GB1569137A - Over-exposure indicator - Google Patents

Description

(54) OVEREXPOSURE INDICATOR (71) We, BALDA-WERKE PHOTO GRAPHISCHE GERATE UND KUNSTSTOFF G.m.b.H. & Co. KG, a Germany Company of 1S20 Steinmeisterstrasse, 498, Bunde 1, Federal Republic of Germany, 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 an overexposure indicator for photographic shutters.

Photographic cameras provided with devices for warning against overexposure are known in the art. However, such devices in the past required the use of separate light-sensitive elements for shutter control and overexposure indication, each element operating in conjunction with its own electronic trigger circuit. In the case of cameras which are equipped with an electronic shutter and a warning device for underexposure, such an arrangement to provide overexposure warning signals results in the use of three trigger circuits and three light-sensitive elements.

In one conventoinal arrangement known in the art, sliding contacts are controlled by the shutter release button so as to switch the various circuits in succession. Thus, in such arrangement the overexposure cir cuit is actuated first, the underexposure cir cuit is actuated second, and the shutter timing is operated next. Such an arrangement using contacts in this manner, how ever, has an inherent low reliability in operation. Furthermore, the four possible positions of the shutter release button give rise to confusion and produce operating problems thereby for the user of the camera.

The present invention is applicable to cameras with electronic shutter timing and an aperture setting means. In such cameras, if the aperture is altered, the electronic timing must be altered correspondingly. This is achieved by altering the aperture in front of the light-sensitive element. The adjustment is carried out by moving a graduated light-reducing filter in front of the light-sensitive element or by adjusting a resistor within a circuit in the camera.

When a user of a camera selects an aperture setting, he may know that the shutter speed will be correspondingly faster or slower to provide correct exposure. He does not, however, know when the fastest shutter speed has been exceeded. Thus he does not know when the aperture is so large under intense light conditions that the shutter cannot adapt.

This is due to the condition that the maximum shutter speed is limited by the mechanical parts.

Obtaining an indication that over-exposure conditions prevail is advantageous to the user of a camera. At the same time, such an indication can be used to establish the largest aperture for reduced depths of field and to determine the fastest shutter speed when photographing moving objects, for example Accordingly, indications of over-exposure conditions that may prevail also serve the purpose of providing substantially increased control over the photographic processes and reducing thereby the amount of photographic failures.

According to the present invention there is provided shutter control and over-exposure indicator apparatus, for use in a photographic camera, comprising an electromagnetic shutter closure timing device, light intensity sensing means, two-state switching means having an output for selectively energising and driving said device to delay or allow shutter dosure depending on the signal level of said output of the switching means, which will correspond to the state thereof, said switching means being responsive, on initial camera operation for shutter release, to a light intensity representative signal from said sensing means to assume one or the other of its states according to whether that signal is above or below a predetermined value related to overexposure conditions, said switching means further being operative to switch between its states only if said light intensity representative signal does not indicate overexposure conditions, such switching occurring after a time interval dependent on said sensing means and serving to close the shutter, and indicator means also connected to the said output of the switching means and serving to indicate when sensed light intensity corresponds to overexposure conditions, preferably prior to actual shutter release.

Such an indicator is operable in conjunction with the mechanical parts of the photographic shutter. In preferred embodiments the light intensity sensing means comprises an electrical resistance which varies as a function of the light intensity impinging thereon. The switching means conveniently comprises a Schmitt trigger circuit and the indicating means may have a transistor operating as a switch to control energisation of indicating means in the form of a lamp.

The switching means may be arranged to control electromagnetic means for controlling shutter operation when the light intensity is satisfactory.

In one preferred embodiment of the invention the light intensity sensing means comprises a photo-resistor connected in series with a fixed resistor and an electrical switch for the purpose of forming a voltage divider. A tap on the voltage divider is connected to the input side of a Schmitt trigger circuit having an output side connected to the coil of a solenoid which attracts an armature when energized.

The output side of the Schmitt trigger circuit is also connected, through a fixed resistor, to the base of a switching transistor. The collector of the transistor is connected to an indicating lamp which lights- when the transistor is in the conducting state.

When the ambient light conditions are satisfactory, the operation of the electrical switch causes the Schmitt trigger circuit to assume an output state in which the coil of the solenoid is energized while the transistor is maintained in the turned-off state.

The energization of the solenoid then allows normal shutter operations to be carried out.

In the event that over-exposure conditions -prevail, the Schmitt trigger circuit does- not assume the state in which the solenoid is energized. Instead, operation of the electrical switch causes the transistor to be turned on to the conducting state and the-indicating lamp lights to provide the user' of the camera with a warning that the shutter has been set for an over-exposure.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings, of which: Figure 1 is a partial schematic view of representative shutter elements which may be used in conjunction with the over-exposure indicator of the present invention; and Figure 2 is an electrical schematic circuit diagram that shows the electronic elements and their interconnections to provide an indication when over-exposure conditions prevail.

Referring to Figure 1 of the drawings, a photographic camera 10 is provided with a shutter release member 11 which is arranged to actuate an electrical switch 12 upon being depressed. The release member 11 is, furthermore, provided with an actuating rod-shaped member 13 which engages one arm 14 of a pawl 15. The latter has a second arm 16 engaging a projecting arm 1 7a of a striker member 17 which has three other projecting arms, 17b, 17c and 17d, spaced clockwise around the member.

In proximity of the projecting arm 1 7c of the striker member 17 are shutter arm 18 and holding arm 19 of a shutter blade 20.

In proximity to the projecting arm 1 7b of the striker member are a coil 21 of a solenoid 22 provided with an armature 23 which is pivotable about a pintle 24. As shown in Figure 1, when the projecting arm 17a of the striker member is engaged by the arm 16 of pawl 15, the projecting arm 17b supports the armature 23 against the end of solenoid core 22a.

In operation of the elements of Figure 1, depressing of the release member 11 causes the pawl 15 to move in a counter-clockwise direction resulting in the release of the striker member 17 from being held by the arm 16 of the pawl. After the pawl 15 has rotated to the extent that the arm 16 no longer engages the respective projecting arm 17a of the striker member 17, a spring 25 attached to arm 17d causes the striker member to rotate in a clockwise direction so that arm 17b no longer supports armature 23. If the coil 21 is not energised at this time the armature 23 will be released from the solenoid core 22a, and will pivot counter-clockwise. If, on the other hand, the coil 21 is energised, the armature 23 remains in the contact position shown in Figure 1.

When the striker member 17 rotates in a clockwise direction upon being released from the holding action of the pawl 15, the projecting arm 17c of the striker member 17 actuates the shutter arm 18 and there by causes actuation of the shutter blade 20 against the action of a spring 27, If the solenoid 22 is energized at that time, the holding arm 19 rides over the slanted or inclined surface of a projecting member 28 (such as a lug) connected to armature 23, and latches behind it, thus holding the shutter blade 20 in the actuated position. Upon the expiration of a predetermined time interval, the solenoid 22 becomes de-ener gized and the armature 23 is released. As previously mentioned, this results in a rotation of the armature 23 and member 28 connected thereto in a counter-clockwise direction. The rotation of armature 23 in this manner results in the release of the holding arm 19 so that the spring 27 may snappingly return the shutter blade 20 to its initial position.

When the striker member 17 is rotated clockwise by spring 25 upon being released from the holding action of pawl 15, a switch 29, previously held closed by projecting arm 17d of the striker member, is also opened.

The electrical switch 12 and 29, as well as the coil 21 of the solenoid, are interconnected in accordance with the circuit diagram of Fig. 2. As shown in this circuit diagram, switch 12 serves to connect the circuit to a power source having positive and negative terminals 30, 31, such as a battery. Connected across the power source when switch 12 is closed is a photoresistor 32 connected in series with the switch 29 and a fixed resistor 33. A capacitor 34 is connected in parallel with the series combination of the switch 29 and the fixed resistor 33. The photo-resistor and fixed resistor 33 form a voltage divider with a tap 35 connected to the input of a trigger circuit 36 which may, for example, be in the form of a Schmitt trigger. The output of the Schmitt trigger circuit is connected to the coil 21 which becomes energibed when the trigger circuit has switched states to make the output 37 at its low potential level.

The output 37 of the Schmitt trigger 36 is also connected, through a resistor 38, to the base of a transistor 39. The collector of the transistor 39 is connected to an indicating lamp 40 which lights when the transistor is in the conducting state. The emitter of the transistor 39 is connected to the negative terminal 31 of the power source.

In operation of the circuit of Fig. 2, the closure of switch 12 by the release button 11 causes a potential to appear at the tap 35 of the voltage divider 32, 33. The fixed resistor 33 has a magnitude selected so that, when satisfactory ambient light conditions prevail, the potential at tap 35 has a magnitude which will cause the Schmitt trigger to assume a first state in which its output, line 37, is low. Under those light conditions the photo-resistor 32 assumes a resisttance within a predetermined range of resistance values and thereby provides corresponding voltage drops within that range.

Accordingly, the magnitude of resistor 33 can be selected relative to the range of values of the photo-resistor 32 corresponding to satisfactory light conditions so as to provide a potential at the junction or tap 35 which will result in the desired state of the trigger circuit 36.

With the trigger circuit 36 in that state, the coil 21 of the solenoid becomes energized as a result of the output 37 being low. At the same time this low potential on the output 37 of the trigger circuit 36 holds the transistor 39 in the turn-off state.

The actuation of the release button 11, however, also causes counter-clockwise rotation of the pawl 15, as above described, and upon release of the striker member 17, the switch 29 becomes opened.

When switch 29 opens, the capacitor 34 commences to be charged. The capacitance value of the capacitor 34 is selected so that, as it charges, the voltage potential at the tap 35 reaches a sufficiently high level to cause the trigger circuit 36 to switch states so that its output 37 is then at its high potential level. Upon such switching of states of the trigger circuit 36, the low potential on the output 37 is removed and the solenoid 22 is thereby de-energized. As a result, the armature 23 becomes released and the lug 28 connected thereto is rotated so as to also release the holding arm 19 of the shutter blade 20. The latter is thereby returned to its initial position by the spring 27. The maximum shutter speed is attained when the Schmitt trigger circuit is in the state in which the solenoid is de-energised before the shutter blade is fully opened so as to allow the blade to close immediately.

The resistance of the photo-resistor 32 varies with the light intensity to which the resistor is exposed and thus controls the charge rate of capacitor 34. Thus, the resistance value of the photo-resistor varies inversely with the impinging light intensity. When the ambient light is of an intensity which would result in an overexposure condition, the resistance of the photoresist or 32 is so low that the voltage potential on tap 35 is positive to the extent that trigger circuit 36 immediately assumes the state in which the potential on line 37 is high on closure of switch 12 by the release button 11. As a result, the coil 21 of the solenoid does not become energized. However, under those conditions, the base of the transistor 39 is sufficiently positive, on closure of switch 12, for the transistor 39 to conduct, whereupon the lamp 40 lights and indicates the overexposure condition.

Thus, lamp 40, in conjunction with the single transistor 39 which is operated by the same trigger circuit that operates the solenoid 22, provides an indication, and hence a warning, of the overexposure setting.

It will be further appreciated that the invention makes it possible readily to establish the largest practicable shutter aperture for a given or reduced field depth and thus to determine the fastest satisfactory shutter speed, as is required when moving objects are to be photographed. In this way, moreover, greater control over operating conditions is provided with diminished risk of failure.

WHAT WE CLAIM IS: 1. Shutter control and over-exposure indicator apparatus, for use in a photographic camera, comprising an electromagnetic shutter closure timing device, light intensity sensing means, two-state switching means having an output for selectively energising and driving said device to delay or allow shutter closure depending on the signal level of said output of the switching means, which will correspond to the state thereof, said switching means being responsive, on initial camera operation for shutter release, to a light intensity representative signal from said sensing means to assume one or the other of its states according to whether that signal is above or below a predetermined value related to overexposure conditions, said switching means further being operative to switch between its states only if said light intensity representative signal does not indicate overexposure conditions, such switching occurring after a time interval dependent on said sensing means and serving to close the shutter, and indicator means also connected to the said output of the switching means and serving to indicate when sensed light intensity corresponds to overexposure conditions.

2. Apparatus according to claim 1, wherein the indicator means is operable prior to actual shutter release.

3. Apparatus according to claim 1 or claim 2, wherein the light intensity sensing means includes an electrical resistance which varies as a function of the light intensity impinging thereon, 4. Apparatus according to claim 1, 2 or 3, wherein the switching means is a Schmitt trigger circuit.

5. Apparatus according to any preceding claim, wherein a transistor is connected to said switching means, and the indicating means comprises lamp means connected to the transistor, said lamp means being lighted upon the transistor becoming conductive.

6. Apparatus according to claim 5, wherein the transistor is rendered conductive and lights the indicating lamp when the switching means is in that state corresponding to the light intensity sensed being above said predetermined value, and is inhibited from being in the conducting state when the switching means is in the other of two said states.

7. Apparatus according to any preceding claim, which further comprises capacitor means connected to the light intensity sensing means for applying a signal to the switching means to cause the switching means to change from the state it has assumed when the light intensity sensed is below said predetermined value.

8. Apparatus according to any preceding claim, wherein the light intensity sensing means comprises a photo-resistor having a resistance varying inversely with the intensity of light impinging thereon, and a fixed resistor connected in series with the photo-resistor and forming a voltage divider therewith, the junction of the photo-resistor and the fixed resistor being connected to the input of the switching means.

9. A shutter control and overexposure indicator for use in photographic cameras substantially as herein described with reference to and as shown in the accompanying drawings.

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

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. Thus, lamp 40, in conjunction with the single transistor 39 which is operated by the same trigger circuit that operates the solenoid 22, provides an indication, and hence a warning, of the overexposure setting. It will be further appreciated that the invention makes it possible readily to establish the largest practicable shutter aperture for a given or reduced field depth and thus to determine the fastest satisfactory shutter speed, as is required when moving objects are to be photographed. In this way, moreover, greater control over operating conditions is provided with diminished risk of failure. WHAT WE CLAIM IS:

1. Shutter control and over-exposure indicator apparatus, for use in a photographic camera, comprising an electromagnetic shutter closure timing device, light intensity sensing means, two-state switching means having an output for selectively energising and driving said device to delay or allow shutter closure depending on the signal level of said output of the switching means, which will correspond to the state thereof, said switching means being responsive, on initial camera operation for shutter release, to a light intensity representative signal from said sensing means to assume one or the other of its states according to whether that signal is above or below a predetermined value related to overexposure conditions, said switching means further being operative to switch between its states only if said light intensity representative signal does not indicate overexposure conditions, such switching occurring after a time interval dependent on said sensing means and serving to close the shutter, and indicator means also connected to the said output of the switching means and serving to indicate when sensed light intensity corresponds to overexposure conditions.

2. Apparatus according to claim 1, wherein the indicator means is operable prior to actual shutter release.

3. Apparatus according to claim 1 or claim 2, wherein the light intensity sensing means includes an electrical resistance which varies as a function of the light intensity impinging thereon,

4. Apparatus according to claim 1, 2 or 3, wherein the switching means is a Schmitt trigger circuit.

5. Apparatus according to any preceding claim, wherein a transistor is connected to said switching means, and the indicating means comprises lamp means connected to the transistor, said lamp means being lighted upon the transistor becoming conductive.

6. Apparatus according to claim 5, wherein the transistor is rendered conductive and lights the indicating lamp when the switching means is in that state corresponding to the light intensity sensed being above said predetermined value, and is inhibited from being in the conducting state when the switching means is in the other of two said states.

7. Apparatus according to any preceding claim, which further comprises capacitor means connected to the light intensity sensing means for applying a signal to the switching means to cause the switching means to change from the state it has assumed when the light intensity sensed is below said predetermined value.

8. Apparatus according to any preceding claim, wherein the light intensity sensing means comprises a photo-resistor having a resistance varying inversely with the intensity of light impinging thereon, and a fixed resistor connected in series with the photo-resistor and forming a voltage divider therewith, the junction of the photo-resistor and the fixed resistor being connected to the input of the switching means.

9. A shutter control and overexposure indicator for use in photographic cameras substantially as herein described with reference to and as shown in the accompanying drawings.