Abstract:
A focusing mount includes a focusing frame fixed to a focus tube with at least one lens having an optical axis which is connected to a constraining frame by prismatic joints for enabling movement of the focusing mount along the optical axis without any twisting, lateral, rotational or swing movements.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an improved lens mounting system which provides all the benefits of a simple sliding tube but without the associated drawbacks, and more particularly to a focusing mechanism which moves a focusing frame along the optical axis of each lens system linearly by constraining lateral, rotational and swing movements. 
     2. Description of the Prior Art 
     There are many ways to focus a lens. This invention concerns the focusing of a lens by means of mechanically changing its position along a fixed path, which maintains the orientation of the lens and follows the optical axis of the lens. This movement may be achieved by manual operation, or it can be motorized. This invention does not cover methods which involve using electronic or other means to change the shape or optical properties of a lens to achieve change in focus. 
     In an imaging device, when a lens is moved along its central optical axis, it is desirable for it not to shift or tilt with respect to this axis. This is because an imaging device requires an image capturing film or sensor to be placed in a fixed position at a predetermined focal plane. If the lens tilts or shifts, its optical axis and focal plane will tilt or shift with it. For conventional devices, it is not practical to move the image capturing sensor in concert with any tilt or shift of the lens. Any uncontrolled shifting or tilting will degrade the quality of the focus. Even a small tilt in the orientation of the lens will result in an image becoming blurred even when the lens has been moved to its correct point of focus on the optical axis. This means that the lens will effectively have to move along one optical axis which ends with the predetermined position of the image capturing sensor on the focal plane. 
     It is also desirable for the movement of the lens to be achieved with maximum efficiency to facilitate ease of use and economy of space. This is especially important where there is motorization or automation of the focusing mechanism. 
     Many systems have evolved to move a lens along its optical axis without shifting or tilting. This invention concerns systems which consist of a fixed constraining frame and a moving focusing frame on which the lens is mounted. The focusing frame, constrained by the constraining frame, guides the lens along its optical axis. 
       FIG. 1A  depicts a concentric sliding tube lens focusing system consisting of a rigid outer focusing tube and a rigid inner focusing tube. An outer focusing tube  01  serves as a rigid housing to which a fixed constraining frame  02  is attached. An inner focusing tube serves as a moving focusing frame  03 . A lens  04  is mounted in the focusing frame  03 . The focusing frame  03  slides in and out of the constraining frame  02 . Movement in all other axes other than the optical axis are constrained. The lens  04  cannot shift or tilt on its optical axis. It does however rotate. This movement of the lens relative to the focal plane  05  achieves the desired lens focusing function of the system. An image capturing device such as a CCD or film can be situated on the focal plane  05 . A simple example of such a focusing device can be found in a basic pirate&#39;s telescope. 
     In practice, if there is full contact between the outer cylindrical surface of the focusing frame  03  and the inner cylindrical surface of the constraining frame  02 , a large amount of friction will be introduced into the system. 
     For this reason, such systems are built with contact points at the two ends to support the inner focusing tube and locate the optical axis. The rest of the facing surfaces do not need to be in contact. This eliminates the possibility of the inner focusing tube being supported inadvertently at some point between the two end points, thereby shortening the length of the constraining frame. It also reduces friction and allows one tube to slide freely inside the other. The outer support tube functions as a constraining frame, because only the two contact points at either end are used for supporting the inner tube. The tube is merely a convenient shape for the device. It is unnecessary and difficult to manufacture rigid concentric tubes of sufficient precision to slide in and out of each other with full contact. 
     The key to a smoothly working concentric tube focusing system is the ratio of the diameter of the constraining joint as defined by the focusing frame to the length of the constraining frame. It is the two points of support at the extreme ends of the constraining frame which do the work of keeping the inner focusing frame in place while it is being moved. 
       FIG. 1B  depicts a modified version of the focusing system in  FIG. 1A  with a constraining frame  10 , focusing frame  11  and lens  04 . It is good practice to design the tubes so that the ratio in length (A) of the constraining frame  10  to the diameter (B) of a constraining joint  07  is approximately 7 to 1 or greater. Otherwise, the focusing frame  11  carrying the lens  04  can rock within the constraining frame  10 . The result would be that the lens  04  can deviate from its ideal position on the optical axis.  12 . 
     In fact, depending on the size of the gap between the concentric tubes functioning as a focusing frame  11  within a constraining frame  10 , if the ratio of A/B approaches  1 , which would be a short constraining frame length for a given constraining joint diameter, the focusing frame  11  can tilt so much within its constraining frame  10  that it will jam and not slide at all. 
     In theory, one can design a lens tube with a small constraining joint, a long constraining frame and long focusing frame, and this will be a very stable and accurate focusing system. In practice, the focusing system in  FIG. 1B  needs to function as part of an overall imaging device. The constraining frame and focusing frame need to accommodate peripheral as well as principal light paths entering the imaging device, passing through the lens, and falling on the focal plane. The longer the frames, the narrower the angle of view. 
     The designer of the overall imaging device will desire as much room as possible on either side of the lens, and as much flexibility in the angle of view as possible. This can only be achieved by making the constraining frame and focusing frame as short as possible in order to reduce the thickness of the imaging device. This inevitably leads to the problems with a concentric tube system jamming when the ratio of the length (A) of the constraining frame  10  and the diameter (B) of the constraining joint  07  is insufficient. 
     Another implementation of the fixed constraining frame and moving focusing frame method is depicted in  FIG. 1C . A constraining frame  14  with a helical thread and a focusing frame  13  with a matching helical thread are combined to form a strong but movable constraining joint. This constraining joint is strong, and prevents tilt and shift of the lens because it provides axial support with each thread surface. The ratio of constraining frame  14  to the constraining joint can therefore be reduced. Focusing systems using a helical method can have a short outer constraining frame  14  and large inner focusing frame  13  (large constraining diameter). Such a system prevents tilt and shift, but the lens  04  and focusing frame  13  will rotate within the constraining frame  14 . 
     One variation of the helical thread system uses a double helical thread and two anti-rotation pins to move the lens plane in and out without the lens rotating at the same time. 
     In the system using concentric tubes and helical threads, the lens  04  and the focusing frame  13  are situated inside the constraining frame  14 . This is a limitation inherent to the system. 
     Any helical thread, by nature, introduces drag and inefficiency. This is because the outer moving surface must move a long distance in order for the lens to move a short distance. Also, the total contact area is much greater than for a concentric tube system, where the focusing frame is only supported at two ends. 
     A focusing system involving helical threads is inherently more difficult and expensive to make than one using concentric tubes. It requires accurately cutting two mating and interchangeable helical threads. This is not easy to mass-produce. Mass-production in plastic of such high precision male/female helical thread tubes would require highly expensive precision molding equipment. 
     An auto focus lens with helical thread requires a high precision fit in which there is no slack and the fit is not too tight. This helps reduce the power required to drive the focusing mechanism and extends battery life. This balance is very difficult to attain in production. 
       FIG. 2A  is a side view of a slider-and-track based focusing system. A fixed constraining frame  15  is in the form of a pair of tracks which support and constrain a focusing frame  16 , which is attached to and guides a lens tube  17 , in which is mounted lens  04 . The lens tube  17  has a clearance fit in the rigid outer focusing tube  01 . The constraining frame  15  is fixed to the outer focusing tube  01 , but is no longer concentric with it or the lens tube  17 . 
     An axis  18  of the focusing frame  16  is parallel to the optical axis  12 . The tracks which act as the constraining frame may be cylindrical as shown in  FIG. 2A , but may also be in some other shape. There are also track-based focusing systems which only use a single track. The focusing frame  16  can be moved along the constraining frame  15  using various methods, such as a rack and pinion system. 
     The position of the constraining frame  15  in  FIGS. 2A ,  2 B serves to separate the constraining and focusing frame mechanism from the lens  04 . The lens no longer needs to be situated in a pair of concentric tubes, which in turn would have their diameter determined by the size of the lens. This means that within the limits imposed by materials and workmanship, it is possible to design a sliding track with a very small diameter (A) of the constraining joint as defined by the focusing frame  16  and a length (B) of the constraining frame  15  similar to the length B of the constraining frame  08  in  FIG. 1A . 
     In a track-mount based focusing system, there is no rotation of the lens. Tilt and shift are prevented by means of the constraining frame being made of very strong material, and the constraining frame and constraining joint being very precisely engineered to have no slack on the constraining joint while still allowing the smooth movement. This balance is very difficult to attain in production. It would also require the use of materials much stronger than plastic. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide a focusing system for lenses wherein a focusing frame is moved relative to a constraining frame which maintains the movement of the lens or lenses along their optical axis without subjecting the lens to any lateral, rotational or swing movement. 
     Another object of the present invention is to provide a focusing system for lenses where a focusing frame is moved relative to a constraining frame using only linear movements and no twisting motion. 
     Still a further object of the present invention is to provide a focusing system incorporating a focusing frame slotted with a constraining frame with two pairs of sliding joints. 
     Yet an additional object is to incorporate the foregoing objects in a three-dimensional focusing system. 
     A still additional object is to effect the foregoing objects in an effective and efficient manner. 
     The foregoing objects are achieved by means of a focusing frame fixed to a lens-holding focusing tube, and movably connected to a constraining frame. The focusing frame is connected to the constraining frame by prismatic joints (in the preferred embodiments, pin and hole joints and pin and open-slat joints are described) which constitute purely linear motion along the respective joint axis. The joints used in the preferred embodiments of the invention incur very low frictional force. There is no shifting, tilting or rotation (pitching or yawing) of the focusing frame as it moves relative to the constraining frame, so movement of the focusing frame is always along the optical axis of the lens. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a cross-sectional schematic side view of a concentric sliding focusing system according to the prior art. 
         FIG. 1B  is a variation of the view shown in  FIG. 1A  according to the prior art. 
         FIG. 1C  is a cross-sectional schematic side view of a fixed constraining frame and a moving focusing frame being joined by matching helical threads according to the prior art. 
         FIGS. 2A and 2B  are cross-sectional schematic side and front sectional views of a slider-and-track based focusing system according to the prior art. 
         FIGS. 3A ,  3 B and  3 C are respectively front, top cross-sectional and side cross-sectional schematic views of an embodiment of the invention. 
         FIGS. 3D and 3E  are respectively front and side cross-sectional schematic views of a variation of the embodiment shown in  FIGS. 3A-3C . 
         FIGS. 4A-4C  are respectively front, side cross-sectional and top schematic views of another embodiment of the invention. 
         FIGS. 5A-5D  are respectively top and side perspective, front, top and other side perspective and bottom side perspective schematic views of another embodiment of the invention. 
         FIGS. 6A-6D  are respectively top and side perspective, front, top and other side perspective and bottom side perspective schematic views of another embodiment of the invention. 
         FIG. 7  is a top and side perspective schematic view of a variation of the embodiment shown in  FIGS. 6A-6D . 
         FIGS. 8A-8D  are respectively top and side perspective, top, top cross-sectional and another top cross-sectional views of another embodiment of the invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 3A ,  3 B and  3 C depict the invention in its most basic form. 
     In the concentric tube method, the outer tube is the constraining frame. It is fixed to the housing, and thereby provides support and rigidity to the focusing frame, which holds and constrains the motion of the lens along the required optical axis. 
     In  FIGS. 3A ,  3 B and  3 C, a constraining frame  19  is still part of a fixed housing. In fact, the entire housing functions as the constraining frame. This provides support and rigidity and constrains the motion of the lens  04  along the optical axis  12 . However, this constraining frame  19  no longer supports and guides a long, thin inner focusing frame. Instead, the focusing frame is only represented by means of four focusing rails  20 . (As used herein, the term “focusing rail” means a part fixed to the focusing frame, and the term “constraining rail” means a part fixed to the constraining frame.) Each focusing rail  20  has two freely sliding hole and pin joints  21 . This means there are a total of four hole and pin joints  21  at each end of the constraining frame  19 . The four hole and pin joints  21  at the far end from the lens  04  are formed by the end pieces of the focusing rails and bush pieces joined to the constraining frame  19 . The other four hole and pin joints  21  are formed from pins joined to the inside wall of a focusing frame  23  and holes in the walls of the constraining frame  19 . These four pairs of hole and pin joints  21  are constraining joints joining the focusing frame  23  to the constraining frame  19 . Each hole and pin joint  21  allows the focusing rail to slide along an axis passing through the center of the hole, but rotation is prevented by the presence of the other focusing rails joined to the constraining frame  19  by the other pin and hole joints  21 . 
     The lens tube  17 , fixed to the focusing frame  23  consisting of four focusing rails  20 , holding the lens  04  moves in and out of an opening in the fixed constraining frame  19 , but the moving focusing frame, consisting of four focusing rails  20 , wraps around the constraining frame  19  on the outside. The four focusing rails  20  are joined at the end close to the lens tube  17  to form a rigid focusing frame, which slides freely on the four pairs of free-moving pin and hole joints  21 , guiding the lens  04  very accurately along the optical axis  12 . 
     As described earlier, this provides sufficient support and accuracy for motion of the focusing frame relative to the constraining frame without engaging the entire surface area, which would unnecessarily increase friction in the device. It functions like a focusing rail  20 , but derives its strength and freedom of movement from multiple joints with the constraining frame  19 . Due to the long moment arm between the pairs of constraining joints, the reaction forces at the joints are correspondingly light. The frictional force becomes minute. 
     In  FIGS. 3A-3C , the internal diameter of the constraining frame  19  is now represented by diameter (B), which is a fraction of the corresponding diameter (B) of the constraining frame  10  in  FIG. 1B . The length of the constraining frame  19  is represented by length (A). The diameter (B) of constraining frame  19  can be very small and is only restricted by the need for rigidity of a pin sliding in its mating hole. By careful design, the length (A) of constraining frame  19  can be maximized by placing the contact joints between the constraining frame  19  and the focusing frame  23  made up of four focusing rails  20  at the extremes of the constraining frame  19  such that they move along axes which are parallel to the optical axis  12 . This can be achieved without adding substantially to the overall length of the constraining frame  19 . 
     As explained previously, a large ratio of the length (A) of the constraining frame to diameter (B) of the constraining frame gives rise to a focusing frame which does not tilt within its constraining frame and does not rock when sliding with respect to the support frame. 
       FIGS. 3D-3E  depict another embodiment of the system using a constraining frame and four focusing rails. As described previously, the four focusing rails  20  in  FIGS. 3A-3C  are joined at the end close to the lens  04  to form a rigid focusing frame  23  which holds the lens tube  17  and lens  04 . 
     The functionality and strength of the mechanism depicted in  FIGS. 3A-3C  can also be achieved in  FIGS. 3D-3E  by inverting the hole and pin joints  21  on the side close to the lens  04  such that the pins are joined to the constraining frame  19  and the holes are in the wall of the adjoining arm of the focusing frame  23 . 
     The focal plane  05  can also be moved outside the housing, to the other side of the lens  04 . 
     In  FIGS. 4A-4C , two of the four focusing rails forming the focusing frame  23  have been taken away, leaving two at opposite sides of the constraining frame  19 . This structure retains the long moment arm between pairs of constraining joints necessary to have a smooth travel. However, without the bracing pair of focusing rails at 90 degrees, it can pitch, or rotate about the Z-axis while it is moving. A rigid U-piece or constraining frame rails  28  is fitted to wrap around the constraining frame  19  and is pinned to the focusing frame  23 , which it intersects on the top and bottom sides, by means of two slot and pin joints  31 , which only permit limited sliding movement along the Z-axis. A frame around the constraining frame  19  is completed when the two arms of the U-piece  28  are linked up with a shaft  24  and pivoted on two rotating hole and pin joints  29  formed in conjunction with bush pieces in the wall of the constraining frame  19 . The U-piece  28  is therefore linked to both the focusing frame  23  and the constraining frame  19 . 
     The two slot and pin joints  31  prevent the focusing rails  20  making up the focusing frame  23  from pitching (rotating on the Z-axis) while it is moving. In  FIG. 4C , the sliding hole and pin joints  21 ,  22  at either end of the focusing rails  20  making up the focusing frame  23  prevent the focusing frame from yawing (rotating on the Y-axis while it is moving). 
     In  FIG. 5A , the rigid U-piece  28  in  FIGS. 4A-4C  has been split and is now represented by an upper right adjoining piece  32  joined to the upper end of a shaft  33  by a fixed upper left hole and pin joint  34  and a lower right joining piece  35  joined to the lower end of the same shaft  33  by a fixed lower left hole and pin joint  36 . The shaft  33  on the right-hand side passes through two bush pieces  37  on the right-hand side of the constraining frame  19 , which allows the combination U-piece on the right-hand side to freely pivot as if locked in a pin and hole joint. The upper right adjoining piece  32  is shaped like the letter “L,” providing a long lever which can be used for pivoting the U-piece, comprised of the upper right adjoining piece  32 , lower right adjoining piece  35  and shaft  33 , in the bush pieces  37 . 
     The rest of the U-piece is now represented by two identical levers, an upper lever  39  resting on the upper face of the constraining frame  19  and a lower lever  40  resting on the lower face. The right-hand side of the upper lever  39  is joined to the left-hand side of the upper right adjoining piece  32  by an upper pin and open slot joint  41 . The right-hand side of the lower lever  40  is joined to the left-hand side of the lower right adjoining piece  35  by a lower pin and open slot joint  42 . 
     The left-hand side of the upper lever  39  has a hole  43 . The left-hand side of the lower lever  40  also has a hole  44 . There are two bush pieces  45  on the left side of the constraining frame. The upper lever  39  is linked to the lower lever  40  by a shaft  46  on the left-hand side that passes through the holes  43  and  44  aforementioned and the two bush pieces  45 . Depending on the application, the link between the shaft  46  on the left-hand side and the upper and lower levers  39  and item  40  can either be fixed for free floating. 
     In a working device such as an imaging device with a focusing lens, the far end of the upper right adjoining piece  32 , which is shaped like a lever, has an index mark  47 , which would point to a graduated focusing scale  48  as it is moved. This would allow the user to control the focus of the device. The focusing scale  48  will work well as long as the focus markings are synchronized to the actual focal point  06  of the image forming lens  04 . In real world mass-production, there is usually a range of focus points in any given sample of focusing lenses. Even a small discrepancy in the focus point  06  can lead to a misalignment of the index mark  47  on the lever with the focusing scale  48 . To correct this problem, another embodiment of this invention provides an assembly-time method for trimming the focal point  06  of the focusing lens  04  to correct the focus discrepancy. To function properly, this focus trimming method must work independently of the main focusing lever, which is represented by the upper right adjoining piece  32 . 
     The upper lever in  FIGS. 5A-5C  is transformed in  FIGS. 6A-6D  into a short upper-left adjoining piece  50  and an upper differential piece  51  in the middle, which are joined by a rotating hole and pin joint  52 . The lower lever  40  in  FIGS. 5A-5C  is transformed in  FIGS. 6A-6D  into a matching short lower-left adjoining piece  53  and lower differential piece  54  which are joined by another hole and pin joint  55 . 
     In  FIGS. 6A-6D , the short upper-left adjoining piece  50  is joined to a shaft  46  by a fixed upper hole and pin joint  43 . The shaft  46  is passed through two bush pieces  45  joined to the constraining frame  19 , in which it can freely rotate. The other end of the shaft is then joined to the short lower-left adjoining piece  53  by the fixed lower hole and pin joint  44 . The combination of the left upper and left lower adjoining pieces  50 ,  53  and the shaft  46  on the left side becomes a truncated U-piece, which is rigid, but pivots within the constraining bush pieces  45 . 
     The two truncated U-pieces described in  FIGS. 5A-5D  and  FIGS. 6A-6D  above, with a pair of upper and lower differentials  51 ,  54  in between, which prevent pitching of the focusing frame  23 , provide two independent mechanisms for adjusting the focal point  06  of the lens  04 . One mechanism, controlled by the focus lever, is used by the user to focus the lens according to the focus markings. The other mechanism, controlled by the position of the pair of differentials  51 ,  54 , is used to trim the focal point of the lens  04  so that it matches the position of the focusing scale  48 . 
     In  FIGS. 6A-6D , the movement of the U-piece on the right, consisting of the upper right adjoining piece  32 , shaft  33  on the right-hand side, and lower-right adjoining piece  35 , causes the upper and lower differential pieces  51 ,  54  to move in sympathy, which in turn moves the focusing frame  23  along the optical axis  12 . Movement of the U-piece on the left, comprised of the upper left adjoining piece  50 , left-hand shaft  46  and lower-left adjoining piece  53 , also causes the upper and lower differential pieces  51 ,  54  to move in sympathy, which in turn also moves the focusing frame  23  along the optical axis  12 . 
     For any given position of the focusing lever embodied by the upper-right adjoining piece  32 , as determined by the focusing scale  48 , the focal point  06  of the focusing lens  04  can be checked and trimmed by changing the position of the left-hand side U-piece. When the focusing lever is set to a given position, the upper and lower pin and open slot joints  41 ,  42  become two pivot points about which the pair of upper and lower mid-section differential pieces  51 ,  54  can pivot. Once the optimal position has been found, the left-hand side U-piece can be locked down, and the focus trim of the lens unit fixed to coincide with the preset focusing scale  48 , as shown in  FIG. 7 . 
     In another embodiment of the invention, the freely rotating truncated U-piece on the left is made adjustable and lockable. As shown in  FIG. 7 , the upper left adjoining piece  50  in  FIGS. 6A-6D  is transformed into a quadrant-shaped adjustment piece  57 . The quadrant-shaped adjustment piece  57  is provided with a screw-and-track locking mechanism  58  in between the upper left hole and pin joint  43  and the rotating hole and pin joint  52  joining the quadrant-shaped adjustment piece  57  to the upper differential piece  51 . The potential arc of movement of the left-hand side U-piece is constrained by the arc-shaped track in the quadrant-shaped adjustment piece  57 . The screw in the screw-and-track locking mechanism  58  is screwed into the constraining frame  19  and can be tightened to clamp the position of the focus trim once it has been determined. 
     A 3D imaging device works by simulating a pair of human eyes in providing a pair of image capturing lenses to capture a pair of images of the same object from different angles. The difference in the apparent position of the object as seen from different angles is known as parallax. The human brain processes the image pair with parallax, and stereoscopic vision is the result. 
     When the object is relatively far away, the incident light paths entering the pair of eyes are almost parallel, so the person looks straight ahead. When the object is relatively close, the incident light paths entering the pair of eyes will have to be at an angle to the centerline of the object with respect to the pair of eyes. While focusing on a close-up object, the pair of eyes will naturally swivel so that the optical axis of each eye converges toward the object and allows each image to be centered on the focal plane. This is known as parallax compensation. 
     When a basic 3D imaging device is focused on a close-up object, the incident light paths entering the image-capturing lens or lenses will also be at an angle to the centerline of the object with respect to the imaging device. When the 3D imaging device is head on to the object, both images in the pair captured will not be centered on the optical axis. If the device is rotated so that one image in the pair is centered on the optical axis of the image-capturing plane on one side, then the other image will be even further de-centered from the optical axis of the image-capturing plane on the other side. In a real world 3D imaging device, the disadvantage of this is that one or both images in the pair will be inappropriately cropped, and a lot of image information is lost on one or both sides. 
     This invention teaches a new focusing mechanism for a 3D imaging device and a method for linking this focusing mechanism to another mechanism for varying the angle of one or both optical axes in order to allow the device to simulate the parallax compensation function of a pair of human eyes. 
       FIGS. 8A and 8B  show another embodiment of the invention. The right-hand side U-piece, consisting of the upper-right adjoining piece  32 , shaft  33  on the right-hand side, and lower-right adjoining piece  35 , can have a cam  59  attached to the shaft  33  to impart coordinated movement to other mechanical frameworks, such as a mirror holder  60 . Adjustment of focus is tracked by means of a marked focusing scale  48  and achieved through movement of the right-hand side U-piece. This changes the angle of an adjustable mirror  61  which is mounted on an adjustable mirror holder  60  at the same time, which changes the optical axis of the image-capturing framework on the right-hand side. The same movement of the right-hand side U-piece can be linked to more than one adjustable mirror at the same time. 
       FIG. 8C  is a sectional view of a typical 3D imaging device with a twin lens assembly  62  and four light deflection mirrors, which define two image-capturing frameworks situated some distance apart, each with an optical axis  72 ,  73  independent of the other. In  FIG. 8C , one of the four deflection mirrors is the adjustable mirror  61  in  FIGS. 8A-8B , mounted in a mirror holder  60 , pivoting on a hinge joint  63 . A mirror  74  reflects light to mirror  61 , and a mirror  75  reflects light to mirror  71 . The focusing lever as embodied by the upper-right adjoining piece  32  controls the focus of the twin lens assembly  62  and, at the same time, imparts coordinated movement to the adjustable mirror  61  through the cam  59 . A spring  64  is keyed on the mirror holder  60  and reacts against the constraining frame  19  to impart a force to ensure that a cam follower  65 , situated at the tip of the mirror holder  60 , always engages on the cam  59 . 
     The link between the focusing lever as embodied by the upper-right adjoining piece  32  and the adjustable mirror  61  can be calibrated such that the position of the image formed by the adjustable image-capturing framework on the focal plane can be used to indicate the position of the focusing lever and, hence, determine the distance of the object from the imaging device. If the image formed by the adjustable image-capturing framework is viewed in a calibrated optical or digital viewfinder, then the device becomes a rangefinder. This functionality can be achieved with or without the use of electronic components in the device. 
     In 3D photography, an object can be represented as an object plane which has width and different views when photographed from two image-capturing frameworks set apart from each other. In  FIG. 8D , sectional views are shown of the 3D imaging device of  FIG. 8C  with light paths depicting an object plane at infinity, mid-distance and close up. An insert of  FIG. 8D  is also provided for clarity of illustration to show the numbered parts whose positions change in the three diagrams with light paths. 
     In a 3D imaging device, the image formed by the adjustable image-capturing framework and the image formed by the other image-capturing framework will be viewable side-by-side in an optical or digital viewfinder built into the image-capturing device. This pair of images  68 ,  69  is first captured on the focal plane  05 . The relative position of this pair of images  68 ,  69  will indicate whether an object plane  67  is situated at a point on the center axis of the image-capturing device where the optical axes of the two image-capturing frameworks converge. 
     In  FIG. 8D , when the object plane  67  is situated at this point, it will automatically be in focus. In real life, the object plane  67  may not move to coincide with the point where the two optical axes happen to converge. So, in a 3D imaging device, the two optical axes are made to pivot relative to each other so that the point of convergence can be found. This behavior imparts coupled focus-finding capability to the device. This functionality can be achieved with or without the use of electronic components in the device. 
     In practice, the 3D imaging device and adjustable mirror only needs to tilt relative to the centerline and to each other by a very small amount for parallax compensation to be effected. This almost imperceptible displacement is very difficult to display on an accurately scaled drawing. The relative displacements shown in the drawings are exaggerated for clarity of illustration. 
     Also, in practice, the relative length of each adjoining piece forming the U-piece may vary. All directional and positional references such as to the left, right, upper, lower, front, back, and all references to relative length and size are for the ease of understanding of the figures. In other embodiments, the items referenced can be moved to other positions, or transformed into different shapes, or swapped with corresponding items on the other side, and relative sizes and lengths may be varied as long as the basic principles described above are applied. 
     The invention has been described in detail with particular emphasis being placed on the preferred embodiments, but variations and modifications may occur to those skilled in the art to which the invention pertains.