GB1576306A - Focus detecting circuit - Google Patents

Description

PATENT SPECIFICATION ( 11) 1 576 306

( / ( 21) Application No6684/78 ( 22) Filed 20 Feb 1978 ( 19 j ( 31) Convention Application No's 52/017332 ( 32) Filed 19 Feb 19774// 52/075506 27 Jun 1977 4 ' 52/155687 26 Dec 1977 in & E t ( 33) Japan (JP) ( 44) Complete Specification Published 8 Oct 1980 ( 51) INT CL 3 G 03 B 3/10 ( 52) Index at Acceptance H 4 D 711 759 762 764 771 773 782 ( 72) Inventor: SEINAN MIYAKAWA ( 54) FOCUS DETECTING CIRCUIT ( 71) We, ASAHI KOGAKU KOGYO KABUSHIKI KAISHA, a corporation organised and existing under the laws of Japan, of No 36-9, Maeno-cho, 2-chome, Itabashi-ku, Tokyo-to, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in

and by the following statement: 5

This invention relates to a focus detecting circuit.

Focussing systems have been proposed which use a photoconductive element having a peak response at the in-focus position to control a drive motor operating the focus adjustment Problems arise in detecting this peak value with sufficient accuracy under different brightness conditions to ensure accurate focussing 10 According to this invention there is provided a focus detecting circuit, comprising a photo-sensitive element arranged to provide an output signal having an alternating component the magnitude of which reaches a limit value when an image is focussed on it, a DC blocking member connected to the photo-sensitive element, a rectifier for rectifying the alternating component passed by the DC blocking member, and a differentiator connected to 15 the rectifier to differentiate the rectified alternating component.

It is preferred to use as a photo-sensitive element an element which is supplied with light from the image forming plane by means of a rotating reflective system such as an optical fibre which thus continuously scans an annular region of the image forming plane Alternatively an oscillated element could be used, or an element which receives light through an oscillating 20 plate having a hole or slit in it In any event the element output will have an alternating component which is based on the scanning or oscillation frequency, and which component will reach a maximum value when the light and dark portions of the image are correctly differentiated, i e at the in-focus position Away from this position the image becomes progressively blurred and the alternating component reduces towards zero 25 The invention will now be described in more detail, by way of example, with reference to the accompanying drawings, in which:Fig 1 is a circuit diagram of a focussing system embodying the invention and illustrating its principle of operation; Fig 2 shows certain waveforms appearing in the circuit of Fig 1; 30 Fig 3 is a circuit diagram of another system embodying the invention; Fig 4 is a circuit diagram of a further system embodying the invention; and Figs 5 A to 5 F are circuit diagrams of part of the circuit of Fig 4 showing the positions of the switch contacts in various successive operation stages.

In the various embodiments similar elements are given the same reference numerals 35 The focussing system of Fig 1 includes a photo-sensitive element 4, shown as a photodiode, which is positioned to receive light at the desired image forming plane of the image-forming optical system the focus of which is to be adjusted The element 4 provides an output signal which tends to increase as the focus is adjusted towards the in-focus position and takes a maximum value at that position The element is arranged to scan or oscillate over a region of 40 the image forming plane so that, as long as the object to be imaged has contrast in it, the output of the photo-sensitive element presents a waveform schematically shown at R in Fig 1 which varies with the focus adjustment about the in-focus position and has an alternating or AC component which reaches a maximum at the in-focus position Instead of a photodiode, other photo-sensitive elements can be used, for example a photoconductor 45 1,576,306 The photo-sensitive element 4 preferably forms part of a light detecting device of the type described in our British Patent Application No 6682/78 (Serial No 1576305) filed concurrently herewith, to whicch reference should be made.

A capacitor 17 is connected to the output of the photo-sensitive element, the other terminal of which is grounded, and blocks the DC component of the signal from the element 5 4, passing only the AC component, as schematically indicated at S on Fig 1 The AC component is applied through an amplifier 30 and a rectifying diode 24 to a smoothing circuit comprising a capacitor 40 and resistor 41.

From the smoothing circuit the signal is applied to a differentiating circuit which consists of a capacitor 42 and a resistor 43 This provides a differentiating action with a predetermined 10 time constant A comparator 50 has one input connected to a potentiometer 46 for the application of a constant voltage, and its other input connected to a potentiometer 45 which is connected across a floating power source 44 and has one terminal connected to the output of the differentiating circuit A negative output from the differentiating circuit tends to make the comparator output negative and the potentiometer 45 and source 44 provide a bias circuit 15 to bias the output of the differentiating circuit.

The comparator output is applied to the base or control electrode of a transistor 53 to turn the transistor ON when the output of the comparator 50 is positive A relay 54 has its coil 541 connected to the output electrode of the transistor, the collector as shown, and the transistor emitter is grounded Relay 54 has a contact set 542 which in the energised condition of the 20 relay connects a motor 62 to a +B power source, and when the relay is deenergised disconnects the motor The other side of the motor 62 is connected to ground The motor 62 drives the focus adjusting member of the image-forming optical system with which the focussing system is to be used.

It should be noted here that the circuit to the right of diode 24 can be used as the servo 25 mechanism referred to in our aforementioned Application.

The operation of the system of Fig 1 will now be described, reference being made to the waveforms of Fig 2 Initially the potentiometer 45 and source 44 are adjusted so that the comparator provides a just positive output when the voltage at the output of the differentiating circuit, labelled junction (D on Fig 1, is zero It is assumed that the focus adjustment 30 member is initially set to the infinity position and the motor 62 tends to drive it at a steady speed towards the position of minimum range.

Waveform A in Fig 2 shows the voltage at junction , before the differentiating circuit, representing the amplified AC component of the photo-sensitive element output as the focussing adjustment member is moved through its range Since this movement takes place at 35 a steady speed, this waveform also can be regarded as representing the variation of the output of the element 4 with time over a time period required to complete the travel of the focus adjustment member As shown, the adjustment range extends from infinity to 0 5 metre The voltage at ( remains essentially zero from infinity to a point a approaching the in-focus position and then rises between point a and the in-focus position b to a maximum value at the 40 in-focus position If the motor is allowed to continue to drive the focus adjustment member beyond the in-focus position, the element output then decreases to essentially zero at a point c and stays at zero to the 0 5 m position It will be appreciated that if the object to be imaged is nearer or further away than that assumed in Fig 2, the peak of the curve will shift accordingly to right or left 45 Waveforms B in Fig 2 is that obtained at point (D on Fig 1 at the output of the differentiating circuit comprising capacitor 42 and resistor 43 This waveform thus represents the slope of waveform A From point a to shortly before point b the voltage at is increasingly positive, but at the in-focus position b the waveform changes abruptly through zero to a negative value Where the waveform crosses zero represents the precise peak of 50 waveform A Towards point c waveform B again tends to approach zero, as shown.

The comparator 50 is arranged so that as long as the output of the differentiating circuit is zero or positive, transistor 53 remains turned on thus energising relay 54 and supplying power to the motor 62 When the in-focus position is reached, however, the output of the differentiating circuit turns negative and this causes the comparator 50 to provide a negative output 55 which turns the transistor 53 OFF This de-energises the relay 54, disconnecting the supply to motor 62, which stops, leaving the focus adjusting member correctly adjusted so that the image formed by the optical system is in focus on the desired image forming plane The current through the relay coil is shown at waveform C in Fig 2.

Thus accurate focussing is possible, and it will be the more accurate the nearer the circuit 60 stops the motor to the position where the output of the differentiating circuit passes through zero.

The waveforms A and B are also indicated in outline form on Fig 1, for convenience.

Figure 3 shows a modified focussing system in which the elements up to and including the potentiometer 45 are identical to the corresponding elements of Fig 1 and will not be 65 3 1,576,3063 described again.

In Fig 3 a comparator 80 has its inverting input connected to potentiometer 45, and is arranged so as to provide negative output when the voltage at ( is zero or positive and a positive output when the voltage at ( is negative The comparator 80 is provided with a load resistance 81 and has its output connected to a transistor 82 A relay 83 has its coil 831 5 connected to the output of transistor 82 and controls contacts 832 which in the energised condition of the relay couple the transistor output through a normallyclosed manually operable switch 84 to ground and in the de-energised condition connect motor 62 to ground.

The comparator 80 and associated circuitry are connected to the positive supply through a switch 85 and the relay coil 831 and motor 62 are likewise connected to the supply through a 10 switch 86 ganged with switch 85.

The operation of the system of Fig 3 will now be described, also with reference to Fig 2.

Initially the switches 85, 86 are open and the switch 84 is closed The coil 831 will, of course, be de-energised, and the transistor 82 OFF The focus adjustment member is first moved manually to the infinity position and the switches 85, 86 are closed The motor 62 will be 15 energised through contacts 832 and will drive the focus adjustment member so as to cause the voltage at @ to follow waveform A of Fig 2 and the voltage at ( to follow waveform B So long as the output of the differentiating circuit is zero or positive, the output of comparator 80 will be negative, and transistor 82 will remain OFF Once the in-focus point is passed, the comparator output 80 will turn positive, turning transistor 82 ON and thus energising relay 20 coil 831 Contacts 832 now move to disconnect motor 62, thus stopping the motor with the focus adjustment member at the in-focus position As the motor has stopped, the differentiating circuit output will in fact become zero, but relay coil 831 is maintained energised by way of switch 84 and contacts 832 until either switch 84 or switch 86 is opened Thus the in-focus position remains held 25 When the focussing operation is to be repeated, for example if a new object is to be photographed, the focus adjustment member is moved manually to the infinity position and the switch 84 is opened momentarily Coil 831 is now de-energised and motor 62 starts again, repeating the cycle of operation as described above.

In both Figs 1 and 3 there is a switching circuit between the output of the differentiating 30 circuit formed of capacitor 42 and resistor 43 and the motor 62 These switching circuits may be instrumented in other ways than those indicated, in particular transistors may be used in place of the illustrated comparator circuit and the relay.

A more complex focussing system embodying the invention is shown in Fig 4 In this system the focus adjustment member is automatically moved back towards the infinity 35 position before the cycle of operation described above starts The elements leading to the comparator 50 in Fig 4 are identical to the corresponding elements of Fig 1, with the sole exception that resistor 43 is variable to permit adjustment in manufacture.

The circuit of Fig 4 also includes a comparator 70 having an adjustable reference voltage source constituted by potentiometer 66 Two diodes 51 and 57 each have their anodes 40 connected to the output of comparator 50 to prevent mutual interference between the circuits supplied through them, and a diode 71 is connected to the output of comparator 70 Base resistors 52 and 72 are connected to diodes 51 and 71 respectively, and transistors 53, 73 and 58 have their bases connected respectively to resistors 52 and 72 and diode 57 A relay 54 has its coil 541 connected to the output of transistor 53 and has contacts 542 A relay 59 has its 45 coil 591 connected to the output of transistor 58 and has contacts 592 and 593 A relay 74 has a relay coil 741 connected to the output of transistor 73 and has contacts 742 Noise and spark protection diodes 56, 61 and 76 are connected across the relay coils Light-emitting diodes 55 and 75 are connected between the coils 541 and 741 respectively and the positive supply, while coil 591 is connected directjy to the positive supply Motor 62 is connected between the 50 common contacts of contact sets 542 and 742 and has a noise suppression capacitor 63 connected across it Motor 62 is coupled to the focus adjustment member of the imageforming optical system to be focussed.

The movable elements of contacts 542 and 742 are both grounded when coils 541 and 741 are energised, respectively, and are connected to the +B power source when the coils are 55 de-energised Contacts 592 and 593 are operatively associated with each other and serve to ground the emitter of transistor 58 and the base of transistor 53, respectively, when the coil 591 is energised A manually-operable normally-open switch 60 is provided to initiate movement of the objective from the infinity to the in-focus position, as will be described, and a manually-operable normally-closed switch 77 is provided to cause the motor to reverse and 60 drive the focus adjustment member towards the infinity position when a new object is to be focussed.

The operation of the system of Fig 4 will now be described Reference will be made to the circuits of Figs 5 A to 5 F which show part only of the circuit, with certain elements omitted, and illustrate the operation of the switches and relays 65 1,576,306 1,576,306 Initially it is assumed that the focus adjustment member is at the infinity position and the power source is switched on The output of comparator 50 is positive and transistor 53 is turned ON, energising coil 541 and moving contact 542 so as to connect the upper side of the motor as seen in the figures to earth Switch 60 is open, and no current flows through relay coil 591, so contacts 592 and 593 are both disconnected This condition is shown in Fig 5 A 5 The operator then momentarily closes switch 60, as shown in Fig 5 B This grounds the emitter of transistor 58 which turns ON since the output of comparator 50 is positive This connects contacts 592 and 593 Contact 593 turns transistor 53 OFF, thus de-energising relay 54 and connecting the upper side of the motor 62 to the positive + B supply The lower side of motor 62 is grounded through contacts 742, so the motor now starts to move the focus 10 adjustment member away from the infinity position Meawhile contact 592 holds relay coil 591 energised by grounding the emitter of transistor 58 even when the switch 60 is released.

This is the condition shown in Fig 5 C.

As the motor drives the focus adjustment member, the output of the comparator 50 becomes negative, thus turning transistor 58 off an de-energising relay 59 Contacts 592 and 15 593 are now opened However, transistor 53 remains OFF and thus the motor remains energised through contact 542 of the de-energised relay 54 This condition is illustrated in Fig 5 D.

As the in-focus position is passed, the output of comparator 50 turns positive and transistor 53 is again turned ON This energises relay 54, causing the upper side of the motor to be 20 disconnected from the supply and grounded by contacts 542 As a result the motor stops with the optical system correctly focussed The voltage at G) now does not vary so that the output of comparator 50 remains positive and continues to hold transistor 53 ON The optical system remains focussed and can now be used for the desired purpose, e g taking a photograph This condition of the circuit is shown in Fig 5 E, where it is seen that the switches and relays are in 25 the same positions as in Fig 5 A The in-focus condition is indicated to the photographer by illumination of the diode 55.

To photograph another object, the focussing ring must be brought back to the infinity position This is achieved by changing over contact 742 To achieve this, switch 77 is manually opened causing relay 74 to be de-energised and contact 742 to connect the lower side of the 30 motor 62 to the positive supply This causes the motor to reverse in direction and drive the focus adjustment member back towards the infinity position.

While this reverse travel takes place, the coil 741 must be held deenergised This can be achieved as follows During the reverse travel, the output of comparator 70 is negative as the drive motor 62 moves the focus from position b to position a on Fig 2, as the differentiating 35 circuit will provide an output which is the inverse of waveform B When position a is reached the output of the differentiating circuit at point) reaches zero, and comparator 70 is adjusted so that this causes the comparator output to turn positive This turns transistor 73 on, re-energising relay 74 and thus stopping the motor again, in preparation for the next forward drive At the same time, diode 75 is illuminated indicating to the operator that he 40 may now operate switch 60 again.

The comparator 50 and 70 are so arranged that during forward travel comparator 50 changes state as soon as possible after the differentiating circuit output goes negative, and before comparator 70 changes state.

It will thus be seen from the above description that the differentiator ( 42,43) converts the 45 alternating component of the output of the photo-sensitive element ( 4) into a form in which it can control a switching circuit to stop the focussing drive motor at the in-focus position.

The systems illustrated are particularly designed for use in cameras but are applicable to other types of image-forming optical systems.

Claims (8)

WHAT WE CLAIM IS: 50

1 A focus detecting circuit, comprising a photo-sensitive element arranged to provide an output signal having an alternating component the magnitude of which reaches a limit value when an image is focussed on it, a DC blocking member connected to the photo-sensitive element, a rectifier for rectifying the alternating component passed by the DC blocking member, and a differentiator connected to the rectifier to differentiate the rectified alternat 55 ing component.

2 A circuit according to claim 1, including a switching circuit connected to the output of the differentiator for sensing when the differentiator output passes through zero.

3 A circuit according to claim 2, wherein the switching circuit comprises a comparator the output of which changes state when the differentiator output passes through zero, and a 60 relay connected to the comparator output to be energised when the comparator changes state, the relay having a contact arranged to maintain the relay energised until a switch is operated.

4 A circuit according to claim 2, wherein the switching circuit includes a comparator the output of which changes state when the differentiator output passes through zero, a first 65 transistor connected to the comparator output, a first relay connected to the first transistor, a 1,576,306

5 second transistor connected to the comparator, and a second relay connected to the second transistor to be energised thereby on closure of a switch and having a contact set to hold it energised until it is de-energised by the second transistor.

A circuit according to claim 2, wherein the switching circuit can be set into operation in a first mode for focussing operation, and can be set into operation in a second mode for 5 resetting operation.

6 A circuit according to claim 5, wherein the focussing operation is terminated as the differentiator output passes through zero, and the resetting operation is terminated as the differentiator output approaches near zero.

7 A focus detecting circuit substantially as herein described with reference to the 10 accompanying drawings.

8 A focussing system comprising a drive motor for driving a focus adjustment member of an image forming optical system, and a focus detecting circuit in accordance with any of the preceding claims connected to control the drive motor and to terminate its operation at the in-focus position 15 REDDIE & GROSE Agents for the Applicants 16 Theobalds Road, London, WC 1 X 8 PL Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1980.

Published by The Patent Office, 25 Southampton Buildings, London, WC 2 A l A Yfrom which copies may be obtained.