Image Scanning System With Signal Integration Time Control

1. Field of the Invention

This invention relates to an image scanning system, and more particularly to improvements in the control of the signal integration time of image sensing means in an image scanning system using signal integration type image sensing means.

2. Description of the Prior Art

In recent years, a one-dimensional or two-dimensional signal integration type image sensing means such as a charge-coupled device (CCD) has become used in various fields. Such an image sensing means produces an output linearly proportional to the intensity of incident light and therefore its dynamic range is narrow. To cause the image sensing means to respond to a variation in the intensity of a wide range of incident light, a method has been proposed in which an upper limit reference level determined by the saturation level of the output of the sensing means and a lower limit reference level determined with noise such as dark current taken into account are preset, the output level of the sensing means relative to these reference levels is determined by level determination means and the signal integration time of the sensing means is stepwise changed by integration time control means so that the output level of the sensing means is within a predetermined range of levels, thereby enlarging the dynamic range.

Heretofore, design has been made such that the controllable longest integration time or the controllable shortest integration time or the medium integration time is preselected as an initial integration time set by the integration time control means. However, if this has been done, for example, in a system wherein the controllable shortest integration time is set as the initial integration time, where the intensity of the light incident on the sensing means is low, a considerable time has been required until an output of appropriate level is obtained from the sensing means by successively changing the integration time to prolong it by the cooperation of level determination means and integration time control means. Conversely, in a system wherein the controllable longest integration time is set as the initial integration time, a similar demerit has occurred where the intensity of the incident light is high. On the other hand, in a system wherein a medium suitable integration time is set as the initial integration time, such inconvenience is alleviated to some extent, but where a variation in intensity of light over a very wide range is the object, such system is still insufficient and a similar demerit has occurred when the intensity of the incident light has been very high or very low.

The present invention has been made in view of such inconveniences peculiar to the prior art and has as its primary object the provision of a novel integration time control method which, as a method of controlling the signal integration time of signal integration type image sensing means in an image scanning system using such sensing means, can clear off the inconveniences peculiar to the prior art regarding the control of the signal integration time of the sensing means.

It is another object of the present invention to provide improvements in an image scanning system using signal integration type image sensing means and adapted to stepwise control the signal integration time of the sensing means (i.e., the amount of the electrical signal to be produced and stored for each image scanning) on the basis of the output level thereof, whereby the time required for the setting of an integration time corresponding to the intensity of incident light can be greatly shortened and accordingly, for a certain intensity of incident light, an output of appropriate level can be obtained from the sensing means in a very short time.

To achieve such objects, according to the present invention, there is provided a radiation sensing system or an image scanning system having the following characteristic construction. The system includes: signal integration type radiation sensing means producing an electrical output capable of indicating the distribution pattern of incident radiation; means for measuring radiation substantially equivalent to the radiation incident on the sensing means and producing an electrical output capable of indicating the intensity thereof; and means for controlling the signal integration time of the sensing means on the basis of the output of the measuring means and the output of the sensing means.

In a preferred embodiment of the present invention, the integration time control means is designed to determine the initial integration time on the basis of the output of the measuring means, and thereafter stepwise control the integration time on the basis of the output of the sensing means with the initial integration time as the starting point.

Also, in one embodiment, the integration time control means is designed to use the peak level of the output of the sensing means as the basis of the determination as disclosed, for example, in U.S. Pat. Nos. 4,305,657 entitled "Range Finding Device", 4,283,137 entitled "Image Scanning System" and 4,329,033 entitled "Distance Detecting Device and Focus Control System Utilizing the Same" all of which were assigned to the same assignee of the subject application, in controlling the integration time among a plurality of preset different integration times on the basis of the output of the sensing means, but may also use the average level of the output as the basis of the determination or, as disclosed, for example, in U.S. Pat. No. 4,004,852, may use as the basis of the determination data obtained by quantizing the output of the sensing means.

Thus, according to the present invention, the intended purposes are completely achieved and there is provided a radiation sensing system or an image scanning system which is very excellent in responsiveness, that is, which can obtain the output of proper level pf the sensing means in a short time.

Other objects and features of the present invention will become apparent from the following detailed description of an embodiment thereof taken in conjunction with the accompanying drawings.

FIG. 1 is a model view for illustrating a method of controlling the signal integration time of signal integration type image sensing means which is the base of the present invention.

FIG. 2 is a model view showing an embodiment in which the image scanning system according to the present invention is applied to the focus detecting device of a single lens reflex camera.

FIG. 3 is a circuit diagram showing the construction of the electric circuitry of the camera shown in FIG. 2, displayed in halves in FIG. 3A and FIG. 3B.

FIG. 4 is a timing chart showing the relation between the output of a 3-bit up-down counter shown in FIG. 3 and the signal integration time.

FIG. 5 is a timing chart showing various control signals produced by a timing control circuit shown in FIG. 3.

Reference is first had to FIG. 1 to describe the method of controlling the signal integration time of image sensing means which is the basis of the present invention. In FIG. 1, V.sub.MAX and V.sub.MIN are upper and lower limit reference levels between which the sensing means operates linearly. This figure shows the manner in which the accumulating time is changed between t.sub.1 and t.sub.8 so that the output level of the sensing means lies between the upper limit and lower limit reference levels.

Reference is now had to FIG. 2 and so on to describe an embodiment in which the image scanning system according to the present invention is applied to the focus detecting device of a single lens reflex camera. In FIG. 2, LS designates a picture-taking lens, FP denotes a film, QM designates a quick return mirror having a light-transmitting portion LP in the center thereof, and SM denotes an auxiliary mirror for downwardly reflecting the transmitted light from the light-transmitting portion LP of the mirror QM. The light beam reflected by the auxiliary mirror SM enters a prism PM, where it is divided into three light beams which respectively enter the three image scanning sections SD.sub.1, SD.sub.2 and SD.sub.3 of the image sensing means SD. These three image scanning sections SD.sub.3, SD.sub.1 and SD.sub.2 are adapted to receive the incident lights respectively at a position optically equivalent to the surface the film FP, a position slightly forward of the equivalent position and a position slightly rearward of the equivalent position due to the action of the prism PM. Each of the image scanning sections SD.sub.1, SD.sub.2 and SD.sub.3 has a one-dimensional array of a plurality of signal integration type light-receiving elements as will later be described. FS designates a focusing screen, CL denotes a condenser lens, PN designates a penta prism, EL denotes an eyepiece lens, and ML designates a metering condenser lens which condenses light on a metering photoelectric conversion element PE. MT designates a display meter. As conceptionally shown in broken line in FIG. 2, the initial setting of the signal integration time of the image sensing means SD may be effected by the output of the metering photoelectric conversion element PE.

Referring to FIG. 3, SD.sub.3 is the middle image scanning section of the image sensing means SD disposed so as to receive light at a position optically equivalent to the surface of the film FP and, in the present embodiment, the signal integration time of the image sensing means SD is controlled on the basis of the output of the image scanning section SD.sub.3. S.sub.1 -S.sub.n designate light receiving elements (arranged one-dimensionally) included in the image scanning section SD.sub.3, and D.sub.1 and D.sub.2 denote dummy elements masked, for example, by a mask MS(which can be formed by evaporation of Al) for dark current detection. FA.sub.1, FA.sub.2, . . . , FA.sub.m (m=n+2) designate integration clear gates responsive to the high of an integration clear signal ICG to clear the charges stored in the light-receiving elements S.sub.1 -S.sub.n and dummy elements D.sub.1, D.sub.2. FB.sub.1, FB.sub.2, . . . , FB.sub.m denote charge shift gates responsive to the high of a shift pulse SH to shift to charge transfer analog shift registers CA.sub.1 -CA.sub.2m the charges stored in the light-receiving elements S.sub.1 -S.sub.n corresponding to the integrated amount of the light incident thereon and the charges corresponding to the dark currents stored in the dummy elements D.sub.1 and D.sub.2. The analog shift registers CA.sub.1 -CA.sub.2m are of the two-phase driven type operated by clock pulses .0..sub.1 and .0..sub.2, and the transferred charges thereof are put out as voltage information at the last stage through a charge-voltage converting circuit comprising resistors R.sub.1, R.sub.2, R.sub.3 and FET FC.sub.1, FC.sub.2.

AG.sub.1 designates an analog gate for taking out only the signals obtained by the dummy elements D.sub.1 and D.sub.2, of the output of the image scanning section SD.sub.3, and a holding capacitor C.sub.1, a resistor R.sub.4 and a buffer amplifier BP subsequent to the analog gate AG.sub.1 together constitute a dark current signal holding circuit. The resistor R.sub.4 is a resistor for forming a low-pass filter together with the capacitor C.sub.1, and is not always necessary. Resistors R.sub.5, R.sub.6, R.sub.7, R.sub.8 and an operational amplifier OP.sub.1 together constitute a differential amplifier circuit as a dark current compensating differential circuit, and this circuit puts out a dark-current-compensated true scanning output VF by subtracting the dark current signal components obtained by the dummy elements D.sub.1 and D.sub.2 which are held by the dark current signal holding circuit from the scanning output including the dark current components obtained by the light-receiving elements S.sub.1 -S.sub.n.

AG.sub.2 designates an analog gate for taking out only the signals corresponding to the light-receiving elements S.sub.1 -S.sub.n, of the output of the differential amplifier circuit. PD denotes a peak detecting circuit (which may also detect the average value) for detecting, for example, the peak value (hereinafter referred to as VP) of the signal obtained through the analog gate AG.sub.2. PH designates a peak holding circuit for holding the peak value VP detected by the peak detecting circuit. R.sub.9, R.sub.10 and R.sub.11 denote voltage dividing resistors for obtaining the voltages V.sub.MAX and V.sub.MIN corresponding to the upper limit and lower limit reference levels described in connection with FIG. 1. CP.sub.1 designates a comparator which compares the held value VP of the peak holding circuit PH with the upper limit reference voltage V.sub.MAX and puts out a high level signal when VP>V.sub.MAX and puts out a low level signal when VP.ltoreq.V.sub.MAX. CP.sub.2 denotes a comparator which compares the held value VP with the lower limit reference voltage V.sub.MIN and puts out a high level signal when VP<V.sub.MIN and puts out a low level signal when VP.gtoreq.V.sub.MIN. IV.sub.1 designates an inverter for inverting the output of the comparator CP.sub.1. The output of the inverter IV.sub.1 is imparted to an up-down counter UDC(which is a binary up-down counter of 3-bit construction) as a signal for controlling the counting mode of this up-down counter UDC. The up-down counter UDC is set so as to assume an up count mode by the high of the output of the inverter IV.sub.1 and to assume a down count mode by the low of the output of the inverter IV.sub.1. OR.sub.1 denotes an OR gate for taking the logical sum of the output of the comparator CP.sub.1 and the output of the comparator CP.sub.2. EX denotes an exclusive OR gate for taking the exclusive-or of the 3-bit outputs Q.sub.1, Q.sub.2, Q.sub.3 of the up-down counter UDC and the output of the inverter IV.sub.1. AN.sub.1 designates an AND gate for taking the logical multiple of the output of the OR gate OR.sub.1, the output of the exclusive OR gate EX and the counting pulse CP from a timing control circuit TCC to be described. The output of the AND gate AN.sub.1 is imparted to the up-down counter UDC as the count clock of this up-down counter UDC. The exclusive OR gate EX is for preventing the resetting of the up-down counter UDC when the integration time has been set to the shortest time t.sub.1 or the longest time t.sub.8 and further the shift information toward the short time or the long time side has been put out from the comparator CP.sub.1 or CP.sub.2 and for fixing the integration time to the currently set shortest or longest integration time. The relations between the 3-bit outputs Q.sub.1 -Q.sub.3 of the up-down counter UDC and the eight stages of integration time t.sub.1 -t.sub.8 designated thereby are as shown in FIG. 4.

TCC designates a timing control circuit for generating various control pulses and control signals in accordance with the timing chart shown in FIG. 5. CP is the counting pulse of the up-down counter UDC generated once each time the signals of the image scanning sections SD.sub.1 -SD.sub.3 are read out (that is, the integration time controlling pulse). A.0..sub.1 is a gate controlling signal for the analog gate AG.sub.1 for taking out through the analog gate AG.sub.1 the signals of the portions necessary for detection of dark current during each read-out, namely, the signals corresponding to the dummy elements D.sub.1 and D.sub.2. A.0..sub.2 is a gate controlling signal for the analog gate AG.sub.2 for taking out through the analog gate AG.sub.1 the signals corresponding to the light-receiving elements S.sub.1 -S.sub.n, of the output of the differential amplifier circuit OP.sub.1, during each read-out. .0..sub.R is a peak resetting control signal for resetting the peak detecting circuit PD, for example, immediately after each read-out is initiated. .0..sub.H is a peak holding control signal for causing the peak detection value VP before the peak detecting circuit PD is reset to be held by the peak holding circuit PH each time each read-out is terminated. SH is a gate controlling pulse (shift pulse) for the charge shift gates FB.sub.1 -FB.sub.m in the image scanning section SD.sub.3. ICG is a gate controlling signal (integration clear signal) for the integration clear gates FA.sub.1 -FA.sub.m. .0..sub.1 and .0..sub.2 are transfer clock pulses for the charge transfer analog shift registers CA.sub.1 -CA.sub.2m (that is, the analog shift registers CA.sub.1 -CA.sub.2m are of the two-phase driven type, and the shift pulse SH is synchronous with .0..sub.1). RS is a reset pulse for the charge-voltage converting circuit FET FC.sub.1. The above-described signals 2CG, SH, .0..sub.1, .0..sub.2 and RS are also supplied to the other image scanning sections SD.sub.1 and SD.sub.2.

The timing control circuit TCC has the function of controlling the signal integration time (charge accumulating time) of the image scanning sections SD.sub.1 -SD.sub.3 on the basis of the time information designated by the outputs Q.sub.1 -Q.sub.3 of the up-down counter UDC, and specifically realizes the control of the integration time by controlling the period of time from the falling of the integration clear signal ICG to its low level indicated by t in FIG. 5 till the rising of the shift pulse SH between the eight stages t.sub.1 -t.sub.8 in accordance with the conditions of the outputs Q.sub.1-Q.sub.3 of the up-down counter UDC. Thus, the actual integration time of the image scanning sections SD.sub.1 -SD.sub.3 is "the image t+ the duration .DELTA.t of the high level of the shift pulse SH". Incidentally, the image scanning sections SD.sub.1 -SD.sub.3 is of the two-phase driven type as previously described, but the signal of each element thereof is put out in synchronism with .0..sub.1 and the outputting thereof is initiated in synchronism with the shift pulse SH.

Now, in accordance with the improvement of the present invention, a construction for initial setting of the integration time as will hereinafter be described is added to the above-described construction of the image scanning system. In FIG. 3, a metering photoelectric conversion element PE is connected between the input terminals of a high input impedance operational amplifier MP.sub.1. LD.sub.1 designates a logarithmic compression element for logarithmically compressing the photocurrent produced in the photoelectric conversion element PE correspondingly to the brightness of the object to be photographed, and CS.sub.1 denotes a constant current source which, together with a resistor R.sub.20, supplies a bias voltage to the non-inverting input terminal of the operational amplifier MP.sub.1. R.sub.21 -R.sub.23 designate operational resistors, OP.sub.5 denotes an operational amplifier, CP.sub.5 designates a comparator, and Mg.sub.1 denotes an aperture controlling magnet. VR.sub.1 designates a variable resistor in which information "Sv - Tv", i.e., the film speed information Sv minus the shutter time information Tv, and in the operational amplifier OP.sub.5, this information and the object luminance information which is the output of the operational amplifier MP.sub.1 are computed and aperture information is put out to the output of the operational amplifier OP.sub.5. The aperture information is displayed by an aperture information displaying ammeter MT and ah aperture corresponding to the luminance of the object to be photographed is automatically set through a variable resistor VR.sub.2 operatively associated with an aperture adjusting member, the comparator CP.sub.5 and the magnet Mg.sub.1. Vc applied to the non-inverting inputs H) of the operational amplifier OP.sub.5 and the comparator CP.sub.5 is a bias level setting reference voltage.

On the other hand, VR.sub.3 designates a variable resistor operatively associated with a shutter time setting member. The variable resistor VR.sub.3, together with a capacitor C.sub.5, constitutes a timer circuit. SW.sub.10 denotes a count switch adapted to be opened substantially simultaneously with the start of movement of the first curtain of the shutter ST (FIG. 2). When the voltage across the capacitor C.sub.5 reaches a predetermined value, a Schmidt trigger circuit ST is inverted to deenergize a magnet Mg.sub.2 for controlling the second curtain of the shutter ST, thus terminating the exposure. SW.sub.1 denotes a main switch, and E designates a power source. ADC denotes an A/D converter for converting the output of the operational amplifier MP.sub.1, for example, into a 3-bit digital data. OS designates a one-shot circuit. AN.sub.2 denotes an AND gate for taking the logical multiply of the output pulse of the one-shot circuit OS and the A/D conversion termination pulse from the terminal ED of the A/D converter ADC. The output of the AND gate AN.sub.2 is imparted to the preset enable terminal PE of the up-down counter UDC.

Now, in the above-described construction, when the main switch SW.sub.1 of the camera is closed at time T.sub.1 (see FIG. 5), the exposure amount control circuit of the camera starts operating and the one-shot circuit OS generates a pulse which assumes high level during the time from time T.sub.1 till time T.sub.3. Also, the A/D converter ADC starts the A/D conversion of the output of the operational amplifier MP.sub.1 which is the information corresponding to the luminance of the object to be photographed. When the A/D converter ADC terminates the A/D conversion of the object luminance information at time T.sub.2, an A/D conversion termination pulse is generated from the terminal ED thereof and accordingly, by the output of the AND gate AN.sub.2, the preset enable terminal PE of the integration time setting up-down counter UDC assumes high level, whereby the output of the A/D converter ADC is preset to the up-down counter UDC. For example, if the conditions of the output terminals Q.sub.1, Q.sub.2 and Q.sub.3 of the A/D converter ADC are high level, low level and high level, respectively, the output terminals Q.sub.1, Q.sub.2 and Q.sub.3 of the up-down counter UDC also assume similar conditions and accordingly, the first integration time becomes t.sub.6 (see FIG. 4).

On the other hand, upon closing of the main switch SW.sub.1, the timing control circuit TCC starts to put out transferring clock pulses .0..sub.1, .0..sub.2 and reset pulse RS to the image scanning sections SD.sub.1 -SD.sub.3 at time t.sub.1 and renders the integration clear signal ICG high and opens the integration clearing gates FA.sub.1 -FA.sub.m, thereby inhibiting the accumulation of the produced charges in the dummy elements D.sub.1, D.sub.2 and the light-receiving elements S.sub.1 -S.sub.n.

Thereafter, at time T.sub.3, the output of the one-shot circuit OS is inverted to low level, whereupon in response to this signal, the integration clear signal ICG is rendered low as shown in FIG. 5 and the integration clearing gates FA.sub.1 -FA.sub.m are closed, whereby the accumulation of the produced charges in the elements D.sub.1, D.sub.2 and S.sub.1 -S.sub.n is initiated while, at the same time, the accumulating time designated by the outputs Q.sub.1 -Q.sub.3 of the up-down counter UDC (in this case, the accumulating time is t.sub.6) begins to be counted and, when this time counting is terminated, the shift pulse SH is put out. Accordingly, at this point of time, the charge shifting gates FB.sub.1 -FB.sub.m are opened, whereby the charges accumulated in the elements D.sub.1, D.sub.2 and SD.sub.1 -SD.sub.n while said time counting is being effected are introduced through the charge shifting gates FB.sub.1 -FB.sub.m into the corresponding bits of the charge transferring analog shift resisters CA.sub.1 -CA.sub. 2m, whereafter the charges are transferred through the analog shift resisters CA.sub.1 -CA.sub.2m to the charge-voltage converting circuit, where the charges are converted into a voltage and put out as voltage information. The timing control circuit TCC, after having put out the shift pulse SH, again renders the integration clear signal ICG high and opens the integration clearing gates FA.sub.1 -FA.sub.m, thereby inhibiting the accumulation of the produced charges in the elements D.sub.1, D.sub.2 and S.sub.1 -S.sub.n. Now, when the outputting of scanning signals from the image scanning sections SD.sub.1 -SD.sub.3 is thus initiated, the timing control circuit TCC renders the gate controlling signal A.0..sub.1 to the analog gate AG.sub.1 high at a timing whereat the signal corresponding to the dummy elements D.sub.1 and D.sub.2 is put out as shown in FIG. 5, and opens the analog gate AG.sub.1 and thus, the signal corresponding to the dummy elements D.sub.1 and D.sub.2 is held as the dark current signal of the image scanning section SD.sub.3 by the capacitor C.sub.1, and the dark current signal thus held is imparted to one input of the differential amplifier circuit through the buffer amplifier BP. Thus, the differential amplifier circuit then receives at the other input thereof the signal corresponding to the light-receiving elements S.sub.1 -S.sub.n, whereby it puts out said signal minus the dark current signal component, namely, the dark-current-compensated scanning signal VF. On the other hand, at this time, the timing control circuit renders the gate controlling signal A.0..sub.2 to the analog gate AG.sub.2 high during the period of time in which the signal corresponding to the light-receiving elements S.sub.1 -S.sub.n is put out from the image scanning section SD.sub.3 as shown in FIG. 5, thereby opening the analog gate AG.sub.2 and accordingly, of the output of the differential amplifier circuit, the output corresponding to the light-receiving elements S.sub.1 -S.sub.n is imparted to the peak detecting circuit PD. The peak detecting circuit PD is already reset by the reset signal .0..sub.R from the timing control circuit TCC as shown in FIG. 5 during the period of time in which, for example, the signal corresponding to the dummy elements D.sub.1 and D.sub.2, and is imparted the output of the differential amplifier circuit corresponding to the light-receiving elements S.sub.1 -S.sub.n through the analog gate AG.sub.2, whereby it detects the peak value thereof. When the outputting of the signal corresponding to the light-receiving elements S.sub.1 -S.sub.n from the image scanning section SD.sub.3 is terminated, the timing control circuit TCC renders the gate controlling signal A.0..sub.2 low as shown in FIG. 5 and closes the analog gate AG.sub.2 to thereby terminate the peak value detection by the peak detecting circuit PD, whereafter it imparts a hold signal .0..sub.H to the peak holding circuit PH to cause the same to hold the peak detection output VP of the peak detecting circuit PD at this point of time. When the peak value VP is held by the peak holding circuit PH, the comparators CP.sub.1 and CP.sub.2 compare the held peak value VP with the upper limit and lower limit reference voltages V.sub.MAX and V.sub.MIN, and put out the result of the comparison as a high or low logic signal. Generally, good control of the integration time of the image sensing device disposed within the camera can be accomplished by the output of the metering photoelectric conversion element in the camera, but where the metering portion of the metering photoelectric conversion element is not the same as the light-receiving portion of the image sensing device, a scanning output of appropriate level may sometimes not be obtained from the image sensing device for the integration time controlled by the output of the metering photoelectric conversion element.

The present invention solves these problems and provides a method for obtaining a scanning output of appropriate level in a very short time. According to the present invention, the initial integration time is set in an integration time setting circuit by the output of an exposure amount controlling photoelectric conversion element disposed within the camera, and whether the level of a scanning signal put out under such initial integration time is within the range of a predetermined signal level is discriminated and when only (and when) the level of the scanning signal is not within the range of the predetermined signal level, a new setting of the integration time is effected through the integration time setting circuit so that the level of the scanning signal is within the range of the predetermined signal level, whereby a scanning output of appropriate level is obtained in a very short time.

Now, assuming, for example, that the peak hold value VP in VP<V.sub.MIN, the output of the comparator CP.sub.1 is low and the output of the comparator CP.sub.2 is high and accordingly, the output of the inverter IV.sub.1 is high, whereby the up-down counter UDC is set to the up count mode while, at the same time, the output of the OR gate OR.sub.1 becomes high and at this time, the output of the exclusive OR gate EX also becomes high. Accordingly, after the holding of the peak detection output VP of the peak detecting circuit PD by the peak holding circuit PH is completed, a count pulse CP is put out to the up-down counter UDC from the timing control circuit TCC as shown in FIG. 5, whereupon the count pulse CP is imparted to the count input of the up-down counter UDC through the AND gate AN.sub.1 and the up-down counter UDC effects one count up and accordingly, as shown in FIG. 4, the outputs Q.sub.1 -Q.sub.3 thereof become high, high and low, respectively, whereby as shown in FIG. 4, the designated integration time of the solid state image sensing element SP is changed from the shortest time t.sub.6 to the next time t.sub.7. Thus, during the next cycle of scanning, the timing control circuit TCC controls the period of time t from the falling of the integration clear signal ICG to its low level till the rising of the shift pulse SH in accordance with the time t, whereby the integration time of the image scanning sections SD.sub.1 -SD.sub.3 is prolonged from t.sub.6 to t.sub.7 and the level of the scanning output VF obtained through the differential amplifier circuit is caused to rise from this. This integration time changing operation is repeated until the condition that V.sub.MIN .ltoreq.VP.ltoreq.V.sub.MAX is obtained and, when the condition that V.sub.MIN .ltoreq.VP.ltoreq.V.sub.MAX is finally obtained, the outputs of the comparators CP.sub.1 and CP.sub.2 both become low, whereby the output of the OR gate OR.sub.1 becomes low and the supply of the count pulse CP from the timing control circuit TCC to the up-down counter UDC is inhibited by the AND gate AN.sub.1 and thus, at this point of time, the change of the integration time is stopped and the integration time is maintained at this proper time. Of course, if the condition that VP<V.sub.MIN again arises during the time the scanning is repeated under this proper integration time, the integration time will be changed to a longer time by the above-described operation. If, conversely, the condition that VP>V.sub.MAX arises, the output of the comparator CP.sub.1 will become high and the output of the comparator CP.sub.2 will become low and the output of the inverter IV.sub.1 will become low, whereby the up-down counter UDC will be set to the down count mode and will effect one count down by the count pulse CP from the timing control circuit TCC, whereby the integration time will be changed to a shorter time. Thus, through such operation, the integration time of the image scanning sections SD.sub.1 -SD.sub.3 is always controlled to a proper time, namely, such a time that the proper image signal level of V.sub.MIN .ltoreq.VP.ltoreq.V.sub.MAX is obtained.

On the basis of the scanning outputs VF', VF" and VF thus obtained from the image scanning sections SD.sub.1 -SD.sub.3, detection of the focus is carried out in a focus detecting circuit PCK and, when the picture-taking lens is automatically or manually set to the in-focus position, the output of the terminal ED of the focus detecting circuit PCK assumes high level and shutter release becomes possible. The focus detecting circuit PCK thus using the scanning signals from the three image scanning sections SD.sub.1 -SD.sub.3 to effect detection of the focus is fully disclosed in U.S. patent application Ser. No. 310,483 entitled "Focus Detecting System" filed on Oct. 9, 1981, assigned to the assignee hereof, abandoned in favor of continuation application U.S. Ser. No. 572,972, filed Jan. 23, 1984, abandoned in favor of continuation application U.S. Ser. No. 627,488, filed July 5, 1984, and U.S. patent applications Ser. No. 313,579, entitled "Focus Detecting System", which was filed Oct. 21, 1981 and issued as U.S. Pat. No. 4,459,002 on July 10, 1984, U.S. Ser. No. 313,582 "Camera System", filed Oct. 21, 1981, abandoned in favor of continuation application U.S. Ser. No. 631,008, U.S. Ser. No. 313,583 "Focus Detecting System", which was filed Oct. 21, 1981 and issued as U.S. Pat. No. 4,411,504 on Oct. 25, 1983, U.S. Ser. No. 313,584 "Focus Detecting System", which was filed Oct. 21, 1981, abandoned in favor of continuation application U.S. Ser. No. 577,329, which was filed Feb. 8, 1984 and issued as U.S. Pat. No. 4,475,800 on Oct. 9, 1984, and U.S. Ser. No. 313,599 "Signal Processing System", which was filed Oct. 21, 1981 and issued as U.S. Pat. No. 4,437,743 on Mar. 10, 1984, all of which are assigned to the assignee of the present application.

When a shutter release button (not shown) is depressed in this condition, the switch SW.sub.2 is closed and a transistor TR.sub.1 is turned on by an inverter IV.sub.2 and an AND gate AN.sub.3, so that the charge stored in a capacitor C.sub.6 is discharged through a resistor 24 and a magnet Mg3 is energized to start the release of the shutter ST.

According to the present invention, a scanning output of appropriate level can be obtained in a very short time and therefore, where the image scanning system of the present invention is applied, for example, to the focus detecting portion of an automatic focus camera, there can be obtained a great advantage that focusing is accomplished in a short time to enable photography.