Background of the Invention Maintaining a properly focussed image is critically important in film productions. This task is complicated by the fact that many shots require a moving object to be kept in focus or require different objects at different distances from the camera to be in focus at different times during the shot. Typically, on a movie set, the person responsible for focussing the camera, who is called the "first assistant camera"prepares for each shot by carefully measuring the distance to each object which will be in focus during the shot and then carefully marking the focussing ring on the camera lens with the measured distances. All professional lenses have markings which indicate the distance at which the lens is focussed. These markings are not continuous. Even if the precise distance to an object is known it is often necessary to estimate how the focussing ring should be set to focus on the object because the lens will often not have a marking which indicates when the lens os focussed at that precise distance. Focus is particularly critical when using telephoto lenses because, in some cases, one inch (2.5 cm) may mean the difference between soft focus and sharp focus.

During the shot the first assistant camera adjusts the focussing ring to bring each object into precise focus at the desired time. This method works well with immobile objects. If the camera or the elements to be photographed are in motion it becomes very difficult to maintain the proper focus. Furthermore, this method is time consuming. The first assistant camera must make careful measurements before each shot. The enormous costs of production creates a need for a quick and accurate means for maintaining focus during a shot.

The prior art discloses a number of ways to maintain focus in a movie camera. U. S. Patent No. 5,092,670 to Preston discloses an automatic focussing system for use with a motion picture camera. This system incorporates a laser range finder and triangulation to determine the distance to the object and adjusts the camera lens accordingly. The laser range finder is mounted on a tripod that is a fixed distance from the camera. While this system is useful for tracking moving objects, it is not practical for use with a moving camera. Where wide angle lenses are used in tight

quarters it may be difficult to keep the range finder out of the field of view of the camera. Finally the system requires an operator for the range finder. This adds to the cost of filming.

Shenk, U. S. Patent No. discloses an auto-focussing camera incorporating an object presence detecting transducer. The camera automatically focuses on objects within its sonic acceptance angle. The Shenk device does not enable a user to focus on one of several objects within the acceptance angle or to shift the focus between different objects in the sonic acceptance angle.

Bogle et al., U. S. Patent Nos. 4,601,557 and 4,534,629 disclose motion picture cameras having automatic focussing systems. The cameras uses microwave transmitters to transmit signals to targets wearing reactive signal generators. These systems require the objects of focus to wear a signal generator. This is not always practical, for instance when shooting nature documentaries.

Some prior art auto-focus systems include a partially silvered mirror, or the like, which intercepts some of the light passing through the lens of a motion picture camera. Such systems are undesirable because they degrade the optical path. In cinematography it is important to maintain the highest quality optical path possible.

One can compensate somewhat for inaccurate focussing by using a small lens aperture to increase the depth of field of the motion picture camera. This can be undesirable, especially when shooting at night, because a small lens aperture reduces the amount of light that passes through the lens. Using a small lens aperture can make it necessary to provide supplementary illumination.

This adds expense and increases the time needed to set up for a shoot.

Systems which measure distance to an object by measuring the parallax of two laser beams or other parallax indicators typically suffer from the defect that they are only effective on the optical axis ofthe camera's lens. Such systems generally cannot provide indications ofthe distance to objects which are not on this axis.

What is needed is a focus indicator system for a motion picture camera that will allow the person responsible for focussing the camera to quickly verify that a selected object is in precise focus. There is a particular need for such a device that can work with existing camera lenses and cameras.

SUMMARY OF THE INVENTION This invention provides a focus indicator system for a motion picture camera. The focus indicator comprises first and second spaced apart pivotally mounted video cameras. The video cameras can counter rotate about parallel first and second axes respectively. The first and second video cameras have optical axes which cross one another at a point in front of the camera. A monitor displays first and second images generated by the video cameras. The first and second video cameras may be coupled by a coupling to a focus ring on a camera to counter rotate in response to rotation of the focus ring to maintain the crossing point at the distance for which the camera is focussed.

The coupling between the lens and the video cameras preferably comprises an electro- mechanical coupling. Most preferably the coupling comprises a transducer engageable with the focus ring, a control circuit to receive signals from the transducer which indicate the change of position of the focus ring in relation to the focus ring's position when the focus distance is set to infinity, a motor controlled by the control circuit, and a linkage driving counter rotation of the video cameras in response to operation of the motor.

Yet another aspect of the invention provides a method for indicating when a lens of a motion picture camera is focussed on an object. The method comprises providing a focus indicator having first and second video cameras mounted on opposed sides of a lens of a motion picture camera; in response to rotation of a focus ring on the lens pivoting the first and second video cameras about parallel axes so that optical axes of the first and second video cameras cross in front of the lens; and, simultaneously displaying on a monitor first and second images of an object as viewed by the first and second video cameras respectively. The method can be used to focus on objects which are not in the center of a field of view of the lens.

BRIEF DESCRIPTION OF THE DRAWINGS In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way: Figure 1 is a partially schematic top view of a motion picture camera equipped with a focus indicator according to the invention; Figure 2 is a front elevation thereof ; Figure 3 is a side elevation thereof ;

Figure 4A is a schematic view of an alternative embodiment of the invention which has a pair of video cameras mounted on a bracket on either side of a camera lens ; Figure 4B is a view of the alternative embodiment of Figure 4A with the bracket remounted so that the video cameras are above and below the lens; Figure 5A is a top plan schematic view of a pair of video cameras in a focus indicator according to the invention wherein the focus distance is set at a first distance; Figure 5B is a top plan schematic view of a pair of video cameras in a focus indicator according to the invention wherein the focus distance is a second distance less than the first distance; Figure 6A is a view of a monitor screen of the focus indicator with the camera focussed at the first distance of Figure 5A; Figure 6B is a view of the monitor screen with the camera focussed at the second distance of Figure 5B; Figure 6C is a view of the monitor screen showing a very distant object with the camera focussed at infinity; and, Figure 7 is a block diagram illustrating functional components of the invention.

DETAILED DESCRIPTION Apparatus Figures 1,2 and 3 show views of a motion picture camera system comprising a motion picture camera 20 equipped with a focus indicator according to the invention. Camera 20 has a lens 26 which includes a focus ring 28 for adjusting the distance at which camera 20 is focussed.

A separate focus knob 29 (Fig. 3) may be coupled to focus ring 28 so that focus ring 28 can be easily adjusted by hand. Typically a matte box 30 is mounted at the forward end of lens 26. Matte box 30 is typically slidably mounted on a pair of support rods 34 which are themselves mounted on a plate 32. The distance from matte box 30 to the body of motion picture camera 20 (Figure 3) can be precisely adjusted to accommodate different lens lengths. Camera 20 is preferably a professional motion picture camera as is used in cinematography.

The focus indicator comprises a pair of video cameras 38R and 38L (collectively, video cameras 38) which are respectively pivotally mounted on opposite sides of lens 26 for rotation about axes 41R and 41L (collectively axes 41-Figs. 2,4A and 4B) which are parallel to the film

plane of camera 20. Video cameras 38 should be located so that they are not in the field of view of lens 26. Most preferably video cameras 38 are located symmetrically with respect to the centre line H of lens 26.

Video cameras 38 each have a field of view. Each video camera 38 generates an image of its field of view and displays the image on a monitor 42. The images generated by video cameras 38L and 38R are superimposed on monitor 42. Video cameras 38 are preferably compact and lightweight. Video cameras 38 preferably have zoom lenses 39, or interchangeable fixed focus lenses 39, so that their fields of view can be made to be roughly equal to the field of view of lens 26. Video cameras 38 are preferably sensitive so that they capable of creating images for display on monitor 42 under low light conditions. Video cameras 38 have appropriate power supplies.

For example, video cameras 38 may be powered by a battery, or other suitable power source, 45 (Fig. 7). Power connections are indicated by the lines labelled P in Figure 7.

Each video camera 38 has an optical axis 43. Most preferably the optical axes 43 of video cameras 38 and centre line H all lie in the same plane. Video cameras 38 are coupled to focus ring 28 of lens 26 by a coupling indicated generally by 44 (Fig. 7). Coupling 44 is preferably an electro-mechanical coupling. Coupling 44 operates so that optical axes 43 for video cameras 38 are both parallel to the axis H of lens 26 when focus ring 28 is adjusted for focus at infinity. When focus ring 28 is turned to focus lens 26 at another distance then coupling 44 operates to rotate video cameras 38 about axes 41.

When the focus indicator is in use, the optical axes 43 of video cameras 38 cross each other, (as viewed from above) on centre line H, at the distance on which lens 26 is focussed. This distance may be called the"focus distance"of lens 26. While optical axes 43 preferably intersect, it is not necessary for optical axes 43 to intersect one another. One of optical axes 43 may be displaced somewhat relative to the other optical axis 43. The fields of view of video cameras 40 should substantially overlap.

Signals 138R and 138L carrying images from each of video cameras 38R and 38L are carried to a video mixer 49 (Fig. 7). The output 149 from video mixer 49 is displayed on monitor 42. The result is that images from both of video cameras 38 are simultaneously displayed on monitor 42. The first assistant camera can tell whether a particular object is in focus by viewing the images displayed on monitor 42 as is described in more detail below. Instead of connecting

cameras 38 to monitor 42 by wires, each video camera 38 may have a wireless video transmitter for sending its image to a video receiver associated with display 42.

As shown in Figures 5A, 5B, 6A and 6B, a person viewing monitor 42 can tell whether a particular object is in focus or not. If the focus indicator has been properly set up, as described below, and lens 26 is focussed on an object, then the image of that object taken by each of video cameras 38 will coincide on monitor 42. If the object is not in focus then the image of that object taken by one of video cameras 38 will be offset relative to the image of that object as taken by the other one of video cameras 38.

In the preferred embodiment of the invention coupling 44 includes a controller 50 (Fig.

7) which receives input from a transducer 52 coupled to focus ring 28. Controller 50 adjusts the angular position of each of video cameras 38R and 38L about their respective pivot axes 41R, 41L. Controller 50 causes video cameras 38R, 38L to pivoted by the same amount, but in opposite senses, as the focus of lens 26 is adjusted by rotating focus ring 28 so that the optical axes 43 of video cameras 38 cross at the focus distance of lens 26.

As it is desirable that the focus indicator can operate with standard unmodified lenses 26 it is generally desirable for the focus indicator to couple to the outer surface of focus ring 28. In the preferred embodiment, which is illustrated in the drawings, coupling 44 comprises a gear 53 which meshes with teeth on the circumference of the focus ring 28. The focus rings 28 in most professional lenses have suitable teeth. A few different interchangeable gears 53 may be provided to match the focussing rings on different lenses. Gear 53 is supported adjacent focus ring 28 by a suitable bracket 54. Gear 53 is connected to a transducer 52, which may, for example, be a potentiometer, an optical encoder or the like. Transducer 52 generates or modifies an electrical or optical signal in a way which varies as focus ring 28 is turned. Transducer 52 preferably produces a signal which is proportional to the amount of movement of focussing ring 28. This signal is carried from transducer 52 to controller 50.

In the currently preferred embodiment of the invention, the angular position of each video camera 38 is set by one of two servomotors 55R, 55L. Each servomotor turns a member (not shown) on which one of video cameras 38R, 38L is mounted. The servomotors preferably comprise step down transmissions 58 which are coupled to the members on which the video cameras 38 are mounted. This permits video cameras 38 to be smoothly simultaneously rotated

through equal angles but in opposite senses about axes 41R and 41L in response to changes in the focus distance of lens 26. Transmissions 58 preferably have ratios of about 2000: 1 to provide very smooth adjustment of the angular positions of video cameras 38 while permitting video cameras 38 to track reasonably fast changes in focus of lens 26. Transmissions 58 may comprise a worm gear. Servomotors 55 preferably also comprise feedback transducers 59 to provide feedback signals 159 to controller 50.

As shown in Figure 7, where video cameras 38 are positioned by means of servomotors 55, controller 50 preferably comprises a pair of servo controllers 50R and 50L one for each servomotor 55. The servo controllers may comprise, for example, MOTOROLAT^'model No.

MC 33030 dc servo controllers. Each servo controller is connected to receive a signal 152 from transducer 52 and a feed back signal 159 from a second transducer 59. Second transducer 59 is coupled to detect directly the angle of its corresponding video camera 38 and to provide feedback to the controller 50 for that video camera 38. Transducer 59 may be coupled directly to the output of servomotor 55, to the motor of servomotor 55 itself, or to an intermediate point in transmission 58. Each servo controller 50 generates a drive signal 150 which drives its corresponding servomotor 55.

Coupling 44 must be adjustable because, for different lenses 26, the range of rotation and diameter of focus ring 28 can differ. Furthermore, the closest possible focus distance is different for different lenses. In the preferred embodiment of the invention, controller 50 comprises a control 60, which can be used to adjust the amount of rotation of video cameras 38 produced by a given input from transducer 52. Preferably a second control 61 is provided to adjust the offset of the images provided by cameras 38.

In the embodiment illustrated in Figures 1-3, video cameras 38, servo motors 55 and step- down transmissions 58 are mounted on brackets 63, which are received in holders 64 on matte box 30. This permits the focus indicator to be quickly attached to or detached from camera 20.

The focus indicator can preferably be selectively attached to the camera system in at least two orientations. In a first"horizontal"orientation video cameras 38 are on either side of lens 26.

In a second"vertical"orientation video cameras 38 are located above and below lens 26.

Preferably holders 64 are provided on top and bottom of matte box 30 as well as on each side of matte box 30. This permits video cameras 38 to be mounted on either side of lens 26, as shown,

or, in the alternative, mounted above and below lens 26. Positioning the focus indicator with video cameras 38 above and below lens 26 allows the focus indicator to be used in confined areas.

The distance between video cameras 38 is preferably adjustable. When used with a typical motion picture camera 20, video cameras 38 are typically on the order of 14 to 20 inches (35 to 50 cm) apart. If the motion picture camera 20 is being used in a telephoto shot, a greater distance separating video cameras 38 may be preferable to achieve greater accuracy of focus.

Figures 4A and 4B show an alternative embodiment of the invention in which a focus indicator has an L-shaped bracket 65 secured to rods 34. Bracket 65 may be locked in position at a desired point along rods 34 by any suitable locking means. Video cameras 38 are mounted on a bracket 66, which may be connected to bracket 65 in either of two orientations. The focus indicator of Figures 4A and 4B may be mounted with video cameras 38 positioned on either side of lens 26 as seen in Figure 4A, or alternatively above and below lens 26 as best seen in Figure 4B.

In the embodiment of Figures 4A and 4B, the position of video cameras 38 can be moved forward or rearward relative to matte box 30 by sliding bracket 65 forward or rearward along rods 34.

Monitor 42 is located in a position where the first assistant camera (who is responsible for focussing camera 20) can easily view monitor 42. For example, monitor 42 may be mounted at a position above camera 20. In a preferred embodiment of the invention, monitor 42 is detachable.

Where monitor 42 is detachable, monitor 42 may be connected to other components of the focus indicator by a wire. Most preferably, however, the focus indicator includes a wireless video transmitter 70, which transmits an image for displaying on monitor 42, to a compatible receiver 71 built into monitor 42. When monitor 42 is connected to the focus indicator then the image for displaying on monitor 42 is conducted directly to monitor 42 by a suitable cable and transmitter 70 and receiver 71 are not active. Transmitter 70 and receiver 71 may be turned on automatically when monitor 42 is disconnected from the focus indicator or switches may be provided so that they may be manually turned on.

Monitor 42 is preferably mounted in a manner which permits it to be rotated through 90 degrees when the orientation of video cameras 38 is changed from vertical to horizontal, or vice

versa. In the alternative, video cameras 38 may each be housed in a tubular case. The case of each video camera 38 may be received in a tubular holder not shown. Video cameras 38 can then be rotated in their holders so that the images produced by video cameras 38 remain upright on monitor 42 when the focus indicator is moved from a horizontal orientation (shown in Figure 4A) to a vertical orientation (shown in Figure 4B). Detent mechanisms (not shown) may be provided in the holders so that video cameras 38 may be easily rotated into the correct orientation for use in horizontal or vertical orientations.

Method of Use A focus indicator is prepared for use by mounting a desired lens 26 on camera 20 and adjusting the separation distance of video cameras 38 to match the lens and shooting conditions.

If lens 26 is a wide-angle lens or if the shooting location is very cramped then cameras 38 will typically be positioned close to one another. If lens 26 is a telephoto lens then video cameras 38 will generally be positioned farther apart to provide better accuracy. The distances from each video camera 38 to lens 26 are preferably equal.

The first assistant camera next adjusts the zoom of both of video cameras 38 so that the images displayed on monitor 42 are of similar size and perspective to the field of view through lens 26 of camera 20. A single control may be provided to simultaneously zoom each of video cameras 38 so that both cameras 38 will always have the same zoom setting. In the alternative, video cameras 38 may have zoom lenses 39 which are marked with a focal length scale so that they can both be set easily to the same focal length.

Next the first assistant camera sets focus ring 28 to focus lens 26 at infinity and points camera 20 at a distant object. Transducer 52 is set to its position representing focus at infinity.

This causes video cameras 38R, 38L to be oriented so that their optical axes 43 parallel to axis H of lens 26. Control 61 is used, if necessary to superimpose the images of the distant object produced by each of video cameras 38 on monitor 42. The image of a distant object 99 as displayed on monitor 42 is shown in Figure 6C. Gear 53 is then meshed with the teeth on focus ring 28.

Next, the first assistant camera rotates focus ring 28 so that lens 26 is focussed at its minimum focus distance on an object A (Figure 5A) which is located in front of camera 20 on line

L2. The first assistant camera manipulates control 59 to cause controller 50 to turn video cameras 38 until the two images of object A are superimposed on monitor 42. After this has been done, controller 50 will automatically pivot video cameras 38 by equal amounts, in opposite senses, in response to changes in the position of focus ring 28 so that the optical axes 43 of cameras 38 cross at the focus distance of lens 26.

Figure 6A shows the screen of monitor 42 when lens 26 is focussed on a first object A as shown in Figure 5A and the focus indicator has been set up as described above. Several objects B1, B2, B3 are located farther away from lens 26 than object A. Objects B1, B2, and B3 are not in focus. Another object C is closer to lens 26 than object A. Object C is also not in focus. It can be seen that monitor 42 displays two images of each of objects B1, B2, B3 and C. The images are offset from one another. As shown in Figure SA, when lens 26 is focussed on object A, video cameras 38 are each aimed in a different direction. Any object not at the focus distance of lens 26 will appear as a double object on monitor 42.

As seen in Figure 6A, when focus ring 28 is rotated so as to focus lens 26 on objects B1, B2, and B3 which are located farther away than object A then cameras 38 are rotated simultaneously and in opposite directions until their optical axes 43 are more nearly parallel with centre line H. After cameras 38 have been rotated then the images of objects B1, B2, and B3 on monitor 42 as produced by each of cameras 38 coincide, as shown in Figure 6B. The images of object A produced on monitor 42 by each of cameras 38 are no longer aligned with one another and produce a double image on monitor 42. Object C also produces a double image on monitor 42. The first assistant camera can thus tell from viewing monitor 42 that objects B1, B2, and B3 are in focus and objects A and C are not in focus.

It can be appreciated that the adjustability of coupling 44 allows the use of a focus indicator with a wide variety of lenses commonly used in the motion picture industry. No modifications to the lenses are necessary as long as they are equipped with focus rings capable of being engaged by gear 88 or otherwise coupled to transducer 52.

After the focus indicator has been calibrated for the lens 26 being used, as explained above, the first assistant camera will continue to adjust the focus of lens 26 by manually rotating focus ring 28 or by operating a wireless transmitter or other focus driving device to remotely <BR> <BR> <BR> manipulate focus ring 28. As this is done, video cameras 38R, 38L automatically pivot, as

described above. The first assistant camera can see immediately by viewing monitor 42 what objects in the fields of view of cameras 38 (and lens 26) are in precise focus.

The ease with which the first assistant camera can determine where the focus of camera 20 is at any particular time greatly improves the speed and efficiency with which the first assistant camera can maintain focus in a situation where the distance from camera 20 to the object of the shot changes rapidly. The precision with which the focus indicator permits a lens 26 to be focussed in low light conditions can permit shooting with a larger aperture (and therefore a smaller depth of field) than would otherwise be possible. This, in turn, can reduce or eliminate the need for supplementary illumination systems in some cases.

As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example, it will be readily apparent to those skilled in the art that other equivalent systems may be provided to cause video cameras 38 to pivot in opposite senses about parallel axes 41L and 41R as the focus of lens 26 is changed. Controller 50 may take many forms using known circuit elements. Instead of servo motors 55 the invention could be practised, for example, using a stepper motor a small direct current motor, or another actuator capable of smoothly turning video cameras 38 by a finely controlled amount. While motors 55 have been illustrated as rotary motors, motors 55 could comprise one or more linear actuators suitably linked to counter rotate video cameras 38. Instead of providing a separate motor 55 to control each video camera 38 a single motor could be provided to control both cameras 38 simultaneously through a suitable mechanical linkage. The mechanism for allowing the spacing between video cameras 38 to be adjusted may also be varied without departing from the invention.

While it is not preferred, the spacing between cameras 38 may be fixed. While the connection between transducer 52 and controller circuit 50 has been described as an electrical connection, the connection could comprise an optical link instead. Some types of controller 50 may not require feedback from transducers 59. Video cameras 38 may be firmly supported on either side of lens 26 in any suitable way, not just in the manners described above.

Each video camera 38 may comprise a wireless transmitter which transmits a signal capable of being received by a receiver associated with monitor 42. Depending upon the type of monitor 42 which is used it may not be necessary to provide a separate video mixer 49. Monitor

42 may comprise any suitable display capable of displaying images from video cameras 38 in real time. Those skilled in the art will realize that many other modifications and alterations may be made without departing from the broad scope of this invention.

Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.