GB1569170A - Digital exposure meter - Google Patents

Description

(54) DIGITAL EXPOSURE METER (71) We, WEST ELECTRIC CO., LTD., a Japanese body corporate, of No. 3-6, Nagarahama-dori, Oyodo-ku, Osaka City, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to exposure meters, more particularly to a digital exposure meter for use in taking pictures in which luminance of a subject is measured to estimate an appropriate stop value, or F-number, for the combination of a desired shutter speed and a desired film sensitivity to indicate digitally the estimated number.

Instead of a prior art indicating-hand type exposure meter. digital-type exposure meter has been developed in recent years. In such an exposure meter, the senitivity of a film being used and a desired shutter speed are set, and thereafter the luminance of a subject to be photographed is measured to obtain a suitable stop value, which is digitally displayed. The operation of calculating a suitable stop number with the set shutter speed and film sensitivity and the measured subject luminance taken into account is performed in such a manner that the subject luminance is measured in a measuring circuit to which the signal representative of the set shutter speed is applied, the measured analog value of luminance bemg converted into a digital signal, which is in turn combined with the set film sensitivity value through an adder circuit, the resultant number being indicated in a digital form.

This type of exposure meter has the disadvantage that if one desired to find the proper stop number corresponding to a newly set shutter speed differing from the immediately previously set speed the measurement of the luminance of the subject should be made again for the new speed. This is inconvenient in use of the meter.

It often occurs that a picture is required to be taken with its field depth larger than is obtainable with the set shutter speed and aperture. To increase the depth of field, one decreases the aperture (and increases the shutter speed to achieve the correct exposure).

The measurement of the subject luminance must be repeated to each of the combinations of shutter speeds and film sensitivities until the desired stop number is derived. This will reduce the rapidity with which pictures can be taken and can lead to good photographs being missed.

In accordance with the present invention, there is provided a digital exposure meter comprising: (a) timer means for producing a first gating signal whose duration is indicative of a shutter speed hich is presettable, said timer means being arranged to produce said first signal only when a switch connected thereto is turned on; (b) light sensor means connected to said timer means for measuring the brightness of a subject to be photographed and integrating the resulting signal for the duration of the first gating signal to provide a second gating signal; (c) pulse oscillator means operatively connected to said light sensor means for producing a train of pulses for the duration of the second gating signal, the frequency of said pulses being presettable in accordance with film sensitivity; (d) l (d) counter means connected to said pulse oscillator means for counting the number of pulses applied thereto; (e) a first encoder for producing a first binary signal indicative of the pulse count; (f) A second encoder for producing for producing a second binary signal indicative of the preset shutter speed; (g) a third encoder for producing a third binary signal indicative of the preset film sensitivity; (h) first resetting means connected to said second encoder and to said timer means for resetting the second binary signal in the absence of said first gating signal by transmitting a fourth binary signal representative of the difference between two values of said second binary signals, one of said values being representative of the preset shutter speed and the other of said values being representative of a shutter speed set after the occurrence of said first gating signal; (i) second resetting means connected to said third encoder and to said timer means for resetting the third binary signal in the absence of said first gating signal by transmiting a fifth binary signal indicative of the difference between two values of said third binary signals one of which values is representative of the preset film sensitivity and the other of which values is representative of a film sensitivity which is set after the occurrence of said first gating signal; (j) a first added connected to said first encoder for receiving said first binary signal therefrom; (k) a second added connected to said first and second resetting means for receiving said forth and fifth binary signals therefrom for adding the same to each other, said second adder being arranged to transmit a sixth binary signal representaive of and corresponding to the sum of said fourth and fifth binary signals to said first adder so that said first adder transmits a seventh binary signal by adding said first and sixth binary signals to each other; (l) storage means for storing a plurality of stop number signals and connected to said first adder for transmitting an appropriate one of said stored stop number signals in accordance with the value of the seventh binary signal; and (m) indicator means connected to said storage means for indicating a stop number in response to a signal fed thereto.

The invention will be further described with reference to the accompanying drawings, in which: Figure I is a schematic diagram showing a digital exposure meter in accordance with the present invention; Figure 2 is a circuit diagram shown an embodiment of the exposure meter illustrated in Figure 1; Figures 3A-3G are time charts useful in understanding the operations of the exposure meter shown in Figure 2; Figure 4 shows a detailed circuit of the second encoder for use in the meter shown in Figures 1 and 2; and Figure 5 shows a detailed circuit of the third encoder for use in the meter shown in Figures 1 and 2.

Figure 1 shows a digital exposure meter in accordance with the present invention which mcludes a timer 1 developing a gating pulse of which the pulse duration corresponds to a shutter speed set in a second encoder 9, and a light sensor 2 which measures the luminance of a subject and is provided with the gating pulse so as to be energized during the duration of the gating pulse. A V-T converter 3 converts the measured subject luminance provided from the sensor into a corresponding pulse length, this process being called hereinafter V-T conversion. A pulse oscillator 4 generates pulses whose oscillating frequency is associated with a film sensitivity value set in a third encoder 10. A counter 5 is adapted to count and store the pulses transmitted from pulse oscillator 4. A first encoder 6 converts the count stored in counter 5 into its binary coded digital signal in accordance with the apex system, and an adder 7 is arranged to add the digital signal from first encoder 6 to the one from a second adder 8. The second adder 8 is adapted to add the digital signal which is received through a first resetting unit 11 from second encoder 9 encoding a set shutter speed value into its corresponding binary coded digital signal to the digital signal from a third encoder 10 which converts a set film sensitivity value into its corresponding binary coded digital signal. First resetting unit 11 and second resetting unit 12, which are similar to first resetting unit 11, are adapted to reset any "1" digital signal from third encoder 10 and second encoder 9, respectively, to its "0" state in response to the energization of timer 1. A storage 13 is operative in response to the digital signal from first adder 7 to select one of its storage locations, an indicator 14 receiving the signal associated with the location selected in storage 13 to indicate a stop value, or F-number, corresponding thereto. The digital exposure meter in accordance with the present invention, of which the general structure has just been described will be detailed with particular reference to Figure 2.

In advance of measuring the luminance of a subject, a desired shutter speed value, such as 1/60 second, and a desired film sensitivity, such as ASA 100, ar set in second and third encoders 9 and 10 respectively. The second encoder 9 is shown in Figure 4 and third encoder 10 in Figure 5. In setting operation, see Figure 4, a rotary switch SW1, the wiper of which is grounded, has its contacts~1 through 5 connected to be associated with the shutter speeds according to Table I. Assuming that the shutter speed is selected to 1/60 seconds, switch TABLE I shutter contacts terminals speeds of SWl (seconds) el e2 e3 1/15 1 0 0 1 1/30 2 1 1 0 1/60 3 0 1 0 1/125 4 1 0 0 1/250 5 0 0 0 SW1 is set to its contact 3 so as to make output terminals el and e3 ground through diodes D31 and D33, only terminal e2 being provided with the voltage +V through resistor R2.

Hereinafter, a high level output is represented by a "1" state and the grounded level a "0" state. The outputs of terminals e1, e2 and e3 are as shown in Table I by setting rotary switch SW1. Those outputs are applied to an adder circuit At and a latching circuit L1 included in resetting unit 11, which will be descrbribed later. As shown in Figure 2, a variable resistor VR1 in timer 1 is associatedly coupled to rotary switch SW1 in mechanism to make the resistance thereof cooperate with the set shutter speed.

Similarly, third encoder 10 is also arranged in accordance with Table II so as to develop the outputs representing the film sensitivity on terminals eia, e2a and e3a. A variable resistor VR2 in pulse oscillator 4 is mechanically associated with another rotary switch SW2, which is shown in Figure 5.

TABLE II ASA contacts terminals film of SW2 sensitivity eia e2a e3a 400 la 0 0 1 200 2a 1 1 0 100 3a 0 1 0 50 4a 1 0 0 25 5a 0 0 0 The timing unit 1 includes, for example, a conventional monostable multivibrator, from which the duration of the developed gating pulses is associated with time constant T1 of the circuit including a capacitor C1 and variable resistor VR1. Since variable resistor VR1 is thus adjusted associately to the setting operation to shutter speed 1/60 seconds in the second encoder 9, the monostable multivibrator will produce a gating pulse corresponding to the shutter speed i.e. 1/60 second long.

In the circumstances, the actuation of a switch SW1l in timer 1 enables the monostable multivibrator to produce a gating pulse of which the duration is a period of time T1 shown in Figure 3A. The gating pulse is applied to a switch SW12 in subject luminance sensor 2 and a switch SW13 included in V-T converter 3. For example, the switches SW12 and SWl3 may be electronic switches. Switch SWl2 is adapted to be turned ON during pulse duration T1 and switch SW13 turned OFF during the same period. Light sensing unit 2 includes a photo-sensitive device TD, such as a photo-diode, which is supplied with source voltage +V to receive the light dependently upon subject luminance to charge a capacitor C0 along the curve shown in Figure 3B. The charging will stop at the time t2 when switch SW12 is turned OFF. At time t2, the switch SW,3 is turned ON so enabling V-T converter 3 which is generally composed of a conventional constant current circuit In and a conventional comparator Cop. The charge accumulated in capacitor C0 in light sensor 2 is discharged through constant current circuit In along the curve shown in Figure 3C.

The comparator Cop is adapted to detect when capacitor C0 is fully discharged to develop the gating pulse on its output port 51 shown in Figure 3D, whose pulse duration is a period of time T2. This gating pulse is proportional in length to the charge in capacitor CO since comparator Cop has one of its input ports 52 grounded. The gating pulse is fed to pulse oscillator 4.

The pulse oscillator 4 includes a prior art astable multivibrator of which the oscillation frequency depends upon the time constant determined by a capacitor C2 and a variable resistor VR2, which Is associated with third encoder 10 in setting the film sensitivity, as discussed hereinbefore. For example, variable resistor VR2 is adjusted to cause pulses to be generated corresponding to film sensitivity ASA 100. The period for which the pulse oscillator 4 operates is determined by the gating pulse from V-T converter 3 shown in Figure 3D. The pulse oscillator 4 is thus energized during the time interval T2 to produce the pulses, of which the total number depends on the length of the interval, as shown in Figure 3E. In the following it is assumed that the oscillating pulses invoves 32 pulses.

The above-stated operation in which the pulses from pulse oscillator 4 are counted is nothing but the estimation of the amount Bv -Tv +Sv under the APEX system, where Tv is a shutter speed component, Sv a film sensitivity component, Bv the brightness of a subject.

Thirty-two pulses from pulse oscillator 4 are applied to counter 5 to be counted in a binary coded fashion. The counter 5, which may preferably be a 14-BIT BINARY COUNTER (Model MC-14020) manufactured by Motorola Corporation, counts its input pulses in a binary fashion to develop an output from its output port. Examples of the output signal pattern are shown in Table III, in which the application of 32 pulses described above makes the fifth output port 05 enabled to develop a "1" output signal, which is in turn applied to encoder 6.

TABLE III the number of output port input pulses Ot 2 Q 4 Q 6 Q 31 1 1 1 1 0 0 32 0 0 0 0 1 0 33 1 0 0 0 1 0 The first encoder 6 is responsive only to the most significant bits of the signals applied to the input port thereof to develop the signals on the output ports thereof. For example, an 8-BIT PRIORITY ENCODER (Model MC-14532) manufactured by Motorola Corporation may preferably be used as the encoding unit. Examples of the output response thereof are shown in Table IV.

TABLE IV input ports output ports I1 12 I3 I4 I5 I6 I7 Is P1 P2 P3 P4 P5 XXX Xl 0 0 0 0 0 1 0 0 XXX XX 1 0 0 1 0 1 0 0 XXX XXX 1 0 0 1 1 0 0 Note: X represents a don't care state Thus, for example, output signals 000010.... are applied from counter 5 to encoder 6 to develop binary signals 10100 on its output ports. These binary signals are in turn provided to the input ports of first adder 7 and added to the signals provided to the other input ports thereof from second adder 8. The output from adder 7 is in turn applied to counter 13. The added 7 may preferably be a 4-BIT FULL-ADDER (Model MC-14008) produced by Motorola Corporation, and adder 8 may be the same model.

Assuming that the outputs of second adder 8 are all "0", second adder 7 develops signals 10100.... on its output ports. It will be explained why the output signals of second adder 8 are all "0". As will be noted below, it is due to the operation of first and second resetting units 11 and 12. The first resetting unit 11 includes an adder A1, a latching circuit L1 and integrated circuit gates, and second resetting unit 12 includes an adder A2, a latching circuit L2 and gates. The adders A1 and A2 may be type MC-14008 made by Motorola Corporation and the latching circuits L1 and L2 QUAD LATCHES (Model MC-14042) made by the same corporation.

In the illustrated embodiment, it is assumed that the shutter speed is set in second encoder 9 to 1/60 seconds and the film sensitivity is set in third encoder 10 to ASA 100.

According to Tables I and II, output ports e1, e2 and e3 of second encoder 9 provide signals 010 and output ports e1a, e2a and e3a of third encoder 10 provide signals 010 also. Both 010 signals are applied to latching circuits L1 and L2 as well as to one of the input ports of the respective adder A1 and A2. Therefore, the same signal 010 appears on output leads c1, c2 and C3 of latching circuit L1 and output lads d1, d2 and d3 of latching circuit L2. Those signals are both inverted through gates into signals 101, which are in turn applied to the other input ports of respective adders A1 and A2. The signals 101 inverted through the gates and the signals 010 appearing on output ports e1, e2 and e3 of second encoder 9 and ports eia, e2a and e3a of third encoder 10 are mutually added in adders A1 and A2 to be developed in the form of signal 111.

On the other hand, as shown in Figure 3G, the terminal g of timer 1 provides a HIGH level signal, or logical "1", at the time point t2. This HIGH level signal is applied to latching circuits L1 and L2 as well as adders A1 and A2. In response to the signal, latching circuits L and L, keep from former outputs on terminals c1, c2 and C3, and d1, d2 and d3, respectively, for example/60 seconds and ASA 100, even if a new shutter speed and a new film sensitivity are set in second and third encoders 9 and 10, that is, the same output signals 010 are maintained. When addersA1 and A2 receive the "1" signals, which are are added to the above indicated signal 111, the adders A1 and A2 produce signals 000, respectively. Thus, second adder 8, which is provided with the output signals 000 from the respective adders A1 and A2, will in turn develop binary signals 000 in their respective output ports.

The output signals 10100... from first adder 7 are then transferred to storage 13, which selects, in response to the signals applied thereto, one of the memory locations thereof which is associated with the suitable stop number to produce the signal corresponding to the location thus selected. As the storage 13, a 1024-BIT READ ONLY MEMORY (Model MC-14524) manufactured by Motorola Corporation may be used. In accordance with the example shown in Table V, the application of signals 10100... from first adder 7 would cause location No. 5 to be selected to produce signals 00101... on its output port Q1 through Q8. These signals are associated with the selected location and applied to indicator 14.

Thus, indicator 14 receives the signals to display digitally the proper stop number, for example, F 4.0, in its illuminating devices, such as light emitting diodes (LED).

TABLE V locations output ports F Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Qg 3 2.0 0 1 0 0 1 0 0 0 0 4 2.8 0 1 0 0 1 0 0 0 1 5 4.0 0 0 1 0 1 0 0 0 0 6 5.6 0 1 0 0 0 0 0 0 0 As discussed above, a suitable stop number for the measured subject luminance is determined with the shutter speed and film sensitivity set in second and third encoders 9 and 10, respectively, taken into account, and the determined value is indicated in a digital form. If another stop value for a newly set shutter speed and/or a newly set film sensitivity is required, only setting the new data in second and third encoders 9 and 10 permits the suitable stop number to be displayed in indicator 14 without measuring the luminance of the subject again. This is achieved by modifying the stop number which has been obtained in the immediately prior measurement of the subject luminance, for the new shutter speed and film sensitivity set in the encoders by the amount which is estimated in second adder 8. The estimated difference in stop value is in turn applied to first adder 7.

For the purpose of explanation, it is assumed that a subject luminance was measured with the shutter speed set to 1/60 seconds to indicate its appropriate stop number and thereafter the shutter speed is changed to 1/30 seconds. While the value 1/60 seconds is set, the second encoder 9 produces signals 010 on its output ports in accordance with Table I. When the shutter speed is changed to 1/30 seconds in encoder 9, the encoder will develop signals 110.

Since the signals 010 associated with 1/60 seconds are still held on its output terminals in latching circuit L1, as described above, adder A1 addes in effect both signals 110 and 101 to each other in response to the application of a "1" signal provided from timer 1 on its terminal g. Then, signals 100 appear on the output terminals of adder A1.

In this situation, if the setting of third encoder 10 is not changed, that is, the film sensitivity is the same, the signals 100 from resetting unit 11 are added to the signals 000 from adder A2 through second adder 8, of which the output is added to the signals from first encoder 6 through first adder 7. The resultant signals, which are in turn applied to storage 13, permit the memory location corresponding to the signals 0100...., for example No. 6, to be selected, so that the corresponding stop value will be indicated in the indicator 14.

Similarly, a new setting of the film sensitivity in encoder 10 leads to displaying its suitable stop number in the indicator. If both shutter speed and film sensitivity are concurrently changed in the encoders, an appropriate value of the aperture, such as F5.6, will be derived.

In the digital exposure meter in accordance with the present invention, as discussed above, once a subject field has been measured in luminance, a suitable stop number can be obtained with the single measurement even if the shutter speed and/or the film sensitivity are changed.

As far as the output signals from counter 5 are not reset, the repeated actuation of switch SW11 in timer 1 permits the above operations to be repeated so that pulse oscillator 4 oscillates. Superimposed counting is performed in counter 5 to enable a multiple i.e.

cumulative measurement. For example, assume that with an electronic flash unit in which the flash is effected by discharging the charge accumulated in its main discharging capacitor through flash bulb, on a trial operation of the flash unit, the meter indicates the stop value as being F1.4. If the maximum aperture of the lens is F2.0, to achieve the desired exposure one may operate the flash unit a number of times (twice in the case of an indicated required aperture of F1.4). The illustrated meter, being capable of indicating the cumulative exposure over a number of flashes, can indicate the required aperture to produce a given exposure with a number of flashes one after the other. This is achieved by not resetting counter 5, between the multiple flashes.

For achieving the above mentioned operation, a suitable flash unit is connected to the switch SW1l included in the timer circuit 1 for receiving a trigger signal. When the switch SW1l is turned on, a triggering signal is applied to the flash unit so that the flash bulb of the ash unit is energized. The luminance sensor 2 senses the brightness of the subject to be photographed for a predetermined period of time defined by the output signal for the timer rdzcuit 1. Since the light emitted from the flash bulb is given to the subject, the brightness of he subject is much higher than before and thus the luminance sensor 2 produces an output signal indicative of the brightness of the subject. Accordingly, the counter 5 counts the number of pulses applied from the pulse oscillator 4, where the number of the pulses is in proporation to the brightness of the sensed subject. Therefore, the indicator 14 displays a stop number to be used in a camera when the flash bulb is energized once.

If the stop number obtained by the exposure meter is smaller than the minimum value of a camera to be used, the operator turns on the switch SW1l again to sense the brightness of the subject again while the flash bulb is energized again in the same manner. The pulse oscillator 4 produces a pulse train signal in the same manner as the first measurement and the pulses are applied to the counter 5. Since the number of pulses applied to the counter 5 in the first measurement is stored in the counter 5, the counter produces an output signal by adding the second number of pulses produced upon the second measurement to the first number. This means that the output signal of the counter 5 indicates the sum of the brightness of the first and second measurements. Accordingly, the indicator 14 displays a stop number which is larger than before.

If the stop number obtained in the second measurement is within the range of the stop numbers of the camera, the operator will ascertain that the flash bulb should be energized twice. However, if the stop number obtained in the second measurement is still smaller than the minimum stop number of the camera, the operator continues the measurement of the brightness in the same manner, turning on the switch SW11 to obtain a suitable stop number. With this operation, the operator may ascertain how many times the flash bulb should be energized to obtain adequate brightness.

WHAT WE CLAIM IS: 1. A digital exposure meter comprising: (a) timer means for producing a first gating signal whose duration is indicative of a shutter speed which is presettable, said timer means being arranged to produce said first signal only when a switch connected thereto is turned on; (b) light sensor means connected to said timer means for measuring the brightness of a subject to be photographed and integrating the resulting signal for the duration of the first gating signal to provide a second gating signal; (c) pulse oscillator means operatively connected to said light sensor means for producing a train of pulses for the duration of the second gating signal, the frequency of said pulses being presettable in accordance with film sensitivity; (d) counter means connected to said pulse oscillator means for counting the number of pulses applied thereto; (e) a first encoder for producing a first binary signal indicative of the pulse count; (f) a second encoder for producing a second binary signal indicative of the present shutter speed; (g). a third encoder for producing a third binary signal indicative of the present film sensitivity; (h) first resetting means connected to said second encoder and to said timer means for resetting the second binary signal in the absence of said first gating signal by transmitting a fourth binary signal representative of the difference between two values of said second binary signals, one of said values being representative of the present shutter speed and the other of said values being representative of a shutter speed set after the occurrence of said first gating signal; (i) second resetting means connected to said third encoder and to said time means for resetting the third binary signal in the absence of said first gating signal by transmitting a fifth binary signal indicative of the difference between two values of said third binary signals one of which values is representative of the preset film sensitivity and the other of which values is representative of a film sensitivity which is set after the occrrence of said first gating signal; (j) A first adder connected to said first encoder for receiving said first binary signal therefrom; (k) a second adder connected to said first and second resetting means for receiving said fourth and fifth binary signals therefrom for adding the same to each other, said second adder being arranged to transmit a sixth binary signal representative of and corresponding to the sum of said fourth and fifth binary signals to said first adder so that said first adder transmits a seventh binary signal by adding said first and sixth binary signals to each other; (1) storage means for storing a plurality of stop number signals and connected to said first adder for transmitting an appropriate one of said stored stop number signals in accordance with the value of the seventh binary signal; and

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

**WARNING** start of CLMS field may overlap end of DESC **. ash unit is energized. The luminance sensor 2 senses the brightness of the subject to be photographed for a predetermined period of time defined by the output signal for the timer rdzcuit 1. Since the light emitted from the flash bulb is given to the subject, the brightness of he subject is much higher than before and thus the luminance sensor 2 produces an output signal indicative of the brightness of the subject. Accordingly, the counter 5 counts the number of pulses applied from the pulse oscillator 4, where the number of the pulses is in proporation to the brightness of the sensed subject. Therefore, the indicator 14 displays a stop number to be used in a camera when the flash bulb is energized once. If the stop number obtained by the exposure meter is smaller than the minimum value of a camera to be used, the operator turns on the switch SW1l again to sense the brightness of the subject again while the flash bulb is energized again in the same manner. The pulse oscillator 4 produces a pulse train signal in the same manner as the first measurement and the pulses are applied to the counter 5. Since the number of pulses applied to the counter 5 in the first measurement is stored in the counter 5, the counter produces an output signal by adding the second number of pulses produced upon the second measurement to the first number. This means that the output signal of the counter 5 indicates the sum of the brightness of the first and second measurements. Accordingly, the indicator 14 displays a stop number which is larger than before. If the stop number obtained in the second measurement is within the range of the stop numbers of the camera, the operator will ascertain that the flash bulb should be energized twice. However, if the stop number obtained in the second measurement is still smaller than the minimum stop number of the camera, the operator continues the measurement of the brightness in the same manner, turning on the switch SW11 to obtain a suitable stop number. With this operation, the operator may ascertain how many times the flash bulb should be energized to obtain adequate brightness. WHAT WE CLAIM IS:

1. A digital exposure meter comprising: (a) timer means for producing a first gating signal whose duration is indicative of a shutter speed which is presettable, said timer means being arranged to produce said first signal only when a switch connected thereto is turned on; (b) light sensor means connected to said timer means for measuring the brightness of a subject to be photographed and integrating the resulting signal for the duration of the first gating signal to provide a second gating signal; (c) pulse oscillator means operatively connected to said light sensor means for producing a train of pulses for the duration of the second gating signal, the frequency of said pulses being presettable in accordance with film sensitivity; (d) counter means connected to said pulse oscillator means for counting the number of pulses applied thereto; (e) a first encoder for producing a first binary signal indicative of the pulse count; (f) a second encoder for producing a second binary signal indicative of the present shutter speed; (g). a third encoder for producing a third binary signal indicative of the present film sensitivity; (h) first resetting means connected to said second encoder and to said timer means for resetting the second binary signal in the absence of said first gating signal by transmitting a fourth binary signal representative of the difference between two values of said second binary signals, one of said values being representative of the present shutter speed and the other of said values being representative of a shutter speed set after the occurrence of said first gating signal; (i) second resetting means connected to said third encoder and to said time means for resetting the third binary signal in the absence of said first gating signal by transmitting a fifth binary signal indicative of the difference between two values of said third binary signals one of which values is representative of the preset film sensitivity and the other of which values is representative of a film sensitivity which is set after the occrrence of said first gating signal; (j) A first adder connected to said first encoder for receiving said first binary signal therefrom; (k) a second adder connected to said first and second resetting means for receiving said fourth and fifth binary signals therefrom for adding the same to each other, said second adder being arranged to transmit a sixth binary signal representative of and corresponding to the sum of said fourth and fifth binary signals to said first adder so that said first adder transmits a seventh binary signal by adding said first and sixth binary signals to each other; (1) storage means for storing a plurality of stop number signals and connected to said first adder for transmitting an appropriate one of said stored stop number signals in accordance with the value of the seventh binary signal; and

(m) indicator means connected to said storage means for indicating a stop number in response to a signal fed thereto.

2. A digital exposure meter as claimed in claim 1, wherein said timer means comprises a monostable multivibrator the output pulse width of which is variable in accordance with the set shutter speed.

3. A digital exposure meter as claimed in claim 2, wherein said monostable multivibrator comprises a variable resistor determining the time constant thereof, and wherein said second encoder comprises a rotary switch for selecting the value of said second binary signal, means mechanically coupling said rotary switch to said variable resistor so that the time constant of said monostable multivibrator and the value of said second binary signal are set simultaneously.

4. A digital exposure meter as claimed in any one of the preceding claims, wherein said pulse oscillator means comprises an astable multivibrator the output pulse width of which is variable in accordance with the preset film sensitivity.

5.A digital exposure meter as claimed in claim 4, wherein said astable multivibrator comprises a variable resistor determining the time constant thereof, and wherein said third encoder comprises a rotary switch with which the value of said third binary signal is selected, means mechanically coupling said rotary switch to said variable resistor so that the time constant of said astable multivibrator and the value of said third binary signal are set simultaneously.

6. A digital exposure meter as claimed in any one of the preceding claims, wherein said first resetting means comprises: a latch circuit connected to said second encoder and arranged to store said second binary signal until an occurrence of said first signal; logic gate means connected to said latch circuit for inverting the stored second binary signal and transmitting the inverted signal in the absence of said first signal; and adder means connected to said logic gate means and to said second encoder for adding said second binary signal to said inverted signal for producing said fourth binary signal.

7. A digital exposure meter as claimed in any one of the preceding claims, wherein said second resetting means comprises: a latch circuit connected to said third encoder and arranged to store said third binary signal until the occurrence of said first signal; logic gate means connected to the last-mentioned latch circuit for inverting the stored third binary signal and transmitting the inverted signal upon absence of said first signal; and adder means connected to said logic gate means an to said third encoder for adding said third binary signal to said inverted signal for producing said fifth binary signal.

8. A digital exposure meter constructed and arranged substantially as described herein with reference to the accompanying drawings.