GB2123570A

GB2123570A - Luminance-responsive photographic flash device

Info

Publication number: GB2123570A
Application number: GB08318293A
Authority: GB
Inventors: Yoshiyuki Takematsu
Original assignee: FUJI KEOKI KK; TAKEMATSU Y
Current assignee: FUJI KEOKI KK; TAKEMATSU Y
Priority date: 1982-07-13
Filing date: 1983-07-06
Publication date: 1984-02-01
Also published as: GB8318293D0; IT1197673B; IT8348662A0; JPS5912135U; FR2530353A1; DE3322208A1

Abstract

Flashlight emission from flash tube 10 is inhibited if the luminance detected by photocell 20 is above a preset value, said value being adjustable. The flash tube (10) has a discharge capacitor (9) supplied from a DC-DC converter having an oscillating transformer (3) and transistor (4). The flash is initiated by a trigger circuit (14, 15, 16, 17). A circuit (19) has the photocell (20) and a comparator (24) connected to the base of transistor (4) to inhibit the DC-DC converter and prevent flashes if the luminance of an object to be photographed is above a preset value. The preset value can be adjusted by resistors (21, 22, 23) or by a filter or diaphragm (26), so that the threshold at which flash emission is prevented can be altered, depending on optical factors such as the relative aperture of the objective of the camera, or the film sensitivity. Instead of acting on the DC-DC converter, the circuit (19) can control the trigger circuit (14, 15, 16, 17). <IMAGE>

Description

SPECIFICATION Luminance-responsive photographic flash device The present invention relates to a photographic flash device adapted to allow or prohibit flash emission in response to the luminance of an object to be photographed.

There has already been proposed a photographic flash device including a responsive circuit adapted to detect the luminance of an object to be photographed and to provide an output varying in accordance with such luminance level. This well known flash device is arranged to allow flash emission in response to the change appearing in the output of said responsive circuit when the luminance level of an object to be photographed is higher than a predetermined level and to prohibit flash emission in response to the change appearing in the output of said responsive circuit when a luminance level of an object to be photographed is lower than the predetermined level.However, the changeover of said responsive circuit in this known device depends only upon a single factor, i.e., the luminance of the object to be photographed, and in consequence, the device has had the following disadvantages: (1) Although in practice the relative aperture of the objective varies from camera to camera, flash emission occurs whenever the luminance of the object is detected to be lower than a predetermined level, irrespective of whether the relative aperture of the objective mounted on the camera equipped with the flash device is high or low, and this sometimes causes inappropriately exposed flash light photographs.

(2) Flash emission occurring without exception whenever the luminance of the object is detected to be lower than the predetermined level also causes inappropriately exposed flash light photographs, depending on the film sensitivity, since the exposure condition is governed also by the sensitivity of the film actually loaded into the camera.

(3) Flash emission is not released when the luminance of the object is higher than the predetermined level in this well known flash device. Accordingly, such prior art flash device has been inconvenient in taking a picture against the light.

According to the present invention, there is provided a flash device adapted to be activated in response to the luminance of an object to be photographed, said flash device comprising a responsive circuit adapted to detect the luminance of an object to be photographed and to be changed over from a first state to a second state when the level of the luminance thus detected is higher than a present value, a flash emission control circuit adapted to be activated in response to the output of said responsive circuit to allow flash emission when said responsive circuit is in the first state and to prohibit flash emission when said responsive circuit is in the second state, and means for altering said preset value of said responsive circuit in accordance with optical factors such as the relative aperture of an objective or film sensitivity.

With a preferred arrangement according to the present invention, the preset value at which the state of the responsive circuit changes over can be adjusted depending on various factors such as the relative aperture of the objective and the film sensitivity, so that the flash emission can be enabled or prohibited depending not only upon the luminance level of the object to be photographed but also upon the other optical factors as above mentioned. Thus, the previously mentioned disadvantages (1), (2) and (3) are effectively overcome with such a preferred arrangement.

Preferred embodiments of the present invention will be now described with reference to the accompanying drawings, wherein: Fig. 1 is a circuit diagram illustrating one flash device according to the present invention; Fig. 2 is a rear view of said flash device illustrating means for presetting the responsive circuit included in said flash device; Fig. 3 is a partial rear view of said flash device illustrating another embodiment of said means for presetting the responsive circuit; and Figs. 4 and 5 are circuit diagrams illustrating other embodiments of flash devices constructed according to the present invention.

Referring to Fig. 1 which is a circuit diagram of a photographic flash device constructed according to the present invention, reference numeral 1 designates a battery source, reference numeral 2 designates a source switch, reference numeral 3 designates an oscillatory transformer and reference numeral 4 designates a switching transistor, these respective members constituting together a well known kind of DC-DC converter.

Oscillation of this DC-DC converter is initiated upon closure of the source switch 2. More specifically, upon closure of said source switch 2, the transistor 4 is supplied through a protective resistance 5 with a base current by which said transistor 4 is turned ON and thus the oscillation is initiated. Then the transistor 4 is repeatedly turned ON and OFF under magnetic saturation of the transformer core and thereby the oscillation is continued. A capacitor 6 connected across the base and the emitter of said transistor 4 functions to stabalize such oscillation.

A capacitor 7 and a diode 8 both connected to the output of the DC-DC converter together form a rectifier, to which a main discharge capacitor 9 and a flash discharge tube 10 are connected, in parallel with each other.

A series circuit consisting of a diode 11, a current limiting resistance 1 2 and a neon lamp 1 3 connected also to the output of the DC-DC converter serves as a charge level display circuit adapted to provide a lighting display when the main discharge capacitor 9 has been charged to a level higher than a predetermined voltage value. A capacitor 1 5 connected by a charge resistance 14 in parallel to said current limiting resistance 12 and said neon lamp 1 3 serves as a trigger capacitor.

The trigger capacitor 1 5 forms, together with a trigger transformer 1 6 and a trigger switch 17, a trigger circuit which applies an excitation voltage to the flash discharge tube 10 in the well known manner.

A resistance 1 8 connected in parallel to said charge resistance 14 and said trigger capacitor 1 5 serves as a discharge resistance adapted to cause electric charge of the trigger capacitor 1 5 to be discharged and thereby to prevent the trigger circuit from being activated when said DC-DC converter stops oscillating.

A circuit section 1 9 enclosed by two-dot-chainline serves as a responsive circuit to detect the luminance level of an object to be photographed and comprises a cds photocell 20 and a resistance 21 in series, resistances 22,23 in series and a comparator 24. The comparator 24 receives voltage signals appearing at nodes a, b, in the respective series groups.

When the responsive circuit 19 detects a luminance level of an object to be photographed higher than a predetermined level, the output from the comparator 24 becomes LOW, and is applied through the protective resistance 25 to the base of the transistor 4 which is thereby kept OFF. When the luminance of an object to be photographed is lower than a predetermined level, on the contrary, the comparator 24 now provides a HIGH output with which the transistor 4 is turned ON.

In consequence, said responsive circuit 1 9 stops the oscillation of the DC-DC converter when the object to be photographed has the luminance higher than the predetermined level and allows said converter to continue the oscillation when the object to be photographed has a luminance lower than said predetermined level.

Said responsive circuit 1 9 is so arranged that one of the resistances 21 through 23 is manually adjusted depending upon a relative aperture of an objective actually mounted on the camera, expressed by, for example, F 1.4, F 2, etc. In this manner, the output from the comparatoE4 depends not only on the luminance of the object to be photographed but also on the relative aperture of the objective, and thus the DC-DC converter is allowed or prohibited to oscillate under conditions including also said relative aperture of the objective.

Similarly, one of said resistances 21 through 23 may be adjusted depending on the sensitivity of film actually loaded in the camera so as to control the oscillation of the DC-DC converter under conditions including also the film sensitivity.

Further, an arrangement is also conceivable in which said resistances 21 through 23 are optionally adjustable so that the DC-DC converter may oscillate even when a luminance of an object to be photographed is relatively high and a flash discharge may occur in taking a picture against the light.

It is also possible to take the relative aperture of the objective and the film sensitivity into account through the resistance 21 and the resistance 22, respectively.

Still another arrangement is possible in which the relative aperture of the objective or the film sensitivity may be taken into account through a member 26 placed in front of the cds 20 to limit a quantity of light to be received. Said member 26 may comprise well known filters of various shades or well known diaphragms having a plurality of perforations of various diameters. Adopting such member 26 for limiting the light quantity to be received, an arrangement is possible in which the film sensitivity may be taken into account through said member 26 and the relative aperture of the objective may be taken into account through one of said resistances 21 to 23.

Fig. 2 is a rear view of the flash device according to the present invention, showing manual operation knobs operatively associated with said resistances 21 through 23 or said member 26 for limiting the light quantity to be received. A knob 27 for the light value LV is operatively associated with the resistance 21 and a knob 28 for the film sensitivity is operatively associated with the resistance 22. These knobs 27, 28 are slidably movable along respective graduation lines.

When the device is so arranged that one of the relative aperture of the objective and the film sensitivity is taken into account, said knobs 27, 28 may be operatively associated with each other to adjust one of said resistances 21 through 23, or only the knob 27 may be provided. Similar operative association is possible for the member 26 for limiting the light quantity to be received.

Said knob 27 may be replaced by a knob 29 for the diaphragm as shown by Fig. 3 and said knobs 27 through 29 may be arranged on a suitable position other than the rear side of the flash device.

In the embodiment of the flash device constructed according to the present invention as has been described hereinabove, the responsive circuit 1 9 functions to hold the transistor 4 OFF as long as a luminance of an object to be photographed is higher than a predetermined level and, in consequence, even closure of the source switch 2 causes no oscillation of the DC-DC converter. Accordingly, both the main discharge capacitor 9 and the trigger capacitor 1 5 are not charged and no flash discharge occurs.

For a luminance of an object to be photographed lower than the predetermined level, the comparator 24 of the responsive circuit 1 9 provide a HIGH output sufficient to turn the transistor 4 ON and oscillation of the DC-DC converter is initiated upon closure of the source switch 2. The respective capacitors 9, 1 5 are thereby charged and the flash discharge tube 10 thus emits a flash upon closure of the trigger switch 1 7.

If there is a sudden increase in the luminance of the object to be photographed, which is encountered, for example, if an outdoor photograph is to be taken immediately after an indoor photograph by flash light, both the main discharge capacitor 9 and the trigger capacitor 1 5 are certainly in their charged states when the DC-DC converter stops oscillating, but the charge in said trigger capacitor 1 5 is now discharged through the discharge resistance 1 8 and even closure of the trigger switch 1 7 causes no activation of the trigger circuit. Thus, the flash discharge tube 10 emits no flash.

In such a case, the current is prevented by the diode 8 from flowing from the main discharge capacitor 9 into the trigger capacitor 1 5.

Fig. 4 shows another embodiment of flash device constructed according to the present invention, in which a transistor 30 connected to the base of the transistor 4 is controlled by the responsive circuit 1 9 and arrangement of the rest of the elements is similar to the arrangement in the circuit of Fig. 1.

In this embodiment, the transistor 30 has applied to its base the LOW output from the comparator 24 and is turned OFF when the luminance of an object to be photographed is higher than the predetermined level, so that the transistor 4 is held OFF and the DC-DC converter is prevented from oscillating even when the source switch 2 is closed. For an object to be photographed which has a luminance lower than the predetermined level, the comparator 24 provides the HIGH output sufficient to turn the transistor 30 ON and the transistor 4 can be repeatedly turned ON and OFF. Thus, the DC-DC converter is released to oscillate upon closure of the source switch 2.

Fig. 5 shows still another embodiment of the flash device constructed according to the present invention in which the trigger circuit is disabled as the output of the responsive circuit 1 9 varies and thereby a flash emission is prohibited. It should be understood that the rest of the elements of the circuit are arranged in the same manner as in the embodiment of Fig. 1. In the embodiment shown by Fig, 5, a PNP transistor 31 and the serially connected current limiting resistance 12 and neon lamp 13 are respectively connected in parallel to the trigger capacitor 1 5, with said transistor 31 being controlled by the output from the comparator of the responsive circuit 1 9.

In this way, the LOW output provided from the comparator 24 when the luminance of an object to be photographed is higher than the predetermined level turns the transistor 31 ON and simultaneously the electric charge in the trigger capacitor 1 5 is discharged through this transistor 31 and, in consequence, the neon lamp 1 3 is put out. Accordingly, the flash emission of the flash discharge tube 10 cannot be initiated even when the trigger switch 1 7 is closed, since the trigger circuit is not activated. When the luminance of the object to be photographed is lower than the predetermined level, the comparator 24 provides the HIGH output and the transistor 31 is, therefore, held OFF. The flash emission of the flash discharge tube 10 is thus enabled when the trigger switch 1 7 is closed.

As will be obvious from the aforegoing description, the responsive circuit 1 9 adapted to detect the luminance level of an object to be photographed provides an output depending upon various factors such as the relative aperture of the objective and the film sensitivity, so that the luminance level of the object to be photographed can be detected in connection with the particular relative aperture of the objective actually mounted on the camera and with the particular sensitivity oi film actually loaded into the camera. The flash discharge tube is then enabled to initiate flash emission on the basis of the luminance level thus detected.

Claims

1. A flash device adapted to be activated in response to the luminance of an object to be photographed, said flash device comprising a responsive circuit adapted to detect the luminance of an object to be photographed and to be changed over from a first state to a second state when the level of the luminance thus detected is higher than a preset value, a flash emission control circuit adapted to be activated in response to the output of said responsive circuit to allow flash emission when said responsive circuit is in the first state and to prohibit flash emission when said responsive circuit is in the second state, and means for altering said preset value of said responsive circuit in accordance with optical factors such as the relative aperture of an objective or film sensitivity.

2. A flash device according to claim 1 wherein said means for altering said preset value includes a manually adjustable knob.

3. A flash device according to claim 2 wherein said means for altering said preset value includes a plurality of manually adjustable knobs.

4. A flash device according to any one of the preceding claims wherein the output of the responsive circuit is connected to enable or prevent oscillation of a DC-DC converter which is arranged to supply power for the emission of a flash.

5. A flash device according to any one of claims 1 to 3 wherein the output of the responsive circuit is connected to enable or prevent activation of a trigger circuit which is arranged to initate flash emission.

6. A flash device substantially as any described herein with reference to the accompanying drawings.