1. Field of the Invention
This invention relates to automatic focusing systems, and more particularly to automatic focusing systems in which light is projected onto an object to be photographed and its reflection is received to form focusing adjustment.
2. Description of the Prior Art
The in-focus detectors, in which light is projected onto an object to be photographed and the reflection from the object is received by a sensor to determine whether an objective is in focus or out of focus on the object, are known as the so-called "active" type.
A conventional example of such detectors is shown in FIG. 1, wherein a light emitter 1, such as an infrared light-emitting diode, transmits a train of light signals toward an object 2 to be photographed; as its energization, the light emitter 1 is driven b a drive circuit 7. 3 is a projection lens. The reflection from the object 2 is collected by a lens 4 and received by a pair of photoreceptors 5 and 6 such as SPCs. On receipt of the outputs A and B from the photoreceptors 5 and 6, a synchronous detecting circuit 11 produces outputs representing A+B and A-B, respectively, which are then applied to an in-focus judging circuit 12. Using the output of the circuit 12, a servo system 13 controls the operation of a lens.
A voltage-controlled oscillator (VCO) 8 is provided for determining the timing of energization of the infrared light-emitting diode 1 and the timing of synchronous detection. For note, Vref at an input of VCO 8 is a reference voltage for determining the frequency of oscillation (usually 10 KHz or thereabout).
Here explained is the operation of the synchronism detecting circuit 11. Among the light signals received by the photoreceptors 5 and 6, there is a very large amount of noise resulting from foreign illumination. If such noise is present, no realistic use is possible. So, the outputs of the photoreceptors may be integrated in a positive direction during a period when the infrared light-emitting diode is driven, and then in the negative direction during another period when it is not driven, thereby the D.C. noise resulting from solar illumination or the like is removed.
This method is, however, unable to remove the other noise (for example, cyclic noise).
Further, when the same object is shot by two or more cameras whose focusing systems have the same characteristic at nearly, or exactly, the same time, it will very often happen that as the light projected from one camera, upon reflection from the object, not only is transmitted back to that camera, but strays into another camera, the latter is caused to operate in a wrong way.
To eliminate such problems, in most of the state-of-focus detectors, the light emitter is pulsed at a frequency of about 10 KHz, and the synchronism detecting circuit is rendered operative in synchronism with the energization of the emitter, thereby that component of the output of the photoreceptor which results from the non-harmonic foreign illumination with the frequency of the light emitter is not detected as the noise. If the scene is illuminated by fluorescent lamps or similar lighting of periodic nature, however, it is very hard to remove all influence of the foreign illumination. Also, as has been mentioned above, if the same scene is shot by a number of cameras of which at least two have the same kind of automatic focusing system, it is unavoidable that the probability of occurrence of a faulty operation in either of the cameras is highly increased. In case when the cameras used are still cameras, if there is a slight time lag between the actuations of release of two cameras, the probability of the faulty operation will be lowered regardless of their having the same kind of focusing system. But in the case of video cameras and cine cameras where shots require continuous focusing on an object whose distance from the camera is varying during the time it is photographed, or even in the case of still cameras, when a motorized shooting is carried out, the probability of occurrence of such a faulty operation will become very high, and the number of photographic situations where the use of such focusing system gives a fatal drawback to the camera will be highly increased.
FIG. 2 illustrates what faulty operation occurs in terms of the waveform of a focusing control signal. Assuming that the light emitter is driven by a train of pulses on line "a" whose duty ratio is 50%, while the same scene is illuminated by a lighting which is pulsed as shown on line "b", then the reflection-back light is detected, if the foreign illumination is not pulsed, in the form of a similar signal to that shown in line "a", but because of the foreign illumination being of periodic nature, suffers an error shown on line "c" when the light emitter is energized, and on line "d" when it is unenergized. As a result, on assumption that the reflection is received by the photoreceptors and their outputs are integrated to derive a focusing control signal, the supurious signals "c" and "d" cause the output of the integrator to take the form of a signal shown on line "e". For note, a time interval T is a minimum period in which the rising edges of the signals "a" and "b" coincide with each other, so that the signals "c", "d" and "e" change their waveforms recyclically taking the time interval T as a period.
The output of the integrator takes a value of zero at the end of the period T as shown in line "e", but it is at intermediate points P1 and P2 on the way that the output ripples with appreciable values of positive or negative sign. These ripples are finally introduced to the focusing control signal, so that a faulty operation of the camera occurs. In FIG. 2, owing to the ratio of the frequencies fa and fb of the signals "a" and "b" taken at a value of 3:2, respectively, the period T appears to get a small value. But there are many actual situations where the frequency fb comes to be very close to the frequency fa so that the width of the period T is usually very long. Thus, the probability that the signal "e" is not zero, that is, an error signal is formed, is very high.
As has been described above, the active type state-of-focus detector has a high probability of forming spurious signals from the periodic disturbing components of the illumination such as those of the ambient light source or the light emitter in the other detector of the same kind to interfere the automatic focusing operation. Therefore, the prior known detector has had a drawback of not insuring the high reliability and accuracy of automatic focusing control. Such a drawbacks, besides the emitter in the form of producing light, salient to the form of producing supersonic wave.