Method and apparatus for controlling and stabilizing the position of an iris diaphragm in a video camera, using two successive control loops

Abstract: Disclosed are an image pickup apparatus and method and system for controlling the exposure of the image pickup apparatus. The image pickup apparatus includes a circuit (12) for detecting a voltage corre­ sponding to a current value of an iris, a circuit (15, 16) for generating a voltage corresponding to a desired iris value, a circuit (18) for comparing the two voltages, and a circuit (7) for controlling the iris based on the result of the comaprison so as to keep the iris value at the desired value, in which the exposure start time of an image pickup device (1) is changed in an increment/decrement of one horizontal scanning period and/or in an increment/­ decrement of a period shorter than one horizontal scanning period to thereby make it possible to perform manual-iris mode automatic exposure control.

Description: BACKGROUND OF THE INVENTION The present invention relates to an image pickup apparatus and a method for controlling the exposure of the image pickup apparatus. In particular, it relates to exposure control for use in a video movie camera having an electronic variable shutter. Recently, electronic variable shutters for electrically changing the exposure period of time of image pickup apparatus have been made available, and video movie cameras with a new exposure control system such as a programmed automatic exposure (AE) system or the like have been marketed. An electronic variable shutter used in a CCD image pickup apparatus is discussed in NIKKEI MICRODEVICE, No. 10, pp 63-64, Oct. 1987. An electronic variable shutter with a programmed AE system used in an MOS image pickup apparatus is discussed in IEEE Transactions on Consumer Electronics, vol. CE-33, No. 3, pp 249-255, Aug. 1987. A technique for shortening the exposure period of time to a value less than a value designated by an operator when detecting an iris value larger than a predetermined value is disclosed in the U.S. Patent No. US-A-4,959,727. SUMMARY OF THE INVENTION Conventional exposure control systems in video cameras (or video movie cameras) using the aforementioned electronic variable shutters are described with reference to Figs. 1 and 2. Fig. 1 is a circuit block diagram showing a conventional video camera having a manual-exposure time mode automatic exposure function (hereinafter automatic exposure function is called "AE function") in which the exposure period of time is manually set. When a user operates a control button 9, an exposure time changing circuit 10 constituted by a microcomputer or the like starts. As a result, the output pulse of an image pickup device drive circuit 11 changes so that the exposure period of time of an image pickup device 1 is fixed to a desired value. On the other hand, the output signal of the image pickup device 1 is amplified by an amplifier 2 and then supplied to a detector 4. The signal quantity is detected by the detector 4 which supplies a voltage corresponding to the signal quantity to a comparator 6. The comparator 6 compares the voltage corresponding to the signal quantity with a reference voltage supplied thereto from a constant voltage source 5 so as to control the iris of a lens 8 through an iris motor 7. According to the aforementioned construction, the iris can be controlled automatically in a suitable exposure period of time designated by a user, so that the output signal level of the amplifier 2 can be kept at a predetermined level, and, at the same time, the output signal level of a signal processing circuit 3 can be kept at a proper level. Fig. 2 is a circuit block diagram showing another video camera which is constituted by adding a programmed AE function to the conventional video camera depicted in Fig. 1. In Fig. 2, same reference numerals as those used in Fig. 1 designate same as those parts in Fig. 1. In Fig. 2, reference numeral 12 designates an iris value detection circuit which is, for example, constituted by a position detecting element such as a Hall element disposed in the vicinity of the iris motor 7 and an amplifier so as to generate an output voltage corresponding to the iris value. When a programmed AE mode is selected by a user through operating a control button 13, an exposure time changing circuit 14 is given the programmed AE mode so that the exposure period of time is changed automatically corresponding to the iris value. An example of the input-output characteristic of the iris value detecting circuit 12 is shown in Fig. 3. By using a position detection element such as a Hall element, the iris value can be converted into a voltage as described above. An example of the relationship between the iris value and exposure period of time in the programmed AE mode is shown in Fig. 4. In this example, the exposure period of time is controlled as follows. When the quantity of incident light increases so that the iris value exceeds F4, the exposure period of time is automatically fixed to 1/120 sec. When the quantity of incident light increases further so that the iris value exceeds F5.6, the exposure period of time is automatically fixed to 1/250 sec. When the quantity of incident light, on the other hand, decreases so that the iris value is not more than F2.8, the exposure period of time is automatically returned to 1/120 sec. When the quantity of incident light decreases further so that the iris value is not more than F2, the exposure period of time is automatically returned to 1/60 sec. By the aforementioned control, the exposure period of time can be set to a value in a range of from 1/250 to 1/120 sec. in outdoor or in a well-lighted room for the purpose of recording a moving subject sharply. The problem of blur (hereinafter referred to as "iris related blur") due to a diffraction phenomenon occurring when intensive light enters a significantly closed iris can be eliminated because the iris in the exposure period of time of 1/120 to 1/250 sec is open more widely by one to two stop values than the iris in the exposure period of time of 1/60 sec. In the aforementioned conventional examples, the iris could not be fixed by the user, so that it was impossible to attain manual-iris mode AE for performing exposure control while changing the exposure period of time corresponding to the iris value designated by the user. Further, the problem of iris related blur could not be eliminated sufficiently. Due to this, there arises a problem in that the focus cannot be adjusted when intensive light enters the full frame. From the prior art (see Fig. 1) an iris positioning device is known which is based on detected luminance while the exposure time is manually set. Another prior art (see Fig. 2) is also an iris positioning device based on detected luminance wherein the exposure time is automatically changed corresponding to the iris value by satisfying the well-known rule that there is a reciprocal relationship between exposure time and iris value. The value which is indicative for the luminance is called "light value". An exposure meter measures light values. AE systems of still cameras are designed such that the operator sets the exposure time manually whereupon the iris assumes a position automatically to balance the proper light value. Other AE still camera systems are also known wherein the iris is manually set whereupon the exposure time assumes a value automatically to balance the proper light value. For the universal application of a camera it is sometimes desirable to fixedly set the iris and let the exposure time automatically adjust, whereas in other situations it is desirable to fixedly set the exposure time and let the iris automatically adjust. US-A-4 300 167 discloses automatically controlling the iris of a video camera through a feedback loop wherein an iris position is controlled in accordance with a desired light level and a light level detected from the camera's image sensor output. It is an object of this invention to provide an image pickup apparatus and method and system for controlling the exposure of the image pickup apparatus employing a minimum of interlocked components to stabilize the iris in a desired position in order to eliminate during motion picture pickup any fluctuations which unavoidably occur during camera manipulation. The above and other objects are solved by applying the features laid down in the independent claims 1 and 9. Further features and aspects of the method and apparatus and enhancements thereof are defined in the associated subclaims. If through whatever circumstances during motion picture pickup the iris fluctuates, the implemented circuitry compensates these fluctuations and stabilizes the iris position. Apart from this stabilizing effect, the implemented circuitry is a suitable means to converge the actual iris position to the target iris position. It is a further object of this invention to control iris and exposure time of a movie camera in an either manual or automatic mode, and to set limits to iris and exposure time in order to meet criteria concerning, for example, field depth. To attain the above objects, a desired iris value is generated, a current iris value of an iris is detected, the detected current iris value is compared with the desired iris value, and the iris is controlled based on the result of the comparison so that the iris value is stabilized, i.e., is kept to the desired iris value. Thus, iris control is attained through the steps of: Generating a desired iris value of an iris; detecting a current iris value; comparing the two values; changing the iris based on the result of the comparison; and limiting the desired iris value. When the iris is limited so that the iris value does not exceed a predetermined value, the level of the output signal of the image pickup device is automatically adjusted by the exposure period of time and/or the amplification gain when output signal amplification control is provided. In this manner iris related blur due to diffraction is eliminated. By setting and keeping the iris, a desired iris value can be selected by the user. Further, the level of the output signal of the image pickup device can be kept in a proper level by automatically controlling the controlled variables other than the iris, that is, by controlling the exposure period of time and/or the amplification gain of an output signal amplifier. BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages of the present invention will be apparent from the following description taken in connection with the accompanying drawings, wherein: Figs. 1 and 2 are block diagrams of conventional image pick-up apparatus (video cameras); Fig. 3 is a graph showing an example of the input-output characteristic of an iris value detecting circuit; Fig. 4 is a graph showing the relationship between the iris value and exposure period of time in programmed AE; Fig. 5 is a block diagram of an image pickup apparatus as an embodiment of a video camera which is exemplary for the setting of an iris and maintaining its value; Figs. 6 and 7 are diagrams showing examples of an important part of the image pickup apparatus depicted in Fig. 5 and in the following figures implemented in analog and digital circuits, respectively; Fig. 8 is a block diagram of an image pickup apparatus as another embodiment of the present invention; Fig. 9 is a pulse waveform diagram showing an image pickup device driving signal; Fig. 10 is a timing chart showing increment/ decrement amounts of exposure period of time; Figs. 11 through 13 are views for explaining control in the case where the embodiment in Fig. 8 is realized by use of a microcomputer; Fig. 14 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Figs. 15 and 16 are views for explaining control in the case where the embodiment in Fig. 14 is realized by use of a microcomputer; Fig. 17 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 18 is a diagram showing an example of the configuration of the variable-gain amplifier depicted in Fig. 17; Fig. 19 is a diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 20 is a graph showing an example of a control curve in conventional programmed AE; Figs. 21 through 23 are graphs showing examples of control curves in new programmed AE which can be realized by the embodiment of the image depicted in Fig. 19; Fig. 24 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 25 is a graph showing an example of exposure control which can be realized by the embodiment of the image pickup apparatus depicted in Fig. 24; Fig. 26 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 27 is a graph for explaining exposure control which can be realized by the embodiment of the image pickup apparatus depicted in Fig. 26; Fig. 28 is a block digram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 29 is a graph showing exposure control which can be realized by the embodiment of the image pickup apparatus depicted in Fig. 28; Fig. 30 is a view showing an example of the appearance of the image pickup apparatus according to the present invention; Fig. 31 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Figs. 32 and 33 are flow charts showing the operation of the image pickup apparatus in this embodiment; Fig. 34 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 35 is a graph showing the relationship between the output voltage of an iris position detecting element and the iris value; Fig. 36 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; Fig. 37 is a flow chart showing the operation of the image pickup apparatus in this embodiment; Fig. 38 is a block diagram showing a further embodiment of the image pickup apparatus according to the present invention; and Fig. 39 is a graph showing an example of the light intensity dependency of the iris value and exposure period of time in a further embodiment of the image pickup apparatus according to the present invention. DESCRIPTION OF PREFERRED EMBODIMENTS An embodiment of the present invention will now be described with reference to Fig. 5. In this figure the same reference numerals as those used in Figs. 1 and 2 designate the same parts. Reference numeral 16 designates an iris voltage changing circuit for generating a desired iris voltage. The iris value detecting circuit 12 detects a current iris voltage, and a comparator 18 compares the current iris voltage from the iris value detecting circuit 12 and the desired iris voltage from the iris voltage changing circuit 16. The iris motor 7 is controlled on the basis of the result of comparison in the comparator 18 so that the iris value is set to the desired one. A control button 15 is arranged so as to be operated by a user and forms a part of a circuit for changing over the control mode between automatic iris control and manual iris control and for designating an iris value at the time of manual iris control. When the user selects manual iris control, an iris voltage corresponding to the iris value designated by the user is issued as a desired iris voltage from the iris voltage changing circuit 16. When the user selects automatic iris control, the output voltage of the iris voltage changing circuit 16 is made automatically up/down based on the result of the comparison by means of a comparator 17 between the signal quantity from the detector 4 and the reference value from the constant voltage source 5 until the signal quantity from the detector 4 and the reference value from the constant voltage source 5 become substantially. coincident with each other. According to the present invention, not only the iris value can be controlled automatically, but also it can be set to a desired iris value designated by the user. Fig. 6 is a detailed circuit diagram specifically showing the detector 4, comparator 17, the iris voltage changing circuit 16 and the control button 15 in the embodiment of Fig. 5 and following figures. In Fig. 6, reference numerals 50 through 65 designate resistors, 66 a variable resistor, 67 a switch, 68 through 70 capacitors, 71 through 76 NPN transistors, and 77 through 80 PNP transistors. The automatic iris control and manual iris control are selected by turning the switch 67 to the sides B and A, respectively. A voltage Vs supplied from a constant voltage source 5 is used as the above-mentioned reference value for the signal quantity. When automatic iris control is selected, the signal quantity or output of the detector 4 is compared with the reference value and then a voltage corresponding to the result of the comparison is issued from the iris voltage changing circuit 16. When manual iris control is selected, a desired iris value is set through the variable resistor 66 and then the set iris value is put out from the iris voltage changing circuit 16. Fig. 7 shows an example in the case where the iris voltage changing circuit 16 of Fig. 5 and following figures is constructed as a digital circuit. In Fig. 7, reference numeral 161 designates a binary coding circuit, 162 an up-down counter, 163 a digital-to-analog converter (hereinafter abbreviated to "D/A"), 164 a clock generator, 165 a switch, and 166 a controller. When the user selects automatic iris control through the control button 15, the switch 165 is turned to the side A. The up-down counter 162 performs up-counting and down-counting based on the level (HIGH/LOW) of the output voltage of the comparator 17. The result of the counting is put out from the data output and converted into an analog signal by the D/A 163. Through the successive up/down counting operation, the output voltage of the D/A 163 is finally made to be substantially equal to the iris voltage which is made to be a desired signal quantity. When manual iris control is selected through the control button 15, the switch 165 is turned to the side B, the up-down counter 162 takes in an iris voltage corresponding to a desired iris value through its data input and load terminals and then outputs the iris value through its data output terminal. In the following, another embodiment of the present invention is described with reference to Fig. 8. In this embodiment, same reference numerals as those used in Fig. 5 designate same as those parts in Fig. 8. This embodiment shows an example of a camera in which manual-iris mode AE can be attained. The output of the comparator 17 is also connected to an exposure time changing circuit 21. When the user selects manual iris mode AE through a control button 19, the signal quantity is controlled suitably by changing the exposure period of time while keeping the iris value to a value designated by the user. A control circuit 20 keeps one of the iris voltage changing circuit 16 and the exposure time changing circuit 21 to the value designated by the user and performs automatic control upon the other based on the output of the comparator 17. An image pickup device driving circuit 22 serves to change signal sweep-out timing in increment/decrement of 1H and finely adjust the signal read-out timing over 1H period, so that the exposure time can be selected in an increment/decrement finer than 1H. Fig. 9 shows an example of the output pulse of the image pickup device driving circuit 22 of Fig. 8. In Fig. 9, HD represents a horizontal synchronizing signal and SUB represents a substrate voltage. SG represents a pulse indicating signal transfer timing from a photoelectric element to a vertical register. As described in NIKKEI MICRODEVICE, No. 10, pp 63-64, Oct. 1987, the signal charge in the photoelectric element can be swept out to the substrate by raising the level of SUB in a horizontal blanking period. The period of time from the point of time when last sweeping-out is finished to the point of time when the level of SG becomes high is the exposure period of time. When the level of SG becomes high, the potential of the vertical register is lowered so that the signal charge is read out from the photoelectric element to the vertical register. As shown in the drawing, the timing of increasing the level of SG can be selected freely in the period of 1H as long as the timing is in the vertical blanking period. Accordingly, a continuously variable shutter can be attained. The gate of SG (signal read-out gate) does not always exist in the image pickup device 1. In general, the potential of the vertical register is lowered by adjusting the clock of the vertical register to the highest of three values at the time of point when the level of SG becomes high. Fig. 10 shows an example of an increment/decrement amount ΔT for changing the exposure period of time T. Because the increment/decrement amount ΔT decreases as the exposure period of time decreases, the signal quantity change when one increment/decrement is caused in the range of 262H (1/60 sec.) to 1H (1/16000 sec.) is set to be within a range of 1% of the signal quantity before the increment/decrement. Accordingly, the displacement of the signal quantity from a predetermined center value of the signal quantity is not more than 1% even if the exposure control is performed while changing only the exposure period of time. Thus, stable signal quantity control can be attained. In the following, exposure control based on the operation of the sweep-out timing and exposure control based on the operation of both the sweep-out timing and the read-out timing are described with reference to Figs. 11 through 13 in the case where this embodiment is realized by use of a microcomputer. Fig. 11 is a timing chart of pulses related to an electronic shutter. In Fig. 11, the same pulses as those in Fig. 9 are referenced correspondingly. In the drawing, the exposure period of time is the period from the sweep-out point of time Ts to the read-out point of time Tr. The sweep-out point of time Ts is variable in an increment/decrement of ΔTs (=1H). The read-out point of time Tr is variable in an increment/decrement of ΔTr within a range between Tmn and Tmx in 1H within a vertical blanking period. The period of time between Tmn and Tmx is equal to the period of 1H. First, exposure control based on the operation of the sweep-out point of time Ts is described with reference to the flow chart of Fig. 12. The quantity of light is judged first. New Ts is then prepared by adding ΔTs to Ts or by subtracting ΔTs from Ts, based on the result of the judgment. Accordingly, the minimum increment/decrement amount in exposure control is ΔTs (=1H). Next, exposure control based on the operation of both the sweep-out point of time Ts and the read-out point of time Tr is described with reference to the flow chart of Fig. 13. The quantity of light is judged first, and the phase of the read-out pulse is judged. Assuming now that Tr is equal to Tmn, Tr is changed to Tmx because of the necessity for carry and then ΔTs is added to Ts. The period of time between Ts and Tmn is equal to the period of time between Ts + ΔTs and Tmx. Then, Tr is shifted by ΔTr. As a result, the minimum increment/decrement amount in exposure control is ΔTr so that fine exposure control can be made. Hardware for attaining this embodiment can be constructed by using a known logic technique. In the following, a further embodiment of the present invention is described with reference to Fig. 14. In this embodiment, same reference numerals as those used in the foregoing embodiments designate same as those parts in those foregoing embodiments. This embodiment shows an example of an AE camera operable in manual-iris mode and constructed by using an image pickup device driving circuit 26 having a terminal for controlling the exposure period of time in an increment/decrement of every horizontal scanning period (1H) and another terminal for finely tuning the exposure period time over 1H. A comparator 23 has two output terminals. A signal from one output terminal of the comparator 23 is supplied to the iris voltage changing circuit 16 and an exposure time fine tuning circuit 25. A signal from the other output terminal of the comparator 23 is supplied to an exposure time changing circuit 24. The gain in the latter output of the comparator 23 is set to be lower than that in the former output thereof, because the latter output is used for rough exposure time control in an increment/decrement of 1H. Upon detecting an input signal from the comparator 23 in a predetermined range, the exposure time changing circuit 24 stops the rough (1H-increment/decrement) exposure time control and operates the exposure time fine tuning circuit 25. The exposure time fine tuning circuit 25 and the image pickup device driving circuit 26 finely tune the signal charge read-out timing from the photoelectric element to the vertical register based on the output of the comparator 23. Although the range of the fine tuning requires at least 1H, the range can be enlarged to the order of several horizontal scanning periods (several Hs). An example of exposure control based on the operation of both the exposure time changing circuit 24 and the fine tuning circuit 25 is described with reference to Figs. 15 and 16. Fig. 15 is a timing chart similar to Fig. 11. Fig. 16 is a flow chart showing the operation of the comparator 23, the exposure time changing circuit 24 and the exposure time fine tuning circuit 25 in the case where this embodiment is realized by use of a microcomputer. Features of this example are as follows. (1) The exposure time changing circuit 24 serves to control the phase of the sweep-out pulse Ts and, on the other hand, the exposure time fine tuning circuit 25 serves to control the phase of the read-out pulse Tr. (2) The difference between the current exposure light quantity and the proper exposure light quantity is calculated and the obtained value is classified into a group of large difference and a group of small difference. (3) When the difference is large, the current exposure light quantity is approached to the proper exposure light quantity by using the exposure time changing circuit 24. At this time, the exposure time fine tuning circuit 25 fixes the phase of read-out pulse SG to T CN as a middle phase of Tmn and Tmx and then keep the phase in this state. (4) When the difference is small, the controlling procedure as shown in Fig. 13 is carried out, carrying out a main control by the exposure time fine tuning circuit 25. When the phase of the read-out pulse satisfies the relation Tr=Tmx or Tr=Tmn, the exposure time fine tuning circuit 25 informs the exposure time changing circuit 24 of this fact to thereby shift the phase of the sweep-out pulse Ts by ΔTs. The image pickup device driving circuit 26 in this embodiment has an advantage in that the circuit can be provided in the form of a simple logic circuit, compared with the image pickup device driving circuit 22 in Fig. 8. In the following, a further embodiment of the present invention is described with reference to Fig. 17. In this embodiment, same reference numerals as those used in the foregoing embodiments designate same as those parts in those foregoing embodiments. This embodiment shows an example of a camera in which manual-iris mode AE is attained by using the image pickup device driving circuit 11 for controlling the exposure period of time in an increment/decrement of 1H. Control in an increment/decrement of 1H may be carried out as shown in Fig. 12. Rough exposure time control in an increment/decrement of 1H is carried out by the comparator 23, an exposure time changing circuit 27 and the image pickup device driving circuit 11. The signal quantity is finely tuned by a variable-gain amplifier 28 provided in the outside of the rough exposure control loop so that the signal quantity is kept constant. The variable-gain amplifier 28 is different from a conventional variable-gain amplifier used for increasing the gain at the time of shortage of light quantity. More specifically, the variable-gain amplifier 28 can operate even in the case where the iris is not in a fully opened state. The conventional variable-gain amplifier has an AGC eliminator by which the gain is minimized when the iris is closed even in the least amount (in other words, when light quantity is not insufficient). However, the variable-gain amplfier in this embodiment has no AGC eliminator. An example of the variable-gain amplifier is shown in Fig. 18. The output signal level can be kept constant by negative-feeding back the output of a detector 282 to a gain control terminal of a variable-gain amplifier 281 after comparing the output of the detector 282 with a constant voltage source 284 in a comparator 283. In the following, a further embodiment of the present invention is described with reference to Fig. 19. In this embodiment, same reference numerals as those used in the foregoing embodiments designate same as those parts in those foregoing embodiments. This embodiment shows an example of a camera in which all the exposure control of manual exposure control, manual-exposure time mode AE, manual iris mode AE and programmed AE can be attained. A control circuit 30 receives the information from a control button 29 operated by the user, the iris value information from the iris value detecting circuit 12 and the exposure time information from the exposure time changing circuit 21 so that programmed AE control based on various programs can be carried out. Fig. 20 is a graph with the exposure period of time as the abscissa, showing the same control curve as that in conventional programmed AE in Fig. 4. Figs.. 21 through 23 are like graphs with the exposure period of time as the abscissa, showing examples of control curves in new programmed AE. Fig. 21 shows programmed AE in which the iris value is changed according to the information of exposure period of time, as opposed to Fig. 20. Fig. 22 shows programmed AE in which both the iris value and the exposure time are operated continuously and interlockingly. Fig. 23 shows programmed AE in which automatic iris control and automatic exposure time control are switched over at a suitable iris value (for example, F4). In any case, programmed AE can be attained by the configuration of the embodiment of Fig. 19. In the following, a further embodiment of the present invention is described with reference to Fig. 24. This embodiment shows an example in which an iris value limiting circuit 31 is added to the embodiment of Fig. 8 to thereby limit a desired iris value not more than a predetermined value. The exposure time is controlled automatically corresponding to the limitation of the iris value to thereby keep the signal level constant. An example of the exposure control is shown in Fig. 25. In this example, when the subject is so lighted that the iris value reaches F8, the iris value is forcedly fixed to F8 regardless of the exposure period of time designated by the user. That is, the signal level can be kept constant by shortening the exposure time. In the case of a 1/2 inch optical system, iris related blur caused by diffraction hardly occur in the iris condition of about F8. Accordingly, a camera with no iris related blur can be provided. In the following, a further embodiment is described with reference to Fig. 26. In this embodiment, same reference numerals as those used in the foregoing embodiments designate same as those parts in those foregoing embodiments. This embodiment shows an example in which an iris value limiting circuit 31 is added to the embodiment of Fig. 19 to thereby limit a desired iris value to not more than a predetermined value. In this embodiment, iris related blur can be prevented even in the case where the user selects programmed AE. An example of the exposure control is shown in Fig. 27. Even in the case where programmed AE is selected, the iris value is forcedly fixed to F8 when the subject is so intensively lighted that the iris value reaches F8. That is, the signal level can be kept constant by shortening the exposure period of time. In the following, a further embodiment is described with reference to Fig. 28. In this embodiment, same reference numerals as those used in the foregoing embodiments designate same as those parts in those foregoing embodiments. This embodiment shows an example in which the iris value limiting circuit 31 in Fig. 26 is replaced by a limiting circuit 32 for limiting the exposure period of time not shorter than a predetermined value as well as limiting the iris value. Even in the case where the user selects a specific iris value in manual-iris mode AE mode, exposure control is carried out as follows. When intensive light enters so that the exposure period of time is shortened to a predetermined value, the exposure period of time is forcedly fixed to the predetermined value so that exposure control is carried out by increasing the iris value. By providing a limitation on the exposure period of time as described above, the scale of the image pickup device driving circuit 22 as shown in the embodiment of Fig. 8 can be reduced. An example of the manual-iris mode AE exposure control is shown in Fig. 29. In this exmaple, the exposure period of time is limited so as not to be shorter than 8H (1/2000 sec). If the increment/decrement ΔT of exposure period of time is designed according to the example of Fig. 10, it is sufficient that the increment/decrement is equal to H/16. Accordingly, in this case, the image pickup device driving circuit 22 can be constructed in the form of a simple logic circuit, compared with the case where the increment/decrement is H/128 or less. In the following, an implementation of a control panel is described with reference to fig. 30. This figure shows an example of external appearance of a video movie camera having control buttons for selecting four exposure control modes, namely, programmed AE, manual-iris mode AE, manual-exposure time mode AE and manual exposure control, as shown in Figs. 19, 26 and 28. A group of button switches are provided in one side of a casing of a movie camera 100 and between an object lens and a cassette cover 106. When, for example, the slide switch 101 is set to the left, full automatic exposure control is selected so that programmed AE starts. When the push switch 102 is pushed after the slide switch 101 is set to the right, the iris value is fixed to a set value so that manual iris mode AE starts. When the left-right push switch 104 is then pushed to the left, the iris is changed so as to be closed. When the left-right push switch 104 is pushed to the right, the iris is changed so as to be opened. When the push switch 102 is pushed once more, the iris control is returned to automatic as a programmed AE mode. When the push switch 103 is pushed in this condition, the exposure period of time is fixed to a set value so that manual exposure time mode AE starts. When the left-right push switch 105 is pushed left or right in this condition, the exposure period of time is changed. When the push switch 102 is then pushed to set the two switches to manual, a manual exposure control mode starts so that both the iris value and the exposure period of time can be designated suitably by operating the left-right switches 104 and 105 by the user. Indications such as indication of the iris value and exposure period of time, indication of the exposure control mode, alarming indication at the time of manual exposure control, and the like, may be made in the vicinity of the switches 101 to 105 or may be made in an electronic view finder. In any case, desired one of the exposure control modes can be easily selected by the user through the control buttons arranged as shown in Fig. 30. In the following, a further embodiment is described with reference to Figs. 31 through 33. This embodiment shows an example in which a manual-iris mode AE function and a function (iris stopper) of preventing iris related blur are provided in a camera for performing auto-iris exposure control. In Fig. 31 reference numeral 301 designates an iris, 302 an iris motor, 303 an iris position detecting element (Hall element) mounted on the iris motor, 304 an amplifier for amplifying the output of the Hall element, 305 an image pickup device in which the exposure period of time or electronic shutter speed can be controlled continuously in one horizontal scanning period or less, 306 an image pickup device driving circuit, 307 a camera signal circuit, 308 an iris detection circuit, 309 a comparator, and 310 a reference voltage source. A portion 311 surrounded by broken lines shows a microcomputer. In this embodiment, processing by the microcomputer 311 is shown as a functional block diagram. Reference numerals 312 to 318 designate terminals. Reference numeral 319 designates a signal input circuit (binary coding circuit) in which a signal is taken in the microcomputer 311 based on a timing pulse applied on the terminal 318. Reference numeral 320 designates an up-down counter for performing counting-up/ down of data based on the result (high/low) of comparison in the comparator 309. Reference numeral 321 designates an A/D converter for performing A/D conversion or iris value information. Reference numeral 322 designates a comparator for comparing data of the up-down counter 320 and iris value data with each other. Reference numeral 323 designates an output device for sending out a high-level or low-level voltage based on the result of the comparison in the comparator 322. The aforementioned A/D conversion, comparison and sending out of the result of the comparison are carried out in the period of timing pulses applied on the terminal 314. Reference numeral 324 designates an iris drive circuit for opening/closing the iris 301 based on the high- or low-level voltage sent out from the terminal 313. Reference numeral 325 designates a processing mode change-over switch which receives, through the terminal 316, an instruction related to the AE mode designated by the user, and changes the processing mode in the microcomputer. Reference numeral 326 designates an exposure time controller which gives an instruction related to exposure period of time or shutter speed to the image pickup device driving circuit 306. In the following, the operation of the respective parts is described based on the signal flow. Both mode setting and shutter speed and iris value setting are necessary for the operation. For example, the settings are carried out by connecting two switches (not shown) to the terminal 316. When, for example, three modes of ordinary AE, manual-shutter speed mode AE, manual-iris mode AE can be carried out, these AE modes are selected successively based on a signal obtained from one of the switches connected to the terminal 316. "Ordinary AE" herein used is equivalent to exposure control in a home video camera having no electronic shutter function. For example, this is equal to manual-shutter speed mode AE of 1/60 sec in an NTSC signal method image pickup apparatus. In the manual-shutter speed mode AE mode, the shutter speed is selected through the other switch connected to the terminal 316. For example, the shutter speed is selected in the order of 1/60 sec, 1/100 sec, 1/250 sec and 1/500 sec. In the manual-iris mode AE mode, iris values of, for example, F1.2, F2, F2.8 and F4, are selected successively through the switch. In the case where the ordinary mode or manual shutter speed mode AE mode is designated by the user, the exposure time control circuit 126 issues an instruction for the image pickup device driving circuit 306 to fix the exposure period of time of the image pickup device to 1/60 sec or a value designated by the user and neglects the output of the comparator 309. Assuming now that initial data A is stored in the up-down counter 320, the iris value takes a value of A because the iris is controlled by a loop constituted by 324-302-303-304-321-322-323 so as to be closed when the iris value data in 321 is more than A, in other words, when the iris is opened and, on the other hand, so as to be opened when the iris value data is less than A. Of course, a detection voltage obtained from the detection circuit 308 in the aforementioned condition takes a value corresponding to the iris value A. Through comparison between the detection voltage and the reference voltage 310, the data of the up-down counter 320 is rewritten. Assuming now that the data in the counter is rewritten from A to B, the iris is controlled again so that the data in 321 becomes substantially equal to B. As a result, the iris value is controlled so that the output voltage of the detection circuit 308 becomes substantially equal to the reference voltage 310, and, then, automatic exposure is carried out at a shutter speed designated by the user. Accordingly, manual-shutter speed mode AE is attained. Fig. 32 is a flow chart of the aforementioned procedure. In the following, the case where the manual iris mode AE mode is designated by the user is described. In this case, the data of the up-down counter 320 is fixed to a data C corresponding to an iris value designated by the user regardless of the result of the comparator 309. As a result, the iris value is fixed to a value to make the data in 312 C, that is, fixed to an iris value designated by the user. In the aforementioned condition, the exposure time controller 326 issues an instruction to change the shutter speed based on the result of the comparator 309. As a result, the shutter speed is controlled so that the output voltage of the detection circuit 308 becomes substantially equal to the reference voltage 310. Accordingly, manual iris mode AE in the iris value designated by the user is attained. Fig. 33 is a flow chart of the aforementioned procedure. In the following, the iris stopper is described. In the manual-iris mode AE mode, the iris value designated by the user is limited within a range, for example, about F11, in which iris related blur is not remarkable. In the ordinary mode or manual shutter speed mode AE mode, the iris value is calculated from the result of the A/D conversion of the Hall element output and fixed to F11 by forcedly changing the processing of the microcomputer to the manual iris mode AE mode when the iris value exceeds F11. That is, when the iris value exceeds F11, exposure control based on the electronic shutter is carried out while the iris value is fixed to F11. According to the aforementioned exposure control, for example, in the case where the image of a brightly illuminated subject requiring the iris value of F16 is picked up, the iris value and the electronic shutter speed become F11 and about 1/120 sec, respectively. Accordingly, there is no occurrence of iris related blur. In the aforementioned embodiment, two loops exist. One is a small loop for feeding back the result of comparison between the actual iris value and the data in the up-down counter 320 as a target value. The other is a large loop for feeding the result of comparison back to the iris drive circuit while changing the data in the up-down counter 320 as a target iris value based on the comparison between light quantity obtained from the iris value determined by the small loop and a reference value. Feedback sampling periods are determined by timing pulses applied onto the terminals 314 and 318, respectively. In this case, it is important that the small loop control period S 1 is shorter than the large loop control period S 2 , so that the actual iris value can be controlled stably in the small loop period with respect to the target iris value changed by the large loop control period. Accordingly, smooth iris driving can be made. As described above, according to this embodiment, both a manual-iris mode AE function and an iris stopper function can be provided. In the following, a further embodiment of the present invention is described with reference to Fig. 34. This embodiment is based on the embodiment of Fig. 31 and improved in iris response speed and stability. In this embodiment, therefore, the output voltage of the detection circuit as representing exposure light quantity is converted from an analog signal to a digital signal in the microcomputer so that the control loop feedback gain is changed corresponding to the difference between the obtained voltage and the reference voltage or the ratio of the two voltages. Reference numerals 201 to 226 in Fig. 34 correspond to reference numerals 301 to 326 in Fig. 31, respectively. In Fig. 34 reference numeral 227 designates an A/D converter for performing A/D conversion on the detection output signal and 228 an A/D converter for performing A/D conversion on the reference voltage 210. Reference numeral 229 designates a comparator for comparing the results of the two A/D conversions with each other to generate the difference therebetween or the ratio thereof. The operation of the respective parts is described in the case where a lens having a maximum aperture F value of 2 is used and intensive light requiring the iris value of F16 suddenly enters into the lens with the iris almost fully opened in the ordinary mode. In the stable condition before the entrance of intensive light, the output voltage of the detection circuit 208 is substantially equal to the reference voltage 210. Accordingly, the difference between the results of conversion of both the output and reference voltages is zero, and, in other words, the ratio therebetween is 1. At this time, the target iris value data in the up-down counter 220 is equal to the current iris value data in the A/D converter 221. Accordingly, the iris is kept in a stable state. When intensive light now enters the lens, the output voltage of the detection circuit 208 becomes largely different from the reference voltage. A threshold is provided in the comparator 229 in advance. When, for example, the difference is within one iris stop (±6dB), an instruction to shift the data of the up-down counter 220 by one bit is given. When, for example, the difference is within a range of from one to two stops (±12 dB to ±6 dB), an instruction to shift the data by two bits is given. When, for example, the difference is more than two stops, an instruction to shift the data by 4 bits is given. In this embodiment, therefore, the iris moves at a speed four times as high as the normal speed before the iris value reaches F8; the iris moves at a speed twice as high as the normal speed when the iris value is within a range of from F8 to F11; and the iris moves at the normal response speed and stabilized after the iris value exceeds F11. In the manual-iris mode AE mode, the iris moves in the same manner as described above. When the output voltage of the detection circuit is largely different from the reference voltage, the quantity of change of the exposure period of time is increased. As the difference decreases, the quantity of change approaches to that at the stable state. According to the aforementioned embodiment, both the speed and stability in the automatic iris function including a manual-iris mode AE function can be attained at the same time. In the following, a further embodiment of the present invention is described with reference to Figs. 35 through 37. In this embodiment, the iris response speed and stability are improved on the basis of the embodiment of Fig. 31, similarly to the embodiment of Fig. 34. To this end, in this embodiment, an iris judgment means is provided in the microcomputer to divide the iris value into a plurality of areas so that the control loop feedback gain in each area is changed. In the case where the iris is controlled based on digital data, the reciprocating motion of the data within a range of ±1LSB is repeated in every control period in the stable state. It is, therefore, necessary that the A/D conversion bit number is a bit number enough to make the change of light quantity caused by the ±1LSB reciprocating motion not more than the detection limit. Fig. 35 shows dependency of the Hall element amplifier output voltage on the iris value. As the iris is reduced, the output voltage change decreases relative to the iris value change. Accordingly, in the case where the iris is controlled continuously based on the comparison between the current iris value and the target iris value as in the present invention, it is necessary to determine a quantization bit number from the slope of the Hall voltage with respect to the light quantity change in the condition in which the iris is reduced to the minimum value in the iris control range of the image pickup apparatus. When an iris stopper as shown in the embodiment of Fig. 31 is provided, a quantization bit number necessary for obtaining sufficient resolution power in an iris value related to the stopper must be determined. In the case where the iris is controlled based on the thus determined quantization bit number, redundant bits occur when the iris is opened to F2 or F2.8 so that the iris response becomes very slow though the data change by one LSB is repeated in one control procedure. In this embodiment, therefore, the quantity of change of the target iris value in one control procedure is changed based on the current value by using the areas set in the iris judgment means provided in the microcomputer to thereby change the data in the up-down counter 420 in such a manner as follows. When, for example, the iris value is within a range of from the maximum aperture state to F2.8, the third lowest bit of the data in the up-down counter 420 is changed. When, for example, the iris value is within a range of from F2.8 to F5.6, the second lowest bit of the data in the up-down counter 420 is changed. When, for example, the iris value is F5.6 or more, the lowest bit of the data in the up-down counter 420 is changed. Alternatively, the data in the up-down counter may be shifted by the bit number equal to the value designated by the respective area in the condition that the value of the respective area is determined in advance in such a manner as follows. A value of 4 is given to the area in the range of from the maximum aperture of F2.8; a value of 3 is given to the area in the range of from F2.8 to F5.6; a value of 2 is given to the area in the range of F5.6 to F8; and a value of 1 is given to the area in the range of from F8 to F16. Accordingly, both the speed and stability in iris response can be attained at the same time. Reference numerals 401 to 425 in Fig. 36 correspond to reference numerals 301 to 325 in Fig. 31, and the operation of those parts designated by reference numerals 401 to 425 are the same as that of those designated by those corresponding reference numerals 301 to 325, respectively. In Fig. 36 reference numeral 427 designates an iris value judgment means in which the quantity of change obtained by the up-down counter 420 and the exposure time control circuit 426 in one control procedure is changed corresponding to the iris condition. Fig. 37 is a flow chart showing the control in this embodiment. By combination of this embodiment and the embodiment of Fig. 34, higher speed can be attained. Although this embodiment has shown the case where a microcomputer with software is used, the same effect can be attained in the case where a non-linear characteristic may be given to the amplifier 404 of the Hall element output so that the change of the output voltage of the amplifier with respect to the change of light quantity can be kept constant in every iris value with no use of the microcomputer. In the following, a further embodiment of the present invention is described with reference to Fig. 38. This embodiment is based on the embodiment of Fig. 31 and improved in both iris response speed and stability similarly to the embodiments of Figs. 34 and 36. In this embodiment, therefore, the output signal from the output device 323 in Fig. 31 is sent out in the form of a pulse signal having a duty factor which is changed corresponding to the iris opening/closing instruction. One example of the operation is shown in Fig. 37. In Fig. 38, the diagram (a) is a timing chart showing the data in the up-down counter 320 in Fig. 31, that is, the change of the target iris value, the diagram (b) shows the change of the output pulse signal corresponding to the change shown in the diagram (a), and the diagram (c) shows the change of the actual iris value. In Fig. 38, the iris value coincides with the target iris value during the period between T 0 and T 1 . During the period, pulses of a 50% duty factor are generated from the output terminal 313 in Fig. 31. When the target iris value is changed at time T 1 , the output pulse duty factor is changed. In this embodiment, a pulse of a 100% duty factor or a high-level voltage is generated. When the difference between the actual iris value and the target iris value or the ratio therebetween falls within a predetermined range at time T 2 , the duty factor of the output pulse is changed again. In this embodiment, the duty factor becomes 75%. When the actual iris value coincides with the target iris value at time T 3 , the duty factor becomes 50% again. When the target value is moved reversely in the period between T 3 and T 6 , the output pulse duty factor is changed in the sequence of 50% - 0% - 25% - 50%. According to this embodiment, both the response smoothness and stability can be improved compared with the output method in which high-level and low-level signals are alternated easily in the relationship between the actual iris value and the target iris value. It is a matter of course that an image pickup apparatus more greatly improved in both the response smoothness and stability can be provided by combination of two or more embodiments as shown in Figs. 34, 36 and 38. In the following, a further embodiment of the present invention is described with reference to Fig. 39. This embodiment is related to the proper handling of the image pickup apparatus constructed according to the present invention. Assuming now that the range of light intensity in a general camera is from 100 lux to 100,000 lux, an iris control range of F1.2 to F36 and an exposure time control range of 1/60 sec to 1/60,000 sec must be provided to perfectly realize both the manual-iris mode AE and manual-shutter speed mode AE in the case of use of a lens of the maximum aperture F1.2. However, this is difficult to achieve from the point of view of accuracy. In particular, the electronic shutter determining the exposure period of time can be, in most cases, changed only in an increment/decrement of a predetermined period such as a horizontal scanning period because of structure of the image pick device, limitation by the control circuit, and the like. In this case, there is a predetermined limitation in the exposure period of time which can be controlled. This embodiment is related to a measure as an image pickup apparatus which handles the case where a necessary exposure time control range cannot be maintained. Fig. 39 shows the change of the iris value and the change of the exposure period of time caused by the light quantity in the case where the user selects the manual-iris mode AE mode and designates the iris value of F1. As the light quantity increases, the exposure period of time decreases. When the exposure period of time must exceed the time T 1 limited as the control range for the aforementioned reasons, or in other words, when the light quantity exceeds ℓ 1 , the control mode is switched to the manual-shutter mode AE mode after the exposure period of time is forcedly fixed to T 1 . By the aforementioned method, areas in which exposure control cannot be made in the manual-iris mode can be eliminated. The aforementioned mode switching may be indicated, for exaple, in a finder. Methods for providing the aforementioned various controlling means in the image pickup apparatus are as follows. Although the control means can be constructed in the form of an analog circuit, it is necessary to store information such as a current iris value, a desired iris value, a current exposure light quantity, a desired exposure light quantity, and the like, and perform various calculations based on the information. The objects to be controlled are the iris value and the exposure period of time of the image pickup device, and, at the same time, the processing speed is low and takes a value of the order of kHz. From the point of view of popularization of microcomputer including A/D converters, the system using such a microcomputer is advantageous in cost and the quantity of data processing. According to the present invention, the iris value can be maintained to attain manual iris mode AE and, accordingly, the user can select depth of field. Further, an iris stopper can be provided to sufficiently prevent iris related blur.

Claims: An exposure control method in a video camera having at least two operating modes (A, B) and having an iris of the feedback controlled type, comprising the steps of: generating (5) a predetermined first reference signal to establish a desired luminance signal level; during the sensing of an image by said video camera, continuously detecting (4) the current luminance signal level; in a first comparison operation (17) comparing said desired luminance signal level with said current luminance signal level, and in a first operating mode (B) controlling (7) the iris based on the result of said first comparison which is indicative of a desired iris value, to place said iris into a position wherein the luminance signal of the sensed image generated by the image pickup device (1) of said video camera has a level corresponding to said desired luminance signal level, characterized by further comprising the steps of: during the sensing of an image by said video camera, continuously detecting (12) the current iris position and providing a current iris signal level; performing a second comparison operation (18) to compare said current iris signal level with the result of said first comparison operation (17); and controlling the iris based on the result of said second comparison (18) to place said iris into a position wherein said current iris signal level is stable in a position as indicated by the result of said first comparison. The method of claim 1 characterized by further comprising the steps of: generating (66) a predetermined second reference signal to establish a desired iris signal level corresponding to a desired iris position specified by the user; and in a second operating mode (A) performing said second comparison operation (18) to compare said desired iris signal level with said current iris signal level wherein said resulting luminance signal is cut off (67) the iris control The method of claim 1, wherein the step of controlling said iris is characterized by limiting (31) the iris value not to exceed a predetermined value. The method of claim 1, characterized by a further step of controlling the level of said luminance signal generated by said image pickup device (1) of said video camera by automatically adjusting an exposure period of time and/or the gain of an amplifier (281) for amplifying said current luminance signal. The method of claim 4, characterized by a further step of controlling (21, 22) the exposure period of time of said image pickup device (1). The method of claim 5 being further characterized by a step of controlling said iris to be positioned to an iris value not exceeding a predetermined iris value. The method of claim 5 being further characterized by a step of controlling said exposure period of time by means to limit the exposure time not to undercut a predetermined exposure time value. The method of claim 5 being further characterized by a step of limiting the iris position to a value which does not cause the exposure time to exceed a predetermined time. A video camera comprising: a lens and an iris of the luminance controlled type, control means (7) for driving said iris, an image pickup device (1), means (4) for detecting the current luminance signal outputted by said image pickup device, a first comparator means (17) coupled to said detector means (4) for comparing a level of said current luminance signal with the level of a first reference signal (5) to produce a first control signal indicative of a desired iris value, and driver means (11) for controlling said image pickup device on the basis of an exposure time value, and so that said image pickup device generates a current luminance signal of appropriate signal level; characterized by further comprising: iris value detector means (12) for detecting during sensing of an image by said video camera, a signal representing a current iris value, a second comparator means (18) for comparing said first control signal with said signal representing a current iris value to control said iris driving means (7) based on the result of said second comparison (18) to maintain said signal representing a current iris value in a stable position. The video camera of claim 9, further characterized by means for generating (66) a predetermined secondreference signal representing a second desired iris signal level corresponding to a second desired iris position specified by the user. The video camera of claims 9 and 10, characterized in that said second comparator means (18) is also provided for comparing said second desired iris signal level with said current iris signal level thereby forming a servo control loop to automatically place the iris into a position wherein the current iris position corresponds to the second desired iris position specified by the user. The video camera of claim 11, characterized by a switch (67) to alternatively connect one input of said second comparator means (18) either to the said first control signal or to the said predetermined second reference signal (66). The video camera of claim 9, further characterized by: control means (15) coupled to said control means (7) for driving said iris and for automatically controlling said iris in said first operating mode (B), and for manually setting said iris in a second operating mode (A). The video camera of claim 9, further characterized by: means (21, 22) coupled to said image pickup device (1) for driving said image pickup device to control an exposure period of time of said image pickup device either automatically or manually, and selecting means (19, 20) to select a third operating mode for automatically controlling the exposure time and a fourth operating mode for manually controlling the exposure time. The video camera of claim 14, further characterized by: first control means (21, 30) coupled to said image pickup driving means (22) for controlling said image pickup driving means to limit in said image pickup device (1) the exposure period of time manually or automatically, and second control means (31, 16, 18) coupled to said first control means (21, 30) for limiting an iris value not to exceed a predetermined value. The video camera of claim 15, characterized in that said first and second control means (21, 30; 31, 16) limit an iris value not to exceed a predetermined iris value, and limit an exposure period of time not to undercut a predetermined exposure time value. The video camera of claim 13, further characterized by means (66) for setting a desired iris value in said second operating mode (A). The video camera of claim 9, further characterized by an iris value limiter means (31) coupled to said means (66) for generating a desired value for the iris position, for limiting the current iris value not to exceed a predetermined iris value, wherein said second control means (31, 16) is coupled to said iris driving means (7) for controlling the iris position on the basis of said iris value limiter means (31) to maintain the level of the luminance signal at the output of the image pickup device (1) at a predetermined luminance value. The video camera of claim 18, characterized in that said image pickup driver means (22) is controlled by an exposure time changing circuit (21) based on a result of the comparison performed by said first comparator means (17) wherein a level of the current luminance signal is compared with the level of said reference signal (5) to establish a desired luminance signal level. The video camera of claim 9, further characterized by: A/D conversion means (321) for converting an analog iris signal from said iris detector means (303, 304) into a digital value; counter register means (320) for providing a series of digital values commensurate to the digital values which may be produced by the A/D conversion means (321) and outputting one of said values of said series; input means (319) to cause said counter register means (320) to provide and store a numerical value corresponding to said first control signal; and control means (322, 323, 324) for controlling said iris (301, 302) to be positioned such that said numerical value in said counter register means (320) and the digitized value produced by said A/D conversion means (321) are substantially coincident with each other. The video camera of claim 20, further characterized by: luminance signal generating means (307, 308), as said means (4) for detecting the current luminance signal, for generating a current luminance signal value representing a current quantity of exposure light; means (310) for generating, as said first reference signal (5), a luminance reference signal representing a reference quantity of exposure light; comparing means (309), as said first comparator means (17), for comparing (309) said current luminance signal with said luminance reference signal; and by said counter register means (320) having an up/down counter (162) coupled to said comparing means (309), outputting a numerical value stored in said counter register means (320) which is incremented or decremented in accordance with the resulting value obtained by said comparison (309). The video camera of claims 15 and 21, characterized in that the numerical value which can be inputted (318) to said up/down counter (162) is limited to prevent said numerical value from being reduced beyond a predetermined value. The video camera of claim 21, characterized by an iris value judgement means (427) in which the quantity of change obtained by the up/down counter (420) and the exposure time control circuit (426) in one control procedure is changed corresponding to the current iris position. The video camera of claim 20, characterized in that the said iris control means (414, 423, 413, 424) comprises a means (423) for generating a pulse sequence (b) having a duty factor which changes in accordance with the difference or ratio determined by the second comparator (422). The video camera of claim 24, characterized in that said second comparator (422) compares the number outputted by said up/down counter (162) and the current iris value which is detected by a detector (303) and converted in an A/D converter (421) from an analog signal to a digital value. The video camera of claim 24, characterized by: means (410) for generating a signal corresponding to a luminance reference value representing a predetermined quantity of exposure light; means (416, 425) for generating at least one single target iris value which can be externally selected; and an iris control and driver means (423, 413, 424), as said control means (7) for driving said iris, for controlling said iris (401, 402) in accordance with at least one duty factor provided by said means (423) for generating a pulse sequence and selected (416, 425) from one of: (i) a difference or a ratio between the current luminance value and the luminance reference value, or (ii) a difference or a ratio between a current iris value and the target iris value, or from (iii) the current iris value.

Inventors: IMAIDE TAKUYA, KINUGASA TOSHIRO, KURASHIGE TOMOYUKI, TARUMIZU HIROYUKI, YAMAMOTO NAOKI, IMAIDE, TAKUYA, KINUGASA, TOSHIRO, KURASHIGE, TOMOYUKI, TARUMIZU, HIROYUKI, YAMAMOTO, NAOKI
Applicants: HITACHI LTD, HITACHI VIDEO ENG, HITACHI VIDEO ENGINEERING, INC., HITACHI, LTD.
Application year: 1990
Main classification: H04N5
IPC codes: H04N5