CA1124125A - Auto focus movie camera using periodic range pulses - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
The lens mount of a movie camera is automatically positioned to maintain an image of a subject in focus on a focal plane of a recording station by generating periodic range pulses having a characteristic directly related to subject range, and converting each pulse to a number which represents the focus position of the lens mount. Such number is stored in a first register whose contents vary in response to changes in subject distance at a rate dependent on the pulse repetition rate of the range pulses. The actual position of the lens mount is determined in the form of a second number which is stored in a second register whose contents vary in response to changes in the position of the lens mount at a rate determined by the rate of change of position, Rotation of a reversible motor is initiated when the contents of the registers are unequal, rotation being terminated when the contents are equal. The direction of rotation is in accordance with which of the two registers has the larger contents. Alternatively, rotation is initiated only after it is determined that the contents of one register exceeds the other over a majority of a predetermined number or range pulses, the direction of rotation being determined by which of the two registers has the larger contents a majority of the time over such predetermined number of range pulses.

Description

This invention rclates to an automatic focusing system for a movie camera, and to a camera incorporatlng the same.
Automatic focusing systems or movie cameras are known wherein the lens mount is moved to a location, hereinafter called the focus location, at which an image of a subject is in focus on the focal plane of a recording station, in response to a range pulse generated on receipt of an echo from a subject illuminated with a supersonic burst transmitted from the camera.
Arrival of the lens mount at its focus position keys a transducer and ~he cycle repeats alloWing the lens mount to track a subject whose dis~ance is changing. The tracking of a moving subject by the lens mount is done in steps determined by the ~ime required for the lens mount to move to its new focus position.
In this focusing system, the range pulses define time intervals linearly related to subject distance by a fixed scale actor whereby the lens/
subject function, which establishes the relati.onship between the distance of a subject and the position of the lens mount at which a subject at that distance is in focus, is parametrically related to time. The accumulated output of a scaled clock, whose pulse repe~ition frequency varies in accordanc0 with the tim~ derivative of an approximat-ion of the lens/subject function, is the integ-ral of the derivative of the function itsel so that at the end of a timeinterval defined by a range pulse, the number of pul-ses produced by the ~aled clock will be a number representing the focus position o the lens. This number representing the current focus position is stored in a register whose oontents are subtracted from the conten~s of a register containing a number representing the previous focus position of the lens. The sign of the dif-ference controls the direction of rotation of a reversible motor connected to the lens mount while the magnitude of the difference determines the extent the lens mount has to move from i~s previous focus position to reach the new focus posi~ion. Thus, the absolute position of the lens mount ~i.e., its position relative to a fixed position) is not known. Consequently, any error introduced in the actual positioning of the lens mount by a missed pulse, for example, during one ranglng operation will remain during subsequent ranging operations.
It is therefore an object of the present invention to provide a ne~
and improved au~omatie focusing movie camera having smooth and continuous lens mov~ment.
Another object is ~o provide an automatic focusing system for a movie camera wherein errors that may be introduced during one ranging operation are eliminated by ~ubs~quent ranging operations.
An automatic focusing system according to the present invention in-volves generating periodic range pulses having a characteristic directly re-lated to subject distance, whereby the ocus position o the lens mount foreach pulse is reprcsented by a number, and storing such number in a first register, The contents of this register will thus vary in response to changes in subject distance at a rate dependent on the pulse repetition ra~e of the range pulses, and not in accordance with the time required ~or the lens mount to move from one fo~us position to another as -in the case of the prior art.
The present invention also involves dete~mining the actual position of the lens mount in the form of a number which is stored in a second register whose contents thus vary in response to changes in the position of the lens mount at a rate determined by the rate of change o position, and comparing the magnitudes of the content~ o the reg~sters to determine which register has the larger contents.
In one embodiment o the inventi4nJ rotation of a re~ersible motor is initia~ed when the cont~nts of the regis*ers are unequal and halted when the contents ar~ equal. The direction o ~ota~lon is determined by which of the two registers has the larger contents. In another embodiment of the inventionJ
rotation o the motor is initiated only ater it is determined that the con~
tents of one register exceeds the other over a majority of a predetermined number of range pulses, the direction o rotation being determined by which of the two ~egisters has the larger contents a majority of the tlme over the pre-dete~nined number of range pulses. In the las-t-mentioned embodiment, rotation of the motor is inhibited when the contents o the rcgister are equal.

-2-~ ~L~ 5 By reason of providing for peraodic range pulses ~o be generated, and for the contents of the first counter ko be updated during the ti~e ~hat the lens mount is moving toward a position at wh~oh the subject will be in focus, movement of the lens mount will be smooth and continuous and not con-tain any step~wise discontinuities. Purthermore 3 any counting e~rors arising during a ranglng operation will be corrected on subsequent ranging operations because the absolute position of the lens mount as utilized in its positioning.
When taken together, all of the improvements in the au~omatic focusing system result in a more pleasing display of subjects whose distanc0s change during filmlng.
According to a first broad aspect of the present invention, there : is provided in a camera having a range finder for period~cally generating range parameters related to subject distance~, means for varying the operation of the camera in acGordance with subject distance, and means for peraodlcally providing a cam~ra pa~am~te~ related to the operation o the camera, the im-provement comprising: means for comparing the range parameters with the cam-era parameters; means fo~ sto~ing signal~ representati-~e o the diferences between each of a predetermlned number o range parameters and the came~a pa~ameter in a first reg~s~er when a range parameter is grea~er than the ~am-era parameter and in a second register when a range parameter is smaller than the camera parameter; means for determining whi-ch of the two ~egisters has stor0d the greater number of signals after generating the predetermlned number ofrange parameter~; and means responsive to the determin3tion for actuating the varying means to adjust the camera parameter to correspond to the last of the range parameters generated in thc predetermined number.
According ~o a second b~oad aspect of the present invention~ there i5 provided a method for automatically posltioning the lens of a camera to main-tain an image of a subject in fOGUS on a focal plane of a recording station comprising the steps of: periodiGally determlaing the range of a subject in a photographic sc~ne in the orm of a first number ~epresenting the desired lens poSition for fo~using of the subject on the focal plane; determining ~he actual ~J -3-position of the lens in the form of a seconcl number; comparlng sald first and second numbers ~fter the periodic determinatlon of each first number and stor-ing a signal in a first register when said firs~ number ls greater than sald second and ln a second reglster when sald flrst number is less than said second; determining which of the two registers has a majority of stored signals during a prede~ermined group of successively determined first numbers; and initiating displa~ement of the lens only when one of said registers is deter-mined to have a majority content.
The inventlon will now be described in greater detail with re-erence ~o the acc~npanying drawingsl i-n whlch:
Figure 1 ls a slde view of a movie ca~era showing a schematic of the present invention lncorporated khereinto;
Figure 2 ls a perspective view cf a lens mount for a movie camera showing the encoder mechanism by whlch the positlon of the lens mount on the camera is converted into a number representative of such position;
Figure 3 is a block diagram of one embodlment of the automatic focuslng system according to the present lnventlon;
Figure 4 ls a bloc~ digram of a dii.ferent embodlment of the drive means shown in Figuro 3;
Figure 5 i~ a composi~e timing diagram show~ng the relative positions of various pulses in the system; and Figure 6 is a chart gLvlng an example of the manner in ~hich the majcrity loglc opera~es~in the embodiment of Flgure 4.

DESCRIPTION OF T~E PREFERRED EMBODIMENTS
-~eferring now to Fig. 1, reference numexal lO
designates, in schematic form, an automatic focusing movie camera according to the present invention. Camera lO com-prises a housing ll, on the bottom of which is mounted a handle 12 by which a user holds the camera to record a subject 13 through an adjustable lens mount 14 that directs scene light to a focal plane 15 of a recording station when a shutter mechanism 16 is operated. A diaphragm 17 associated with the shutter mechanism 16, in conjunction with the lens mount 14, establishes the instantaneous amount of light incident on the focal plane 15. The diaphragm opening is ; controlled by an activatable photometer circuit 18 in response to scene light.
Also mounted within housing ll is an automatic focus system l9 which, when activated, is responsive to the distance of the subject to the camera, and to changes in ~- such distance for adjusting the position of the lens mount 14 such as to maintain the image of the subject 13 in ~ocus on the focal plane 15. A switch 24 mounted in the camera handle 12 in connection to an energy source (not shown), controls the operation of the pho-tometer circuit 18 and the automatic focus system l9 in accordance with pivotal movement of an actuator 25 as the handle is engaged by the heel of a user's hand when holding the camera in position to record a sceneO
Additionally, switch 24 also couples the battery to motor run switch 21 -to permit scene recording as explained ~low.

Housing 11 also contains a motor 20 which, when energized through motor run switch 21 responsive to an actuator 23, simultaneously operates the shutter 16 and a film indexing claw (not shown) which intermittently drives the film 22 past the recording station located behind the shutter 16.
Finally, a sighting tube 26 is provided for allowing the camera user to frame the scene being filmed.
In the operation of the camera 10, the user grasps the handle 12 and frames the subject 13 by means of the viewfinder 26. As the user holds the handle 12, the switch 24 is closed by the movement of the ac~uator 25 thereby powering the photometer circuit 18 and the automatic focus system 19. The photometer circuit establishes the proper diaphragm opening in accordance with the amount of light in the scene being photographed while the automatic focus system begins to transmit sequential bursts of ultra-son;c energy toward the subject 13, such bursts being designated by refer-ence numerals 27A and 27B in Figure 1. These bursts are reflected by the subject 13 as indicated at 28A and 28B such that the reflected energy is received by the transducer of the automatic focus system.
It should be understood that ~he echo received from the subject 13 due to the interrogating burst 27A transmitted from the system 19 is received prior to the transmission o~ the subsequent interrogating burst 27B. That is to say, the automatic focus system of the present invention is a mono-pulse system with a desired focus position of the lens being computed in response to each interrogating pulse.
The system 19 determines the time interval between transmission of an interrogating burst 27A and the return of its echo 28A for the purpose of es~ablishing the distance of the subject 13 to the camera. Having established such distance, the system 19, when permitted, is effective to move the lens mount 14 toward a focus position in which an image of the ` subject will be focused on the focal plane 15 upon activation of shutter 16.
As indicated previously, activation of the shutter 16 is selectively carried out when the user depresses trigger 23 thereby closing switch 21 and power-r 6-ing motor 20. On the other hand, the autofocus system 19 remains in opera-tion as long as the user maintains his grasp of the handle 12, and is effective to continuously determine subject range and to cause the lens mount 14 to track changes in subject distance both prior to and during filming.
Details of the automatic focus system 19 are shown in Figures 3 and 5, to which reference is now made. When the switch 24 is closed, power is applied to the components of the system 19 and causes a system cycle programmer 31 to divide-down the high frequency output of an oscillator 32 into a transmit and reset pulse train having the same pulse repetition frequency, but shifted in phase. The transmit pulse train produced at out-put 33, which is designated MESs may have a period in the range 100-200 ms;
but a pulse repetition frequency from 6-8 cycles per second is preferred ~see Figure 5). The reset pulse train produced at output 34, and designated RST, is delayed with respect to the MES pulses by about 50 ms which is greater than the round trip time for sonic energy, under normal conditions of temperature and i ,~
., .,~

pressure, for s~lbjects at a dlstance of about 24 feet from the camera which represents the hyperfocal lens position. This arrangement will allow any echo frorn a subject within 24 feet o~ ~he camera to be received by the syskem 19 in the time interval between successive RST pulses ~see Fagure 5).
Transmit and blanking generator 35, to which the MES pulses and the output of oscillator 32 are applled, operates ~he transducer 36 to transmit periodic interro~ating bursts, two of which are illustrated at 27A and 27B.
An echo from the subject due to the burst 27A, indicated at 28A, is received by the transducer 36 where thc echo is routed to a receiver amplifier 37. The amplifier 37 has a ramp gain characteristic controlled by a ramp gain generator 37A to increase the sensitivity of the system to distant subjects. The output of amplifier 37 i5 detected at a receiver detector 3~ to produce an echo pulse ~indicated by reference numeral 39 in Figure 5), such that ~he time bet-ween a transmit puLse 40 ~Figure 5) and its associated echo pulse 39 is prop-ortional to the dlstance between the subject and camera.
This above-noted time interval is utilized an conjunction with a scaled clock 41 to establish a number represen~ative o the desired focus pos-ition of the lens mount 14. The output o the scaled clock 41 ls a train o pulses whose pulse repetition frequency varies with time in the same manner as the time deri-vative othe lens/subject unction. The output of scaled ~P

clock 41 is integrated by accumula-ting the pulses produced by the scaled clock in a binary counter 42; the contents of the counter 42 at any instan-t representing the inteyral of the time derivative of the lens/subject functi.on evaluated from -the time of -transmit pulse 40 to the instant in question.
Consequently, the contents of the counter 42, when echo pulse 39 occurs, is the definite i.ntegral of the time derivative of the lens/subject function, namely a number representing the focus position of the lens mount 14 for a subject whose distance is determined by the time interval between the transmit pulse 40 and the echo pulse 39.
By means of the detector 38, echo pulse 39 is used to strobe parallel entry shift register 43 to thereby transfer the contents of counter 42, at the instant of the echo pulse 39, into shift register 43. Thus, the transducer 36, the receiver 37, the detector 38, the counter 42 and the register 43 constitute means responsive to the sonic echo for - generating a number representi:ng -the focus position of the lens mount 14. As shown in Fig. 5, shortly after the sonic echo or the re5ulting echo pulse 39 occurs, a reset pulse 44 occurs thereby resetting the scaled clock 41 and the binary counter 42 as well as preparing the transmit and blar.king generator 35 for the next transmit pulse 40. In addition, the ramp gain generator 37A is reset in preparation for the next transmit pulse which causes the above cycle of operations to repeat again so that upon generation of the next echo pulse 39, the contents of counter 42 are again strobed into register 43. As a consequence, the number in register 43 repeatedly varies in response to changes in subject distance at a rate dependent on the pulse repetition rate of the transmit pulses 40.

For determining the actual position of the lens mo~mt, a lens positlon decoder 44 which is preferably in gray code and is described in de-tail with regard to Figure 2, is p~ovided. As shown in Figure 2, lens mount 14 ca~rying objective lens 70 i5 rotatably mounted on a threaded member 71 carried by the camera housing 11 so that rotational movement of the lens mount causes a~ial displacement of ~ens 70. Actually, the pitch of member 71 is se-lected such that considerably less than 360 is required to displace the lens from its e~treme close-up axial position to its infinity or hyperfocal axial position. In order to rotate the lens mount, a gear train 72 is interposed between the motor 51 and the gear teeth carried by the periphery of lens mount 14. A slip clutch connection ~not shown) is interposed between the motor 51 and lens mount 14 to permit overruDning of the motor in the e~ent of a jam or engagement of the lens mount with a stop establishing the limit of axial dis-placement at either terminal position. Mounted on and rotatable with lens mount 14 is a disc 75, carrying shaft encoding lndicia 74 in the form of a gray code. Ope~a~ively associated wi~h the indicia is a plurality of photocells 73 and light sources 76 whereby the output of each photocell 73 provides one bit of information on the angular position of the lens mount. A sufficient number of bits is provided to provide the desired degree of accuracy in determining the linear position of the lens 70.
Turning again to Figure 3, the output of the decoder 44 is applied to a gray-to-binary converter reglster 45 which cons~itutes means responsive to the position of the lens mount for generating a number representative o the actual position of the lens mount.
The register 43 is a first regi~ter of the system and stores a number representative of the focus position of the lens mount, the contents of this irst register varying in response to changes in subject distance at a rate dependent ~n the pulse repetition ~ate of the transmit pulses. Note that the distance of the subject is sampled at a relatively high frequency to ac-commodate movement of human subjects.
The gray-to-binary register 45 consti-tutes a second register of the ' -10-system ~ at stores a number representative of ~he actual position of the lens mount, the contents of this register varying in response to changes in the lens mount position at a rate determined by the rate of change of position.
The rate o change of the contents of register 45 is thus independent of the rate at which th0 contents of register 43 are updated.
The contents of registers 43 and 45 are compared in a magnitude comparator 47 to determine, on a continuous basis, which register contains the larger number. Since each register n~mber is based on the same reference ~i.e., the focus position and the absolute position are measured rom the same reference point), the contents of the registers will be equal when the actual position of the lens moun~ corresponds to the focus position. When the con-tents of one register exceeds ~he other, the actual position of the lens - mount will be changed by an amount equal to the difference between the con-tents. Whether the actual position is on one side or the other of the focus posi~ion will depend, of course, on which register contains the larger number.
With knowledye of the existe~ce of a diference in con-tents as well as identification of -the reyister with the larger number, a drive means 4~ is responsive -to the output of the comparator means for causiny the lens mount 14 to track changes in subject dis-tance.
The comparator 47 has first and second terminals 49 and 50. A signal appears on first terminal 49 only when the contents of the first register 43 exceeds the contents of the number in second register 45. If the numbers in the registers are designated A and B, then a signal will appear on terminal 49 when A > B. Ordinarily, a signal will appear on second terminal 50 only when the reverse relationship between the magnitudes occurs, namely B ? A.
Drive means 48 includes reversible motor 51, opera-tively associated with the lens mount 14 through a mechanical connection 52, a forward motor control 53 and a reverse motor control 54. The forward motor control 53 is responsive to a signal on the first terminal 49 for operating the motor in one direction to thereby move the lens mount in one direction so as to increase the number in the register 45 and thereby ~ending to null the signal on terminal 49. Whether a null condition is reached, however, depends on what ls concurrently happening -to the contents of the register 43. This, of 2S course~ depends on changes in subject distance.
Similarly, a signal on the second terminal 50 causes a reverse motor control 54 to operate -the motor 51 in the vpposite direction, therehy moving the lens mount in the opposite direction and to thereby decrease the number in the register 45. The movement should -tend to nullify the .f~ Z,~

siynal on terminal 50 if the subject is stationary. When a signal appears on neither oE terminals 49 or 50, neither controls 53 or 54 is operated and motor 51 remains stationary.
As indicated above, -this condition exis-ts when the actual position of the lens mount corresponds to the focus posi-tion for the targe-t subject.
As the subject distance changes, the time intervals between successive transmit pulses 40 and echo pulses 39 change in direct relationship thereby causing strobing of the contents of the counter 42 into the register 43 at a diEferent point in time measured from the beginning of each initiation of operation of the scaled clock 41. The number in the register 43 thus changes and the output of the comparator 47 reflects this change, causing the motor controls 53 or 54 to operate in such a manner as to reduce the difference between the contents of the registers 43 and 45 as the actual position of the lens mount approaches the focus position. secause the position of the lens mount 14 is determined in an absolute manner by reason of the decoder 44, any error that may be introduced during a ranging operation, due to missed or added pulses which affect the contents of the registers 43 and 45, is corrected on a subsequent ranging operation. Thus, errors introduced into the system are self-corrected.
In photographing a moving object at a distance that remains substantially constant during a filming interval, mGtion of the object may affect the manner in which the sonic interrogation bursts are reflected at which echo pulses 39 occurs with respect to transmit pulses 40. In addition, a subject may change in range slightly during a filming interval but it may not be desirable to track such change to reduce undesirable ~isual effects when a reproduction of the scene is displayed.
Reduc-tion in hunting of the lens mount 14, and still smoother changes in Eocusing can be achieved by utilizing the circuitry shown in Fig~ 4 which is a modifi-cation of the drive means shown irl Fig. 3. In Fig. 4, a drive means 48A includes a pair of sample~and-hold means 60 and 61, respectively associated with each of terminals 49 and 50 of the comparator 47. Each of the sample-and~hold means includes a serial entry shift register 62, 63 having a pluralit~ of cells (four are shown, but the number is not important) with an input cell 62A of register 62 connected to the terminal 49 while an input cell 63A is connected to the terminal 50. The state of the input cell of a serial ent~y shift register is dependent upon the presence or absence of a signal on the terminal connected to the input cell. Shifting of the contents of one cell in the shift registers to an adjacent cell occurs upon the application of a shift pulse to each of the shift registers as shown in ~ Fig. 4. The shift pulses are obtained by delaying echo pulses 39 as indicated in Fig. 5 by means of a delay circuit 65 whose output is the pulse RCD.
As indicated in Fig. 5, the shift pulses 64 are delayed with respect to the echo pulses 39, such delay being of the order of magnitude of 100 ms. Thus, each time the distance of a subject is interrogated and a number represen-tative of the focus position is generated and stored in regis-ter 43, the contents of each cell in registers 62 and 63 are transferred. This situation is indicated in the chart of Fi~. 6 wherein for a given example,just prior to the firs-t shiEt pulse, the contents (A) of register 43 exceeds the contents (B) of regis-ter 45 with the result that a signal was present in line ~9 (the input to regis-ter 62) when the first shift pulse occurre~. If the state of line 49 when a signal is present is designated ONE, -then shift pulse 6g will transfer O~E into cell 62A. Eig. 6 shows the same situation existed when the second shift pulse occurred. However, when the third shift pulse occurred, the contents of registers 43 and 45 were equal. When the fourth shift pulse occurred, the contents of register 45 exceeded the contents of register 43. Finally, when the fifth shift pulse occured, the contents of register 43 exceeded the contents of register 46. When each of these shi~t pulses occurred, contents of the cell was transferred. Consequently, after the fifth shift pulse occurs, the five cells of the shift register 62 will have the contents listed ln column A while the five cells of the shift register 63 will have the contents listed in column B.
A first majority logic gate 65 is assoc1ated with the shift register 62 while a second majority logic gate 66 -~ 20 is associated with the shift register 63. Each of these logic gates tests the contents of the cells in the registers with which they are associated and produces an output when the majority of the cells in the associated shift register has a given state indicated hy the presence of a signal. In other words, the gates 65 and 66 respectively provide a control signal only when the sample-and-hold means has a ; majority of a predetermined number of samples.
When gate 65 produces a signal, motor control 53 is enabled for causing motor 51 to run in one direction. When ~L~ 5 gate 66 provides an outpu-t, motor control 54 is enabled for causing the motor 51 to run in the opposite direction. By providing a third terminal 67 for comparator ~7 for indicating that equality exists between the contents of registers 43 and 46, a signal appearing in terminal 67 can be used to inhibit the motor control.s 53 and 54. In this way, the presence of an output on one or the other of gates 65 and 66 cause motor 51 to operate, driving the lens mount 14 in a direction that tends to produce equality in the contents of registers 43 and 45. When equality occurs, the lens mount will be positioned at the focus position and the subject will be in focus. At this instant, an inhibit signal produced in terminal 67 will deenergize motor 51 even while the shift registers 62 and 63 continue to sample and hold the states of terminals 49 and 50. In the absence of terminal 67 and the inhibiting signals applied to controls 53 and 54, motor 51 will tend to overshoot .its proper position, and hunting of the lens mount would occur.
Under most conditlons, this is undesirable and the provision : of terminal 67 is preferred.
Since the apparatus of Fig. 4 determines lens movement upon a majority determination of several ranging interrogations, lens movement is not caused by an interfering object in a scene, for example, bv an object crossing the subject or scene of interest, or a stationary object such as, for example, a tree during camera sweep of a more distant scene.
Consequently, it should be reali~ed that the receiver, detector 37, 33 and register 43 provide means for generating a range parameter related to subject distance, the lens moun-t 14 and motor 51 provide means for varying 2~5 the operation of the camera, or at least the lens assembly thereof, in accordance with subject distance, the grey-binary converter register ~5 provide means for providing a camera parameter related to said subject dis-tance, and said magnitude conparator 47 and said motor controls 53 and 54 provide means for comparing said range parameter with said camera parameter and for varying said camera in accordance with the difference determined by said comparing means so as to thereby produce a camera parameter eliminating said difference.
Additionally, the shift registers 62 and 63 provide means for repeatedly storing the difference, and the gates 65 and 66 provide means for determining which of said registers has a majority.
It is believed that the advantages and improved results furnished by the apparatus of the present in~ention are apparent from the foregoing description of the several embodiments of the invention. Various changes and modifi-cations may be made without departing from the spirit and scope of the invention as sought to be defined in the claims that follow.

Claims (11)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a camera having a range finder for periodically generating range parameters related to subject distances, means for varying the operation of the camera in accordance with subject distance, and means for periodically providing a camera parameter related to the operation of the camera, the improvement comprising:
means for comparing the range parameters with the camera parameters;
means for storing signals representative of the dif-ferences between each of of a predetermined number of range para-meters and the camera parameter in a first register when a range parameter is greater than the camera parameter and in a second register when a range parameter is smaller than the camera parameter;
means for determining which of the two registers has stored the greater number of signals after generating the pre-determined number of range parameters; and means responsive to the determination for actuating the varying means to adjust the camera parameter to correspond to the last of the range parameters generated in the predetermined number.

2. The camera of claim 1, wherein the camera parameter is the focus position of an objective lens displaceable for focussing.

3. The camera of claim 2, wherein the varying means is a reversible drive motor operatively connected to the lens.

4. The camera of claim 3, wherein the motor comprises a forward motor control for driving the motor and the lens in one direction upon determination of the other register.

5. The camera of claim 4, wherein the determining means comprises a comparator having first and second terminals respectively connected to the first and second registers.

6. The camera of claim 5, wherein the comparator comprises a third terminal on which a signal appears only when the contents of the first and second registers are equal, the signal on the third terminal serving to inhibit both the forward and the reverse motor controls.

7. The camera of claim 5, wherein the determining means associated with each terminal includes a shift register the output cells of which are connected to such terminal, the contents of each cell in the shift register being shifted in response to pulses related to the range parameters.

8. The camera of claim 1, wherein the determining means is rendered operative upon generation of each of the range parameters.

9. The camera of claim 1, including means responsive to the comparing means for inhibiting the varying means when the last of the predeter-mined number of range parameters equals the camera parameters.

10. A method for automatically positioning the lens of a camera to maintain an image of a subject in focus on a focal plane of a re-cording station comprising the steps of:
periodically determining the range of a subject in a photographic scene in the form of a first number representing the desired lens position for focusing of the subject on the focal plane;
determining the actual position of the lens in the form of a second number;
comparing said first and second numbers after the periodic determination of each first number and storing a signal in a first register when said first number is greater than said second and in a second register when said first number is less than said second;
determining which of the two registers has a majority of stored signals during a predetermined group of successively determined first numbers; and initiating displacement of the lens only when one of said registers is determined to have a majority content.

11. The method of claim 10 including the step of displacing the lens responsive to the initiating displacement step to a position in correspondence with the last produced first number.