GB2131249A

GB2131249A - Auto-focus decision circuit

Info

Publication number: GB2131249A
Application number: GB08325724A
Authority: GB
Inventors: Koji Matsushima; Junji Kajiwara; Fumio Yasui; Yoshiaki Hirao
Original assignee: Konica Minolta Inc; Matsushita Electric Industrial Co Ltd
Current assignee: Konica Minolta Inc; Panasonic Holdings Corp
Priority date: 1982-10-05
Filing date: 1983-09-26
Publication date: 1984-06-13
Also published as: DE3336220A1; JPS5964818A; GB2131249B; GB8325724D0

Abstract

An auto-focus decision circuit wherein a first switch circuit (10), (11) controlling the output of a constant voltage generating circuit (12) for generating an information other than a measured-distance information and a second switch circuit (4), (5) controlling the transmission of an input information to the output side of a buffer circuit (2) receiving the measured-distance information (Vx) are controlled by the output of flip-flops (13), (14) receiving the information other than the measured-distance information, e.g. on exposure, depth of field, so that the output of the constant voltage generating circuit (12) is taken as an auto-focus decision output (VO) when the information other than the measured-distance information is applied, while the first and second switch circuits are controlled so that the measured-distance information (Vx) is taken as the auto-focus decision output (VO) when the information other than said measured-distance information is not applied. <IMAGE>

Description

SPECIFICATION Auto-focus decision circuit This invention relates to an auto-focus decision circuit for a camera, etc., and in particular it relates two an auto-focus decision ci rcuit which generates a signal indicating a stop position of a photographic lens at which it is in focus.

In an ideal auto-focus camera, the distance to the subject is measured by the camera itself, the photographic lens is moved to a position at which it is in focus, and the film is exposed when the shutter button is pushed with the camera directed toward the subject.

Such a camera, although long desired, had not been manufactured until "Jaspin Konica" (brand name of a product of Konishiroku Photo IndustryCo., Ltd.) was manufactured and appeared on the market in 1977. A large obstacle to the manufacture of an auto-focus camera, notwithstanding various proposals made so fartherefore, was that any distance-measuring device thereof was not very reliable.

This remains a substantial problem even now when auto-focus cameras are in wide use. This is because it is practically impossible to realize a distance-measuring device which would enable the correct measurement of distance in any photographic conditions, for any subjectand with any baclçg round.

In one distance-measuring system, for instance, the distance is measured by receiving natural light reflected from the subject (passive distance-measuring system), and in another system the distance is measured by emitting light from a camera and measuring its reflection (active distance-measuring system).The former has the faultthatthe reliability of the measured-distance signal decreaseswhenthe subject is not very bright, while the latter has the fault thatthe reliability ofthe measured-distance signal decreases when the subject is located far away since the level ofthe signal drops with the square of the distance to the subject, and the reliability of the measured-distance signal also decreases due to the lowering of the S/N ratio which occurs when the field ofviewis light.

There are proposals for making use ofthe focal depth of a photographic lens to compensateforthe low reliability ofthe measured-distance signal.

Such being the case, the auto-focus cameras supplied at present are provided with a default mode, which will be described later, in orderto provide successful photography in circumstances wherein the reliability ofthe distance-measuring device can not be expected to be high (for instance when it is dark), or they are complemented by this utilization ofthe focal depth. For this purpose, they are provided with an auto-focus decision circuit which generates a signal indicating the stop position for the photographing lens in accordance with the output of the distancemeasuring device, etc.

Conventional decision circuits of this kind, however, do not necessarily give an optimum stop position for the lens. Moreover, they are complicated, and have no room forthe employment of a buffer circuit having a wide dynamic range.

The present invention is made in view of these points. An object thereof is to furnish an auto-focus decision circuit which enables the stoppage ofthe lens atan optimal position and has a simple constitution, and furnish an auto-focus decision circuit that makes it possible for a buffer circuit to have a wide dynamic range.

The other objects and characteristics of the present invention will be made apparent in the following description, with reference to the drawings.

Figure lisa circuit diagram of the constitution of one embodiment of the present invention; Figure 2 is a graph ofthe relationship between the distanceto a subjectand outputvoltage; and Figure 3 is a circuit diagram of an actual circuit ofthe embodiment of Figure 1.

Figure lisa circuit diagram ofthe constitution of an auto-focus decision circuitwhich is one embodiment of the present invention. In this figure, an output signal (measured-distance signal) Vxfrom a distancemeasuring device (not shown in the figure) is applied to a positive input terminal 3 of a buffer amplifier 2 which is supplied with powerfrom a power source 1.

Switches 4 and 5 are connected in parallel between the output end of the buffer amplifier 2 and ground.

The output end of the buffer amplifier 2 is also connected to the base of a transistor 6, while the collector of this transistor is connected to the power source 1. Resistors 7, 8, ana 9 and a switch 10 are connected in series, and the series circuit of the resistor 7 and the switch 10 is connected in parallel to a switch 11.One end of the resistor9 is grounded, and the voltage outputfrom a constant voltage source pc&commat; 12 is applied on the junction between the switches 10 and 1 The junction between the resistors8and 9 is connected to the emitter ofthetransistor 6, which is connected to the inversion input of the buffer amplifier 2, so that an emiter-follower circuit is formed ofthe transistor6 and the resistor 9.

D flip-flops 13 and 14 receive a signal PF indicating pan-focus mode and a signal DEF indicating default mode, respectively, at the D terminals thereof, while both receiveatiming control signal atSheTte;minals thereof. PanJocus mode means a mode wherein, when the field of view is extremely light, the depth of field is increased, and the focus can be adjusted within a range ofclose-upto infinity.The default mode in an active system is a mode indicating that the light reflected from the subject is so small that the electric signal into which the reflected light is transduced can not be regarded as a correct signal, and it means a modewhereinthe measured-distance signal output from a distance-calculating circuit is ignored and a prepared decision signal is generated when the light relected from the subject is below a certain threshold value. The opening and closing of the switches 5 and 11 are controlled by a Q output from D flipJlop 13, while those of the switches 4 and 10 are controlled by a Q output of D flipflop 14. An inverter 15is provided for inverting the panfocus mode signal PF and applying it to the D terminal ofthe D flip-flop 14, so as to give the pan-focus mode priority over the default mode. The voltage ofthe power source 1 connected to the buffer amplifier 2 and the collector of the transistor 6 is larger than that ofthe constant voltage source 12 An output voltage from the entire circuit is taken out from an outputterminal 16 connected to the junction between resistors 8 and 9.

The following is a description ofthe operation ofthe circuit proposed by the present invention, having the constitution described above.

When the camera is in pan-focus mode, a high-level signal ("1") is applied as PF, the Q output of D flip-flop 13 goes from "1" to "0" synchronously with the fall of theinputoftheTterminalfrom"1"tolowlevel ("0"), so thatthe switches 11 and 5 are turned ON and the transistor 6 is cut off. Therefore, the measureddistance signal Vx applied to the buffer amplifier 2 has no relationship with the output signal VO from the outputterminal 16. When in the default mode, DEF is setto be "1 ", the Q output goes "0" simultaneousiy with the input to the Tterminal falls from "1" to "0", so thattheswitches 10 and 4 are turned ON and the transistor 6 is cut off.

Accordingly, the pan-focus mode and the default mode can be expressed by the following formulas (1) and (2), wherein the output voltage ofthe constant voltage source 12 is Vr, the resistances of the resistors 7,8, and 9 are R1, R2, and R3, respectively, the output voltage in the pan-focus mode is to = Vp, and the outputvoltage inthedefaultmode isVO =Vd.

Vp = (R3 x Vr) / (R2 + R3) ...(1) v,= (Rs X Vr)/(R1 + R2 f Rg)...(2) The values of the distance to the subject corresponding to Vp and Vd in these formulas varywith the focal length ofthe lens, the distance represented bYVp is setto be approximately in the middie between close and long ranges, while the distance represented by Vd is set at infinity or in the vicinity thereof. The relationships between the voltages Vp and Vd and the measured-distance signal Vx, and also the relationship between an output voltage Vm corresponding to close range and the measured-distance signal, are as shown in Figure 2. The priority between pan-focus mode and default mode is decided by the inverter 15, as described above.When the pan-focus mode input PF is "1" in the circut of Figure 1,the default mode input DEF is forced to be "0", and thus priority is given to pan-focus mode. On the other hand, when neither pan-focus mode nor default mode is required, the measured-distance signal Vx appears unchangedattheoutputterminal l6throughthe buffer ampUfier 2,thetransistor 6, and the resistor 9.

A more practical embodiment of this invention than that of Figure 1 will be described in the following with reference to Figure 3. The embodiment in Figure 3 is prepared by constructing the circuit of Figure 1 by semiconductors. The same numerals and symbols are given to parts in Figure 3 which correspond to those in the circuit of Figure 1. In Figure 3, transistors 24,25,26, 27,28,29,31,34 and 35; constant voltage sources 36, 37, and 38; and an oscillation-stopping capacitor 30 are equivalenttothe bufferamplifier2 of Figure 1, and form a PNP Darlington input differential circuit.The characteristic feature of this part of this circuit lies in thatthe output voltage VO, which is the terminal voltage at the resistor 9, can follow the input voltage Vxto the point at which the input voltage Vx is nearly zero bythefunction ofthe constant voltage source 38 and the transistors 34,35 forming a current mirror circuit, and a negative feedback circuit including the emitterfollowertransistor6. In conventional oper ational amplifier, the output voltage V0 does not fall below a level of about 0.6 V, since a PNPtransistor is employed therein as an element equivalentto the ouptut transistor 35.In the circuit of the present embodiment, the output voltage can foilowthe input voltage Vxto the point at which the latter is zero, since the output transistor is an NPN transistor. In the present embodiment, a circuit equivalentto the switches 4 and 5 of Figure 1 is formed of an inverter39 and transistors 32 and 33. When the Output of either Dflip-flop 13 or 14 is "0", both outputs ofthe inverter 39 are "1", so thatthetransistors 32 and 33 are saturated and absorb the currents from the constant voltage sources 37 and 38. This putsthetransistors 6, 34 and 35 in a completely non-operational state. The D flip-flops 13 and 14 and the inverters 15 and 39 are IlLs.

Transistors 10 and 11 are equivalent to the switches 10 and 11 of Figure 1, they are controlled bythe Q output ofthe Dflip-flops 14and 13, respectively. Aconstant voltage source of the present embodiment receives the output of a power source 1 as an input The operation of this circuit of Figure 3 is identical to that of the circuit of Figure 1. That is, the outputvoltage VO is generated atthe outputterminal 16,Vp is generated in pan-focus mode, while Vd is generated in default mode. When in neither of these modes, vex is generated. These outputs are controlled bytiming pulses applied to theTterminals ofthe D flipJlops 13 and 14.

In each ofthe above embodiments, the flip-flops are two circuits. The basic concept of the formation ofthe flip-flops is the same irrespective ofthe number of circuits making up the flip-flops, and thus it can be realized by extending the circuit of Figure 1. Switch circuits and resistors are arranged in series in the constant voltage generating circuit, butthe same function can also be fulfilled by these elements arranged in parallel. The signals in the pan-focus and default modes are referred to herein as information otherthan information on the measured-distance. If a stroboscope is employed, it can also be used as input information.

As described above, the present invention enables the output signal which gives an optimal stop position to a lens, since an output determining the stop position ofthe lens is decided, according to the present invention, not only by information on the measured-distance, but by information on exposure, the depth offield, etc. In addition, by constituting a buffer circuit as illustrated in the embodiments, it can be made to have a wide dynamic range which is the most necessary condition for a constantvoltage source.

Claims

1. An auto-focus decision circuit comprising flip- flops receiving, as input signals, information other than information on a measured-distance, a buffer circuit receiving the information on the measureddistance as an input signal, a constant voltage generating circuit connected between a constant voltage source and ground for generating a voltage corresponding to the information other than said information on the measured-distance, a first switch circuit controlled by said flip-flops and controlling the output of said constant voltage generating circuit, and a second switch circuit controlled by said flip-flops and controlling the transmission of input information to the output side of said buffer circuit, wherein said first and second switch circuits are controlled by the output of said flip-flops controlled by timing pulses, so thatthe outputof said constantvoltage generating circuit is taken as an auto-focus decision output when information otherthan said information on measureddistance is applied, while said first and second switch circuits are controlled so that the information on the measured-distance given to said buffer circuit is taken asthe auto-focus decision outputwhenthe informa- tion otherthan said information on measured-distance is not applied.

2. The auto-focus decision circuit according to claim 1, characterized in that said constant voltage generating circuit is a series circuit consisting of at least two resistors, and the junction between said resistors is connected to the output end of said buffer circuitto form a section fortaking outthe auto-focus decision output.

3. The auto-focus decision circuit according to claim 2, characterized in that said buffer circuit consists of a buffer amplifier and an NPN transistor driven by said buffer amplifier, and the emitter of said transistor is connected to said junction between said resistors, while said second switch circuit is connected between the outputend of said buffer amplifier and the ground.

4. The auto-focus decision circuit according to claim 1,characterized in that a plurality of said flip-flops are provided according to the number of pieces of information otherthan said information on the measured-distance, said first and second switch circuits are also increased to match the number of said flip-flops, and a priority sequence is given to said flip-flops.

5. The auto-focus decision circuit according to claim 4, characterized in that two of said flip-flops are provided and constituted so that one flip-flop is given a signal in a pan-focus mode and the other a signal in a default mode.

6. The auto-focus decision circuit according to claim 1, characterized in that said buffer circuit has an input stage which is a differential circuitformed by connecting PNPtransistors in a Darlington connec tion, the firsttransistor amplifying the output of said differential circuit is ioaded with a constant current, the second transistor forming an output circuit has a base connected to the collector of said firsttransistor and an emitter connected to one input of said differential circuit to form a negative feedback circuit, and a resistor and a constant-currentabsorbing transistor are connected in parallel between the emitter of said second transistor and ground.

7. The auto-focus decision circuit according to claim 6, characterized in that said second switch circuit is constituted so that it cuts off said second transistor and said constant-current absorbing transistorwhen it isturned ON.

8. An auto-focus adjusting device comprising: a rangefinding means which generates a signal corresponding to a distance to an object two be photographed; meansforsetting a photographing lens to a position corresponding to the output signal from the range finding means; a primary judging means which judges whether or not a diaphragm setting is within the range in which fixed focus photographing is possible or not; and a secondary judging means which judges whether or not an output signal from the range finding means is at a level capable of operating an automatic focusing means, wherein the secondary judging means judges only when the primary judging meansjudgesthat a diaphragm setting is not within the range in which fixed focus photographing is possible and the automatic focusing means is operated only when the secondary judging means judges the output signal is at a level capable of operating the automatic focusing means.

9. An auto-focus adjusting device as set forth in claim 8,furthercomprising a meansforsetting a photographing lens to the fixed focal point in the case that the primaryjudging means judges a diaphragm setting to be within the range in which fixed focus photographing is possible.

10. An auto-focus decision circuit, substantially as hereinbefore described with reference to the accompanying drawings.