Focus detecting apparatus

Abstract: A focus detecting apparatus in a camera comprises quantity-of-light detecting means producing an output in conformity with the intensity of an incident light from an object to be photographed, a charge accumulation type light receiving device having a plurality of light receiving elements, each of the light receiving elements receiving the light from the object to be photographed and accumulating therein charges conforming to the light, the light receiving device outputting a plurality of accumulation signals conforming to the plurality of light receiving elements, and calculating means for effecting the focus detection of a photo-taking lens on the basis of the plurality of accumulation signals of the light receiving device. The calculating means controls the charge accumulation time of the light receiving device in conformity with the output from the quantity-of-light detecting means when the focus detecting apparatus effects the first focus detection after the starting thereof, and controls the N th charge accumulaiton time of the light receiving device on the basis of the plurality of accumulation signals obtained by the N-1 th focus detection when the focus detecting apparatus effects the N th ( N ≧ 2) focus detection after the starting thereof.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a focus detecting apparatus in an auto focus camera. Related Background Art A charge accumulation type light receiving element, for example, a CCD image sensor, is used in a focus detecting apparatus in a camera. Where such a kind of sensor is used, the illumination of the element surface varies from a high luminance to a low luminance depending on an object to be photographed and therefore, the accumulation time is varied and controlled so that the signal level to be processed becomes substantially constant. There have been the following methods of controlling the accumulation time. One of them is a method as disclosed, for example, in U.S. Patent No. 4,660,955 wherein the luminance of an object is monitored by a light receiving device differing from an image sensor and accumulation is terminated when the monitoring output reaches a predetermined level. US 4,618,235 discloses a variant of this method in which two integration time limits are provided. Integration is suspended when either the amount of light received by a monitor photo cell exceeds a predetermined value or a predetermined time elapses, whichever occurs first. Besides this method, there is also a method wherein charges are accumulated in advance in an image sensor for a certain time and after the termination of the accumulation, a signal from the image sensor itself is introduced and the next accumulation time is determined in conformity with the signal level. The prior arts as described above have suffered from the following problems. In the former, the image sensor is a CCD or the like and therefore, it is difficult to individually detect a monitoring output corresponding to each light receiving element of the image sensor. As a result, it is unavoidable to monitor the average value of received light in a certain degree of range. If so, when as shown, for example, in Figure 8 of the accompanying drawings, there is an object of high luminance in the fashion of a spot, if monitoring is effected at the average value, there has been the problem that the peak value exceeds the dynamic range of signal processing and accurate focus detection calculation is impossible. Also, in the latter method, accurate focus detection calculation is possible, but such an accumulation time that the sensor output assumes a proper level is obtained from the last accumulation time and the signal level thereof and therefore, there is no accumulation time data in the first accumulation time after the closing of the power source switch, and this has led to the disadvantage that if accumulation is effected with the initial value set in a certain accumulation time, several accumulations must be effected until the sensor output becomes proper and thus, much time is required for convergence. SUMMARY OF THE INVENTION It is an object of the present invention to provide a focus detecting apparatus which accomplishes accurate focus detection. It is a further object of the present invention to provide a focus detecting apparatus of which the focus detection accuracy is not affected by the detected luminance distribution of an object to be photographed and in which the time required from after the power source switch is closed until a focus detection result of predetermined accuracy is obtained is very short. According to the present invention there is provided a focus detection apparatus comprising: a first light detector having a plurality of light receiving elements receiving light from an area to be photographed; a second light detector positioned to receive light from said area to be photographed; processing means for providing a focus detection signal in dependence upon light received by said light receiving elements during an accumulation interval; characterised in that said processing means are so configured that at startup said accumulation interval is determined in accordance with the amount of light received by said second light detector and thereafter said interval is determined in accordance with light received by said light receiving elements during at least one preceding focus detection operation. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 schematically shows the construction of a first embodiment. Figure 2 shows the partial construction of the frist embodiment. Figure 3 is a flow chart illustrating the operation of the first embodiment. Figure 4 is a timing chart illustrating the operation of the first embodiment. Figure 5 schematically shows the construction of a second embodiment. Figure 6 schematically shows the construction of a third embodiment. Figure 7 is a flow chart showing a part of the operation of the present invention in detail. Figure 8 is a graph for illustrating disadvantages peculiar to the prior art. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows an embodiment of the present invention. An object image is projected onto a charge accumulation type light receiving unit (CCD) 1 having a pair of one-dimensional image sensors by an imaging optical system not shown. A photodiode 2 putting out as a monitor output an output corresponding to the amount of exposure of the light receiving unit 1 is provided closely adjacent to the light receiving unit 1, and that output is input from a terminal Vmon to a buffer 4. An analog shift register 3 is disposed in contact with the light receiving unit 1. The outputs of the light receiving element of the light receiving unit 1 are successively put out as a time-serial signal from the terminal Vs of the analog shift register 3 after the termination of accumulation, and are input to an A/D converter 6 through a buffer 5. The outputs of the light receiving elements of the light receiving unit 1 converted into digital values by the A/D converter 6 are successively introduced into a CPU 7. The output of the buffer 4 is input to one input of a comparator 8. The other input of this comparator 8 is connected to a predetermined potential Vref, and the output of the comparator 8 is introduced into the CPU 7. The CPU 7 is connected also to a time counter 9, and is capable of clearing, starting and reading of the counted value of the time counter 9. Also, the CPU 7 is connected to a timing generator (TG) 10 so as to be able to deliver the accumulation starting and terminating signal (INT) of the light receiving unit 1 to the timing generator (TG) 10. The timing generator (TG) 10 outputs a signal INT regarding the accumulation, a signal R regarding the reading-out, a transportation clock C and a shift pulse S so that the light receiving unit 1 can start and terminate the accumulation under the control of the CPU 7 and after the termination of the accumulation, the reading-out can be sequentially effected in the order of the shift of the accumulated charges to the shift register 3 and the transportation of the signal charge in the shift register 3. The detailed constructions of the output portions of the light receiving unit 1, the photodiode 2 and the shift register 3 are as shown in Figure 2. Operation of the thus constructed embodiment will hereinafter be described. Figure 3 shows the operation flow of the CPU, and description will be made in accordance therewith. After the closing of the power source switch (step #1), the CPU 7 first judges whether the accumulate in the light receiving unit 1 for focus detection is the first accumulation after the closing of the power source switch (step #2), and if it is the first accumulation advance is made to step #3, and if it is not the first accumulation, skip is made to step #4. At the step #3, the time counter 9 is reset, and next, at step #5, the time counter 9 is started and the accumulation signal INT is set to "L", and the timing generator 10 is instructed to start the accumulation. At step #6, it is detected that the output of the comparator 8 becomes "H". As soon as the accumulation signal INT from the CPU 7 becomes "L", as shown in Figure 4, the timing generator 10 reports it to the light receiving unit 1, whereby the accumulating operation of the light receiving unit 1 is started. At the same time, the resetting of the monitoring photodiode is released, and as shown in Figure 4, an output corresponding to the quantity of light impinging on the monitoring photodiode (in this example, the exposure amount from the start of the accumulaiton) is produced from the terminal Vmon. The output of the comparator 8 is designed to put out "L" until the exposure amount of the monitoring photodiode reaches a predetermined value, and to put out "H" when said exposure amount reaches said predetermined value. Turning back to the flow of CPU, at step #6, it is detected that the output of the comparator 8 becomes "H" and therfore, when the exposure amount of the monitoring photodiode reaches said predetermined value and the output of the comparator 8 beocmes "H", advance is immediately made to step #7, where the counting by the time counter 9 is stopped and the accumulation signal INT is set to "H". At step #8, the time counter value is read and this is stored as the accumulation time in T. When the accumulation signal INT becomes "H", the timing generator, as shown in Figure 4, shifts the accumulated charges to the analog shift register 3 and therefore, the shift pulse S is given to the shift gate, thereby terminating the accumulation. The charges of the light receiving elements introduced into the analog shift register are successively output as light receiving element signals from the terminal Vs by the transportation clock C. The light receiving element signals successively output from the terminal Vs by this transportation clock C are A/D-converted at one light receiving element unit, and the CPU introduces thereinto these A/D-converted values in succession (step #9), and effects focus detection calculation by the use of this light receiving element data (step #10), and skip is again made to step #1. At the step #9, the maximum value Pmax of the outputs of the plurality of elements of the light receiving unit 1 is detected, and the details of this operation will be described later. In the second and subsequent accumulations, skip is made to step #4, where the next accumulation time is determined. That is, calculation is effected so that of the light receiving element signals A/D-converted at the last time, the maximum value becomes a predetermined next time. Here, the predetermined value, if in the case of 8-bit A/D conversion, is set to 128, and when the last accumulation time is T and the maximum light receiving element signal is the CCD peak value and the next accumulation time is Tnext, the next accumulation time is found by the following equation: Tnext = T x 128 (CCD peak value). Subsequently, at step #11, the time counter is reset, whereafter the time counter 9 is started and the signal INT is set to "L", and the timing generator 10 is instructed to start the accumulation (step #12). Subsequently, at step #13, the time counter value is read out and whether the counted value is equal to Tnext is judged, and if it is equal to Tnext, advance is made to step #15. If the counted value is not equal to Tnext, skip is made to step #13, where the reading-in of the time counter value is repeated. At the step #15, the timing generator 10 is instructed to terminate the accumulation with the accumulation signal INT as an "H" output, and at step #16, Tnext is stored in the accumulation time T for the calculation of the next accumulation time, and skip is made to step #9, where the A/D-converted values of the CCD data are successively introduced and focus detection calculation is effected (step #10), and skip is again made to step #2, where this sequence is repeated. Figure 5 shows the construction of a focus detecting apparatus according to another embodiment, and in Figure 5, portions similar to those in Figure 1 are given similar reference numerals. In this embodiment, a photometering circuit for controlling the exposure amount of film connected to a film exposure amount control circuit for determining the aperture value of a photo-taking lens and the speed of a shutter during the exposure of the film is diverted to a quantity-of-light detecting circuit. The output of a photometering amplifier 41 is input to a film exposure amount control circuit 42 and is also input to the base of a transistor 43. The collector of the transistor 43 is connected to the negative input terminal of the comparator 8 and is also connected to the other end of a capacitor 44 having one end thereof connected to a potential Vcc. The emitter of the transistor 43 is set to such an adjustable potential Vvar that the CCD light receiving element signal level becomes suitable. The capacitor 44 is capable of being short-circuited by a transistor 45. During the non-accumulation, the accumulation signal output INT from the CPU 7 is "H" and therefore, the transistor 45 is turned on through an inverter 46 to short-circuit the capacitor. When the start of the accumulation is ordered from the CPU 7, the transistor 45 is turned off and the capacitor 44 is gradually charged in conformity with the output of the photometering amplifier 41. When as a result of this charging, the potential at the junction between the capacitor 44 and the transistor 43 reaches a predetermined value Vref, the output of the comparator 8 becomes "H". The operation flow of the CPU 7 is similar to that of Figure 1 and therefore need not be described. In a focus detecting apparatus according to still another embodiment, as shown in Figure 6, the output of the photometering amplifier 41 is input to an A/D converter 61 to cause this A/D converter to process the output of the photometering amplifier. The first accumulation time after the closing of the power source switch is predetermined by the CPU 7 before the first accumulation on the basis of the A/D conversion value of the photometering amplifier 41. The second and subsequent accumulations, as in the previous embodiments, are found by calculation from the maximum value of the A/D-converted data and the accumulation time of the CCD at the last time. In the case of this embodiment, the comparator is unnecessary. The operation of detecting Pmax which is the peak value of the CCD at the step #9 of Figure 3 is shown in the flow chart of Figure 7. At step #20, the CPU 7 stores the output P₁ of the first element of one of the pair of image sensors of the light receiving unit 1 into the internal memory thereof. The first element is shield from light so that no light may enter it. At step #21, the content i of the internal counter of the CPU 7 is set to 2. At step #22, whether the content of the internal counter is N+1 is judged. N corresponds to the number of the light receiving elements of one of the image sensors of the light receiving unit 1. At step #23, the output P i of the i th light receiving element is stored into the internal memory. At step #24, the output P i of the i th element is compared with the output P i-1 of the i-1 th element, and if the magnitude of P i is above the magnitude of P i-1 , the content of the internal memory P max is rewritten into P i at step #25. At step #26, the content of the internal counter is incremented by 1. If at the step #22, the content of the internal counter becoems N+1, shift is made to the step #10 of Figure 7. Thereby, the peak value of the CCD is detected. As described above, in the focus detecting apparatus according to the present invention, the first accumulation time after the closing of the power source switch is controlled in conformity with the output of the quantity-of-light detecting circuit and therefore, irrespective of high luminance or low luminance, a substantially proper light receiving element signal level is obtained from the first time. Further, at the second and subsequent times, the next accumulation time is determined by calculation from the light receiving element signal level and the accumulation time and therefore, even in the case of an object to be photographed which has a special luminance distribution, focus detection can be accomplished reliably and the response of focus detection from after the closing of the power source switch can be made quick.

Claims: A focus detection apparatus comprising: a first light detector (1) having a plurality of light receiving elements receiving light from an area to be photographed; a second light detector (2) positioned to receive light from said area to be photographed; processing means (7) for providing a focus detection signal in dependence upon light received by said light receiving elements during an accumulation interval; characterised in that said processing means (7) are so configured that at startup the accumulation interval is determined in accordance with the amount of light received by said second light detector (2) and thereafter said interval is determined in accordance with light received by said light receiving elements during at least one preceding focus detection operation. An apparatus as claimed in claim 1 wherein the first light detector (1) is a charge accumulation type light detector in which the elements thereof accumulate charges according to the amount of light incident thereon. An apparatus as claimed in claim 1 or claim 2 including a counter (9) for measuring the accumulation interval. An apparatus as claimed in any one of claims 1 to 3 wherein at startup the accumulation interval is determined by measuring the time taken for the amount of light received by the second light detector (2) to exceed a predetermined value. An apparatus as claimed in any one of claims 1 to 3 wherein at startup the accumulation interval is determined in accordance with the intensity of light incident on said second light detector (2) at the beginning of the focus detection operation. An apparatus according to any one of the preceding claims wherein for a focus detection operation after startup the accumulation time is determined by modifying a previous accumulation time in dependence upon the peak value output by the light receiving elements in a previous operation. An apparatus as claimed in claim 6 wherein the previous accumulation time is divided by the ratio of the peak value output by a light receiving element in the previous operation divided by the maximum value that can be output by a light receiving element. An apparatus as claimed in any one of the preceding claims wherein the second light detector is a photo metering circuit (41) for controlling the exposure of a film. An apparatus as claimed in any one of the preceding claims wherein said first light detector (1) comprises two arrays of CCD sensors (1). An apparatus as claimed in claim 9 wherein said second light detector (2) is formed on the same semiconductor substrate as said CCD sensors (1), said second light detector (2) being a photoelectric converting device having a light receiving area disposed parallel to said CCD arrays.

Inventors: YAMANO SHOZO, YAMANO, SHOZO
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1989
Main classification: G02B7
IPC codes: G02B7, G03B13