. However, nothing limits the claims or the invention to movie cameras. The disclosure is also applicable to any camera including emulsion film and digital moving image or still image cameras—or, indeed in any optical instrument—for which it is desirable to obtain a fine-tuned focus adjustment. 
         [0024]      FIG. 1  depicts a simplified movie camera incorporating a focus calibration apparatus  105 . A camera lens  110  connects to a camera body  100 . The focus calibration apparatus  105  comprises a rotatable ring  130  and a first threaded disc  140 . The focus calibration apparatus  105  additionally comprises a second threaded disc—not visible in this figure—that is positioned between the first threaded disc  140  and an image plane on the camera. The second threaded disc mounts the focus calibration apparatus  105  to the camera body  100  at a mounting point  120 . The camera lens  110  is mounted onto the first threaded disc  140 . 
         [0025]    The “mountings” carried by the focus calibration apparatus  105  for mounting to the camera and to the lens may be any of a variety of conventional connections known to persons skilled in the art. The mounts may include fixed mounts, like screws, pins, rivets, welds, or solder joints. Or, the mounts may include detachable mounts, like clasps, snaps, bayonet, breach, friction, tabbed, or threaded mounts. Detachable mounts will often be suitable for the camera lens mount. However, focus calibration apparatuses of the present disclosure might also be suitable for a fixed lens, where environmental factors like thermal expansion make fine adjustments to the focal length between lens and sensor advantageous. The focus calibration apparatus may be integral with either the camera body  100  or the camera lens  110 . 
         [0026]      FIG. 2  depicts a movie camera with an exploded view of one implementation of a focus calibration apparatus. A camera body  200  connects to a camera lens  210 . The focus calibration apparatus connects to a mount point  220  on the camera body  200 . A camera-mount disc  242  has a plurality of pins or holes  251  that mesh with complementary mounting structures such as holes  252  on the camera such as on the camera body  200 . Pins, rivets, screws, or other fasteners are all appropriate mechanisms to connect the camera-amount disc  242  to the camera body  200 . Alternatively, the camera-mount disc  242  is secured onto the camera body  200  such as by welding or adhesives, or formed integral with the camera body  200 . As described herein, the mounting to the “camera body” refers to mounting in a manner that fixes the camera mounting such as camera mount disc  242  with respect to the image sensor. This may be achieved by direct mounting to the camera housing, or to a mounting surface on the camera that is distinct from the housing. The camera-mount disc  242  has threads  244  on its outer perimeter that mate with a rotatable ring  230 . 
         [0027]    A rotatable ring  230  threadably engages the threads  244  of the camera-mount disc  242  with threads  234  on the interior of the ring  230 . The rotatable ring  230  has a second set of internal threads  233  into which a lens-mount disc  241  is screwed. The lens-mount disc  241  has exterior threads  243  that mate with the internal threads  233  of the rotatable ring  230 . 
         [0028]    One or two or more pins  261  (e.g. four) rotationally link the lens-mount disc  241  with the camera-mount disc  242 . The pins  261  are inserted through a first plurality of pin holes  262  in the lens-mount disc  241  and a second plurality of pin holes  263  in the camera-mount disc. The pins  261  maintain the lens-mount disc  241  in the same rotational orientation as the camera-mount disc  242 , but are slideably engaged with at least one of the discs  241  and  242  to permit axial adjustment along the optical path. The camera lens  210  mounts to the lens-mount disc  241 . Because the camera-mount disc  242  is fixedly attached to the camera, the lens-mount disc  241  and also the lens  210  are maintained in the same rotational orientation as the camera body  200 . 
         [0029]    In this embodiment, the pitch of the rotating ring&#39;s first set of internal threads  233  and second set of internal threads  234  are oriented in opposite directions. One set of threads are right-handed, and the other set of threads are left-handed. Thus, when the rotating ring  230  is turned by an operator in a first direction, the lens-mount disc  241  and camera-mount disc  242  move longitudinally away from one another, relative to the rotating ring  230 , along the optical axis. The change in the focal length between lens and sensor is the sum of the longitudinal movement along the first set of internal threads  233  and the second set of internal threads  234 . Thus, the precision of the adjustments that can be made in this embodiment depends, among other things, on the size of the threads as will be discussed further below. 
         [0030]      FIG. 3  depicts an assembled focus calibration apparatus. A rotatable ring  330  encloses a lens-mount disc  341 . The lens-mount disc  341  connects to a camera-mount disc, behind the lens-mount disc, with four pins  361 . The four pins  361  are inserted into a plurality of holes  362  in the lens-mount disc  341 . 
         [0031]    The rotatable ring  330  may be provided with any of a variety of surface structures or features to facilitate gripping and rotating the ring  330  to accomplish the fine calibration described herein. For example, ridges, grooves, knurling, or other friction enhancing surface structures or textures may be utilized. 
         [0032]      FIG. 4  depicts a focus calibration apparatus as in  FIG. 3 , illustrating rotational movement  471  of a focus ring  430  to make fine adjustments to the focal length between a camera lens and sensor. The focus ring  430  has two sets of internal, opposed threads. One set of internal threads mates with the threads  443  on the perimeter of a lens-mount disc  441 . The other set of internal threads on the focus ring  430  mates with the threads  444  on the perimeter of a camera-mount disc  442 . The lens-mount disc  441  and the camera-mount disc  442  maintain their rotational relationship to one another with four pins  461  that are placed through holes in each disc. In this figure, the pins  461  are inserted through a first set of holes  462  in the lens-mount disc  441  and a second set of holes  463  in the camera-mount disc. As the focus ring is rotated 471, the opposed threads of the ring push the two threaded discs longitudinally apart  472 , or pull them together if the ring  430  is rotated in a second, opposite direction, without rotational movement of the discs themselves. 
         [0033]    The pitch on the threads  443  of the lens-mount disc  441  and the threads  444  on the camera-mount disc  442  are generally no more than about 3 mm, often no more than about 2 mm, and, in one embodiment, no more than about 1 mm. One revolution of the focus ring  430  having a 1 mm pitch on each end produces 2 mm of longitudinal lens travel—1 mm of travel by the lens-mount disc  441  and 1 mm of travel by the camera-mount disc  442 . Of course, grosser or finer pitches may be suitable, depending on the particular application. 
         [0034]    The rotatable ring  330  may be provided with any of a variety of visual or tactile indicium of the amount of adjustment that has been accomplished. For example, a calibration scale such as a plurality of lines may be provided on the rotatable ring  330  or a non-rotatable adjacent component, with a line or marker on the other of the rotatable ring or non-rotatable structure. The calibration scale may be calibrated to allow a user to make adjustments in the axial length of the optical path either continuously or in increments of 0.001 inches, 0.0005 inches, or other distance. Tactile feedback may be provided by including a plurality of detents between the rotatable ring and a non-rotatable component so that the user may hear and/or feel as the rotatable ring  330  clicks or snaps in predetermined increments as the ring is rotated. 
         [0035]    In general, the focus calibration apparatus will be calibrated to allow changes in the axial length along the optical path between the lens and the sensor in a controllable fashion as low as 0.002 inches, preferably as low as 0.001 inches, and, in some embodiments, as low as 0.0005 inches or less. The total adjustment range for the change in length is generally no greater than about 0.10 inches, and in many applications, the adjustment length will be no more than about 0.020 inches or 0.010 inches or less. Thus, the calibration adjustment achieved by the present invention is not intended as a substitute for conventional focus adjustment achieved by the lens. 
         [0036]    A user calibrates focus by adjusting the focus ring and comparing the focus level to a visual pattern placed at a known distance. Alternatively, the focus calibration apparatus has indices indicated on the barrel, such that the user can determine an objectively correct focus calibration. Indices of this sort are useful, for instance, if the user has a set of lenses used on the same camera, and knows to which index the focus calibration apparatus was set the last time a lens was used. 
         [0037]    In other embodiments, it is possible to double the resolution of the focus ring for a given thread pitch by using only one threaded disc. The simplest way of accomplishing this is to axially fix either the lens-mount disc or camera-mount disc in the focus ring while continuing to permit relative rotation. Either the lens-mount disc or camera mount disc may rotatably travel in one or a plurality of annular grooves, rather than on threads. Thus, the focus ring merely spins about the disc, with no longitudinal travel, as the user adjusts the focus ring. 
         [0038]      FIG. 5  depicts an exploded view of a focus calibration apparatus similar to the one shown in  FIG. 4 , but with twice the sensitivity for a given thread pitch. A focus ring  530  has a first set of internal threads  533  that mate with the threads  543  on the perimeter of a lens-mount disc  541 , as has been discussed. The focus ring  530  has a second set of internal annular ridges and grooves  534  which are parallel to each other and mate with corresponding ridges and grooves  544  on the perimeter of a camera-mount disc  542 . The lens-mount disc  541  and the camera-mount disc  542  are connected with four pins  561 . The pins  561  are inserted through a first set of holes  562  in the lens-mount disc  541  and a second, parallel set of holes  563  in the camera-mount disc  542 . The pins  561  maintain the lens-mount disc  541  and the camera-mount disc  542  in the same rotational relationship even as the user rotates the focus ring  530 . Because the camera-mount disc  542  is fixedly attached to the camera at a plurality of attachment points  551 , the lens-mount disc  541  will maintain its relative rotational orientation to the camera, even as rotation of the ring  530  causes the disc to travel longitudinally. 
         [0039]    Since the internal annular ridges and grooves  534  on the focus ring  530  are parallel with each other, the focus calibration apparatus of this embodiment cannot be assembled by threading the focus calibration ring  530  over the camera-mount disc  542 . Thus, the ring  530  is provided with one or two or more part lines  536  at which the ring is separable into two or more components. In this manner, the ring may be partially or completely opened and mounted over the annular ridges and grooves  544  and thereafter reclosed into an annular structure and bonded such as by welding or other technique at part line  536 . 
         [0040]      FIGS. 6A and 6B  depict two cross-sectional views of a focus calibration apparatus such as that illustrated in  FIG. 3 . A focus ring  630  has a first set of internal threads  643  that mate with the threads on the perimeter of a lens-mount disc  641 . The focus ring  630  also has a second set of internal threads  644  that mate with the threads on a camera-mount disc  642 . The lens-mount disc  641  and the camera-mount disc  642  are maintained in the same rotational relationship with a plurality of axially slidable pins  661 . The pins  661  fit through holes  662  in the lens-mount disc  641  and another set of holes  663  in the camera-mount disc  642 . 
         [0041]      FIG. 7  depicts an embodiment of a focus calibration apparatus that includes a lock such as a clamp for retaining a desired adjustment. A focus ring  730  has two sets of internal threads—each opposed to the other—that mate with the threads on a lens-mount disc  741  and the threads on a camera-mount disc  742 . The lens-mount disc  741  and the camera-mount disc  742  are maintained in the same rotational relationship with four pins  761 . The pins  761  are inserted through a set of holes  762  in the lens-mount disc  641  and another set of holes in the camera-mount disc  742  that are not visible in this figure. 
         [0042]    In addition to these features, which are similar to those present in other embodiments described herein, the focus ring  730  has a clamping mechanism. The focus ring  730  in this embodiment is not complete circle, but has a gap  784 . On one side of the gap are one or two or more receivers  782  that accept corresponding screws  783 . On the other side of the gap are corresponding sockets  781  into which the screws  783  are screwed to tighten the gap  784  in the focus ring  730 . As the screws  783  are tightened, the ring  730  tightens around the lens-mount disc  741  and the camera-mount disc  742 . This tightening action prevents the ring  730  from being inadvertently turned by the user or anything else, causing unintended longitudinal movement of the lens-mount disc  741  and the camera-mount disc  742 . 
         [0043]    Other examples of locking features, besides the clamp depicted in  FIG. 7 , include clasps, locking pins, or switches. Indeed, any of a variety of mechanisms that lock the focus calibration ring in place, such that it cannot be inadvertently adjusted, may be utilized. In other embodiments, no locking feature is built onto the ring, but the ring turning mechanism has high friction, or is detented, such that the ring is unlikely to move without an intentional application of force. 
         [0044]    Various embodiments have been described above. Although described with reference to these specific embodiments, the descriptions are intended to be illustrative and are not intended to be limiting. Various modifications and applications may occur to those skilled in the art.