GB2295239A - Exposure control apparatus of a camera - Google Patents

Description

EXPOSURE CONTROL APPARATUS OF A CAMERA 2295239 The present invention

relates to an exposure control apparatus of a camera in which exposure can be controlled when a strobe (i.e., electronic flash) is used. In particular, the invention relates -to a minimum diaphragm value which can be set in accordance with film sensitivity data and object distance data. More precisely, the present invention relates to an exposure control apparatus in which a program shift can be carried out in a program exposure mode when the strobe is used.

An exposure control apparatus is known which has a program shifting function in which, in the program exposure mode, a combination of the shutter speed and the diaphragm value can be varied without changing the exposure value. In the program shift operation, the shutter speed Tv and the diaphragm value Av are first obtained, and then the obtained values are shifted by a predetermined amount. For example, the program shift is performed in accordance with the following equation; Ev=(Av + "shift")+ (Tv - "shift") Upon taking a photograph with a strobe, the shutter speed has an upper limit defined by a strobe synchronization speed, and a lower limit defined by a camera-shake limit speed which prevents the occurrence of camera-shake. In practice, a minimum limit diaphragm value is set at a value slightly greater than the minimum diaphragm value (on an open aperture side) and a maximum limit diaphragm value is set at a value slightly less than the maximum diaphragm value (on a stop-down side), in view of the characteristics of the associated camera and the photographable object distance thereof, etc. Consequently, when the strobe is used in the program exposure mode, in which a relationship between the shutter speed and the diaphragm value is programmed in advance, the ranges within which the shutter speed and diaphragm values can be set are restricted.

If the calculated exposure value is below a lower limit value which is defined by the camera-shake limit shutter speed and below the minimum limit diaphragm value, the shutter speed and the diaphragm value are set at the camera-shake limit shutter speed and the minimum limit diaphragm value, respectively. If the calculated exposure value is above an upper limit value which is defined by the strobe synchronization limit speed and above the maximum limit diaphragm value, the shutter speed and the diaphragm value are set at the strobe synchronization limit speed and the maximum limit diaphragm value, respectively. Consequently, a program shift cannot be carried out for a bright object (i.e., high luminance) or a dark object (i.e., low luminance).

Furthermore, in the conventional strobe program, the diaphragm (i.e., stop) is controlled In such a manner that, when a film having a high sensitivity is used, the diaphragm is stopped-down by steps corresponding to the increment of film sensitivity to thereby make it possible to take a picture using the strobe at an optimum exposure value within a predetermined object distance, irrespective of the film sensitivity. Consequently, even if a film of high sensitivity is used, the object distance range in which a picture can be appropriately taken using the strobe cannot be extended.

If an object at a close distance is taken using the strobe, a large quantity of light emitted by the strobe is reflected by the object. Accordingly, stopping of the emission of strobe light based on the TTL direct metering is delayed, resulting in a tendency of over-exposure.

An object of the present invention is to provide an exposure control apparatus in which when a high sensitivity film is used in strobe photographing, not only the appropriate strobe-photographable object distance (i.e., distance at which a picture can be effectively taken using a strobe) can be extended, but also strobe photographing can be correctly effected for a close object distance without over-exposure.

Another object of the present invention is to provide strobe program shift exposure control apparatus in which combination of a shutter speed and a diaphragm value can be shifted within a controllable range defined by a strobe synchronization limit speed, a low shutter speed and a maximum diaphragm value, in strobe photographing.

Still another object of the present invention is to provide a strobe program shift exposure control apparatus in which a program shift can be carried out even when a bright or dark object Is photographed using a strobe.

According to the present invention there is provided an exposure control apparatus including a photometering means, a film sensitivity setting means, a calculating means for calculating an exposure value, a diaphragm value and a shutter speed in accordance with the measurement made by the photometering means and the film sensitivity set by the film sensitivity setting means, an object distance detecting means, and a strobe light emitting means, comprising a limit diaphragm value calculating means for calculating a first limit diaphragm value AvMIN= corresponding to the film sensitivity and a second limit diaphragm value AvMINLMT2 corresponding to the object distance detected by the object distance detecting means when strobe light is emitted from the strobe light emitting means, and a minimum limit diaphragm value setting means for comparing the first and second limit diaphragm values AvMINLW and AvMINLMT2 to select the larger diaphragm value as the minimum limit diaphragm -value for use in the exposure control.

According to another aspect of the present invention, there is provided an exposure control apparatus including a photometering means, a calculating means for calculating an exposure value, a diaphragm value and a shutter speed in accordance with the measurement made by the photometering means, and a strobe light emitting means, comprising a strobe program mode in which the shutter speed is set within a range defined by a strobe synchronization limit speed TvMAX and a minimum limit shutter speed TvMIN= less than the strobe synchronization limit speed TvMAX, in accordance with the exposure value when strobe light is emitted from the strobe light emitting means, and the diaphragm value is set within a range defined by a minimum limit diaphragm value AvMIN= greater than a minimum diaphragm value AvMIN of an associated taking lens and a maximum limit diaphragm value AvMAX= less than a maximum diaphragm value AvMAX of the taking lens, and a program shift means for varying the shutter speed and the diaphragm value within respective ranges defined by the minimum shutter 'speed TvMIN and the strobe synchronization limit speed TvMAX, and the minimum diaphragm value AvMIN and the maximum diaphragm value AvMAX, without changing the exposure value calculated by the calculating means, in the strobe program mode, when the calculated exposure value is above a lower limit exposure value Evl which is determined by the minimum limit diaphragm value AvMIN= and the strobe synchronization limit speed TvMAX and below an upper limit exposure value Ev3 which is determined by the maximum limit diaphragm value AvMAX= and the strobe synchronization limit speed TvMAX, respectively.

to still another aspect of the present an exposure control apparatus of a camera including a photometering means, a calculating means for calculating an exposure value, a diaphragm value and a shutter speed In accordance with the measurement made by the photometering means, a strobe light emitting means, and a strobe program mode, comprises a means for replacing the exposure value calculated by the calculating means with a lower limit exposure value Evl which is determined by a minimum limit diaphragm value AvMINLMT, greater than a minimum diaphragm value AvMIN of an associated taking lens, According invention and a minimum limit shutter speed TvMINLM, less than a strobe synchronization limit speed TvMAX, in the strobe program mode, when the calculated exposure value is less than the lower limit exposure value Evl. The exposure control apparatus further comprises a program shift means for varying the shutter speed and the diaphragm value, while retaining the lower limit exposure value Evl in the strobe program mode, when said replacing means replaces the calculated exposure value with the lower limit exposure value Evl.

According to yet another aspect of the present invention, there is provided an exposure control apparatus including a photometering means, a calculating means for calculating an exposure value in accordance with the measurement by the. photometering means, and a strobe program mode, comprising a means for replacing the exposure value calculated by the calculating means with limit exposure value Ev3 which is determined by limit diaphragm value AvMAUMT less than a maximum diaphragm value AvMAX of an associated taking lens and a strobe synchronization limit speed TvMAX, in the strobe program mode, when the calculated exposure value is greater than the upper limit exposure value Ev3, and shift means for varying the shutter speed and the value, while maintaining the upper limit exposure an upper a maximum a program diaphragm value Ev3 - a - in the strobe program mode.

Examples of the present invention will be described below in detail with reference to the accompanying drawings, in which; Fig. 1 is a block diagram of a single lens reflex camera having a strobe program exposure control apparatus incorporated therein, for an embodiment of the present invention; Fig. 2 is a flow chart of a main operation of a CPU in the single lens reflex camera shown in Fig. 1; Fig. 3 is a flow chart of photometering and calculating operations in the single lens reflex camera shown in Fig. 1; Fig. 4 is a flow chart of a program shift-up operation in the single lens reflex camera shown in Fig. 1; Fig. 5 is a flow chart of a program shift-down operation in the single lens reflex camera shown in Fig. 1; Figs. 6, 7 and 8 show a flow chart of a strobe program operation in the single lens reflex camera shown in Fig. 1; Fig. 9 is a flow chart of a program shift operation in a strobe program in the single lens reflex camera shown in Fig. 1; Fig. 10 is a flow chart of a shutter speed limiting operation in the single lens reflex camera shown In Fig. 1; Fig. 11 is a flow chart of a diaphragm value limiting operation in the single lens reflex camera shown in Fig. 1; and, Figs. 12A, 12B and 12C are program diagrams which illustrate strobe program lines and program shift in the single lens reflex camera shown In Fig. 1.

In Fig. 1.which shows main components of a single lens reflex camera having a strobe exposure control apparatus incorporated therein for the present invention, a camera body 1 has a zoom lens 2 attached thereto and an external strobe 3 which can be attached to the camera body 1.

The camera body 1, the zoom lens 2, and the external strobe 3 are provided therein with a main CPU 4, a lens CPU 5, and a strobe CPU 6, respectively. The main CPU 4 is connected to the lens CPU 5 and the strobe CPU 6 by respective bus lines 7 and 8 through respective connectors (not shown) to transmit and receive necessary data to and from the lens CPU 5 and the strobe CPU 6.

The camera body 1 is provided with a switch assembly 11 including a main switch which is actuated to turn a power source ON and OFF, a photometering switch SWS, a release switch SWR, various function switches for setting the ISO sensitivity and selecting an exposure mode from among a manual exposure mode, a program mode, a shutter-priority mode, and an aperture-priority mode, a strobe emission mode selecting switch, and programing switches including an up-switch llu and a down-switch lld, etc. The ON/OFF state of these switches is checked by the main CPU 4.

The camera body I is provided therein with a photometering circuit 12 for metering an object brightness, a strobe synchronization circuit 13 which effects TTL direct metering of object light after commencement of an exposure (i.e., emission of strobe light) to control the emission of strobe light, and a DX-code reading circuit 14 which reads DX-code data representing the ISO sensitivity of the film loaded in the camera. The output signals of these circuits are serit to the main CPU 4.

The camera body 1 is also provided therein with a shutter/diaphragm control circuit 15 which controls and drives a shutter device 16 and a diaphragm device 21 of the zoom lens 2.

The zoom lens 2 is provided with a zoom code plate 22 which is adapted to detect the current focal length and a distance code plate 23 which is adapted to detect the object distance at which a focus lens (not shown) is focused. The lens CPU 5 reads the current focal length data and the current object distance data through the code plates 22 and 23. The lens CPU 5 has an internal ROM in which lens data, such as a maximum diaphragm value Avmax and a minimum diaphragm value Avmin (i.e., open F number) is stored.

The strobe CPU 6 has an internal ROM which stores various strobe data including guide number data GNO and illumination angle data, etc. The strobe CPU 6 outputs the strobe data to the main CPU 4 and -receives strobe emission control signals, such as strobe emission preparation signals, light emission signals, or emission stop signals, etc., from the main CPU 4.

The single lens reflex camera has a strobe program and a program shift function, both being activatable in a strobe mode In which strobe light Is emitted, in addition to the regular photographing function. The strobe program weights the film sensitivity at a value above 0 and below 1. Namely, the strobe program makes it possible to make the best use of a high sensitivity film, that is, to expose an image of an object located at a far object distance at an optimum exposure value by reducing the amount of stop-down.

For an object at a close object distance, the stop-down is effected corresponding to the object distance so that strobe photographing at an optimum exposure value can be achieved.

In the single lens reflex camera of the present invention, the program shift can be carried out when strobe photographing, to expand the ranges of the shutter speed Tv and diaphragm value Av that can be set.

The photographing operation of the single lens reflex camera of the present invention will be discussed below with reference to Figs. 2 through 11 and Figs. 12A through 12C.

The photographing operation is generically controlled by the main CPU 4 in accordance with the program stored in the ROM of the main CPU 4.

The definitions of the symbols referred to in the following discussion and the flow charts shown in Figs. 2 through 11 are as follows. The values of these symbols are all indicated by the apex values.

Lv... exposure value (i.e., correctly adjusted exposure value Ev); AvMIN... minimum diaphragm value (i.e., open aperture of the zoom lens 2: open F number); AvMAX... maximum diaphragm value (i.e., minimum diaphragm value of the zoom lens 2: maximum F number); TvMIN... minimum shutter speed (i.e., minimum shutter speed which can be controlled by the camera body 1 and which corresponds to the camera-shake limit shutter speed Tvf in the strobe photographing mode); - 13 TvMAX... maximum shutter speed (maximum shutter speed which can be controlled by the camera body, 1 and which corresponds to the strobe synchronization limit speed in the strobe photographing mode); AvMIN=... minimum limit diaphragm value (first minimum limit diaphragm value which corresponds to a fixed limit value on the open aperture side in the strobe photographing mode); AvMIMMT2... second minimum limit diaphragm value which corresponds to a limit value on the open aperture side at the close strobe photographing mode; AvMAX=... maximum limit diaphragm value (limit diaphragm value on the stopped-down side in the strobe photographing mode); TvMIMMT minimum limit shutter speed (limit value on the low shutter speed side in the strobe photographing mode); fv... focal length (i.e., apex value of the focal length f (mm) of the zoom lens 2), fv=1ogf/log2; Dv object distance (i.e., apex value of the object distance D (mm)), Dv=1og(D)2/log2; Tvf... camera-shake limit shutter speed (i.e., value corresponding to the focal length f of the zoom lens 2), Tvf=0/4) x fv+ 2; f411 - 14 CPU-MAIN Operation:

The main flow chart of the single lens reflex camera, represented by CPUMAIN, is simplified in Figs. 2 through 11 and will be discussed below.

The CPU-MAIN operation starts when the main switch is turned ON. In the CPU-MAIN operation, the initialization of each port of the CPU 4 and the initialization of the RAM for the photographing operation and automatic focusing (AF) operation are first carried out (steps S101 to S105). Thereafter, intermittent operations are carried out by means of a looms timer and a 4 ms timer.

In the intermittent operations, the ON/OFF states of the switches including the up-switch llu and down-switch lld are input, so that the CPU 4 performs the necessary operations corresponding to the switches which are turned ON (steps S107). In the illustrated flow chart, only the program shift operation is shown. If the up-switch llu and the downswitch lld are turned ON, the program shift-up and program shift-down operations are carried out, as shown in Figs. 4 and 5, provided that the program exposure mode and the strobe program exposure mode are selected, respectively.

If neither the up-switch llu nor the down-switch lld are turned ON, or if the necessary operations corresponding to the switches that are actuated are completed, the - is - automatic focus adjusting operation per se known is carried out to move the f ocus lens group of the zoom lens 2 to the focal position (step S109).

Whether or not the set time of the 4 ms timer is up is checked at step S111. If the set time is up, control proceeds to step S119 to check whether or not the release switch SWR is turned ON.

turned ON, the release shown). Conversely, if If the release switch SWR is operation is carried out (not release switch SWR is not turned ON, control is returned to step S107.

If the set time of the 4 ms timer is not up at step S111, control proceeds to step S113 to check whether or not the set time of the 100 ms timer is up. If the set time of the 100 ms timer is not up, control is returned to step S107. Conversely, if the set time of the 100 ms timer is up, control proceeds to step S115 to input the necessary lens data and the strobe data from the lens CPU5 and the strobe CPU 6, respectively. Thereafter, the photometering and calculating sub-routine shown in Fig. 3 is called to effect the necessary exposure operations at step S117. Thereafter, control is returned to step S107 to repeat the necessary operations discussed above.

Photometering and Calculating Operation:

The sub-routine for the photometering and calculating operation, called at step S117, will be discussed below with reference to a flow chart shown in Fig. 3.

The brightness of the object is metered and the brightness data is set at a predetermined address (step S201). Thereafter, the data of the AE operation is cleared and the initial value is set (step S203). After that, the object distance D input from the lens CPU 5 is replaced with an object distance value Dv (step S205), and the data such as the minimum diaphragm value AvMIN and the maximum diaphragm value AvMAX, necessary to calculate the exposure factor, are set at step S207. Thereafter, the calculation of the exposure value Lv is carried out at step S209.

Whether or not strobe light is to be emitted is checked at step S211. If strobe light is not to be emitted, control proceeds to step S223 to call the sub-routines (not shown) corresponding to the selected exposure modes (steps S225, S227 and S229). Thereafter, the shutter speed Tv and the diaphragm value Av for controlling the exposure are calculated and set (step S231).

If strobe light is to be emitted (step S211) (strobe photographing mode), control proceeds to step S213 to replace the maximum shutter speed TvMAX and the minimum limit shutter speed TvMIMMT with the strobe synchronization speed and a predetermined value, respectively.

Thereafter, corresponding the exposure sub-routines (not shown) to the selected exposure modes are called and the shutter speed Tv and the diaphragm value Av for controlling the exposure are calculated and set as control values (steps S215, S219, S221 and S231). If the program exposure mode is selected, the sub-routine for the strobe program operation (Fig. 6) is called (steps S215 and S219).

If control is returned from the exposure sub-routines, data including the shutter speed Tv and the diaphragm value Av, etc., calculated in the respective exposure sub-routines is set. Thereafter, control is returned to the main routine.

In the illustrated embodiment, although the camera-shake limit shutter speed Tvf is set as the minimum limit shutter speed TvMINLM, another value can be set as the minimum limit ' shutter speed TvMIN=. If the camera-shake limit shutter speed Tvf is above the strobe synchronization speed, the maximum shutter speed TvMAX, the minimum limit shutter speed, and the minimum limit shutter speed TvMIN= are all identical to the strobe synchronization speed.

Program Shift-Up and Shift-Down Operation:

Figs. 4 and 5 show flow charts of the program shift-up and shift-down operations for effecting the programshift setting operation. In the illustrated embodiment, the amount of shift is set with reference to the shutter speed Tv. Namely, a shift of 1 (increment)" refers to an addition of 1 Tv to the shutter speed Tv, and the shift of 1'- 1 (decrement)" refers to a subtraction of 1 Tv from the shutter speed Tv. Shift is represented by an absolute value whose magnitude is the value of the program (P) shift. Whether the shift is an increment or decrement is determined by the program (P) shift direction bit.

If the up-switch llu is turned ON at step S107, control enters the P shift-up operation. If the down-switch lld is turned ON, control enters the P shift-down operation, respectively.

In the P shift-up operation, when the control diaphragm speed Tv is lower than the maximum shutter speed TvMAX (step S301) and the control diaphragm value Av is greater than the minimum diaphragm value AvMIN (step S303), control proceeds to step S305 to perform the shift-up operation. Conversely, if the control diaphragm speed Tv is not lower than the maximum shutter speed TvMAX (step S301) or if the control diaphragm value Av is not greater than the minimum diaphragm value AvMIN, control is returned since, if the shift-up operation is effected (i.e., the shift value is increased), the shutter speed Tv or the diaphragm value Av would become a value which cannot be controlled.

The direction of the shift is checked by the P shift direction bit at step S305. If the P shift direction bit (flag) is "0" (positive direction), the P shift value is increased by 0.5 Tv increments until the P shift value reaches the maximum value of 15 4/8 Tv (steps S305 to S309). Thereafter, control is returned.

Conversely, if the P shift direction bit (flag) is 0111 (negative direction), the P shift value is decreased by 0.5 Tv decrements until the P shift value becomes 0. If the P shift value becomes 0, the P shift direction bit is set at 11 0". Thereafter, 'control is returned (steps S305, S311 to S315).

In the P shift-down operation, when the calculated diaphragm speed Tv is higher than the minimum shutter speed TvMIN (step S351) and the calculated diaphragm value Av is smaller than the maximum diaphragm value AvMAX (step S353), control proceeds to step S355 to perform the shift- down operation. Conversely, if the calculated diaphragm speed Tv is not higher than the minimum shutter speed TvMIN or the calculated diaphragm value Av is not less than the maximum diaphragm value AvMAX, control is returned since, c if the shift-down operation is effected (i.e., the shift value is decreased), the shutter speed Tv or the diaphragm value Av would become a value which cannot be controlled.

The direction of the shift is checked by the P shift direction bit at step S355. If the P shift direction bit (flag) is "l" (negative direction), the P shift value is increased by 0.5 Tv increments until the P shift value reaches the maximum value of 15 4/8 Tv (steps S355, S363, S365). Thereafter, control is returned.

Conversely, if the P shift direction bit (flag) is 11011 (positive direction), the P shift value is stepwise decreased by every 0.5 Tv decrement until the P shift value becomes 0. If the P shift value becomes 0, the P shift direction bit (flag) is set at "10. Thereafter, control is returned (steps S355, 'S357 to S361).

The program shift value and the program shift direction bit set in the above mentioned operations are used in the strobe program shift operation shown in Fig. 9. Note that the unit increment or decrement is not limited to 0.5 Tv and can be a value more or less than 0.5 Tv.

Strobe Program Operation:

Fig. 6 shows a flow chart of a strobe program operation called at step S219 in the sub-routine shown in Fig. 3. In the strobe program of the illustrated embodiment, the minimum limit shutter speed TvMIN= is set in accordance with the focal length of the photographing lens (i.e., taking lens), and the maximum limit diaphragm value AvMAX= and the minimum limit diaphragm value AvMIMMT are set in accordance with the film sensitivity Sv.

Also, the minimum limit diaphragm value AvMIN= is varied in accordance with the object distance.

The shutter speed Tv and the diaphragm value Av are determined in accordance with the maximum limit diaphragm value AvMAUMT, the minimum limit diaphragm value AvMINLMT, the minimum limit shutter speed TvMINLMT, and the maximum limit shutter speed TvMAX (i.e., strobe synchronization limit speed). If the program shift is carried out, the controllable ranges are extended to the maximum diaphragm value AvMAX, the minimum diaphragm value AvMIN, the minimum shutter speed TvMIN and the maximum shutter speed TvMax, of the taking lens.

Figs. 12A, 12B and 12C show examples of the program diagrams obtained by the strobe program operation shown in Fig. 6. In Figs. 12A through 12C, the exposure value Evl, which is determined by the minimum limit diaphragm value AvMIN= and the minimum limit shutter speed TvMINLMT, is the lower limit of the exposure value; the exposure value Ev2, which is determined by the minimum limit diaphragm value AvMIN= and the maximum shutter speed TvMAX, is the second exposure value; and, the exposure value Ev3, which is determined by the maximum shutter speed TvMAX and the maximum limit diaphragm value AvMAXLMT, is the upper limit of the exposure value.

In the strobe program operation, the adjusted film sensitivity SXv is obtained by subtracting the exposure correcting value Xv from the film sensitivity Sv at step S401. Thereafter, the adjusted film sensitivity SXv is limited to the range of 3 (corresponding to ISO 25) to 7 (corresponding to ISO 400) at steps S403 through S409. The reason for this operation is that, if the adjusted sensitivity SXv is above 7 (ISO 400), the maximum limit diaphragm value AvMAXLMT (=SXv+ 2) will be beyond the upper limit of the practicable range (i.e., controllable range). Conversely, if the adjusted sensitivity SXv is below 3 (ISO 25), the maximum' limit diaphragm value AvMAXLMT (=SXv+ 2) will be below the lower limit of the practicable range (i.e., controllable range).

Thereafter, the maximum limit diaphragm value AvMAXLMT is obtained by the following equation, using the adjusted film sensitivity SXv (step S411); AvMAXLMT=SXv + 2 Whether or not the maximum limit diaphragm value AvMAXLMT is greater than the minimum diaphragm value AvMIN is checked (step S413). If the maximum limit diaphragm value AvMAXLMT is above the minimum diaphragm value AvMIN, the maximum limit diaphragm value AvMAX= is not changed. Conversely, if the maximum limit diaphragm value AvMAX= is not above the minimum diaphragm value AvMIN, the maximum limit diaphragm value AvMAX= is replaced with the controllable minimum diaphragm value AvMIN (step S415). Thereafter, whether or not the maximum limit diaphragm value AvMAX= is

below the maximum diaphragm value AvMAX is checked (step S417). If the maximum limit diaphragm value AvMAX= is below the maximum diaphragm value AvMAX, the maximum limit diaphragm value AvMAX= is not changed. Conversely, if the maximum limit diaphragm value AvMAX= is not below the maximum diaphragm value AvMAX, the maximum limit diaphragm value AvMAX= is replaced with the controllable maximum diaphragm value AvMAN (step S419).

Thereafter, the' first minimum limit diaphragm value AvMIN= in the strobe photographing mode is calculated by the following equation (D, at step S421; AvMINLMT=(5- AvMIN)/2+ (Sv - 5)/2+ AvMIN 11) Equation (D indicates that, in the case of film having a high sensitivity, the film sensitivity Sv is weighted with an appropriate weight value (e. g., 1/2) above 0 but below 1 so that the degree (amount) of stop-down is reduced corresponding to the film sensitivity Sv to thereby make it possible to take a picture of an object located at 25 a farther distance at an optimum exposure value.

For instance, if the open diaphragm value of the zoom lens 2 is F4 (AvMIN=4), when the film sensitivity is ISO 100 (Sv=5), the AvMINLMT is equal to 4 1/2, and when the film sensitivity is ISO 400 (Sv=7), the AvMINLMT is equal to 5 1/2, respectively. Note that a value (solution) of "(5 - AvMIN)/2+ (Sv - 5)/2" in equation (1) is usually greater than 0.

There is a possibility that the minimum limit diaphragm value AvMIN= is smaller than the minimum diaphragm value AvMIN. Accordingly, whether or not the minimum limit diaphragm value AvMIN= is less than the minimum diaphragm value AvMIN is checked at step S423. if the minimum limit diaphragm value AvMIN= is less than the minimum diaphragm value AvMIN, the minimum limit diaphragm value AvMIN= is set at the minimum diaphragm value AvMIN at step S425.

In the case of a close object distance, the minimum limit diaphragm value AvMIN= should be varied in accordance with the object distance Dv so that a picture of the close object can also be taken at an optimum exposure value using the strobe. To this end, the second minimum limit diaphragm value AvMINLMT2 is obtained by the following equation a at step S427; AvMINLMT2=5- Dv+ (Sv - 3)/2 Equation (2) indicates that, .. Q in the case of a close object, the film sensitivity Sv is weighted with an appropriate weight value (e.g., 1/2) above 0 but below I so that the degree (amount) of the stop-down is adjusted in accordande with the object distance Dv.

Thereafter, the second minimum limit diaphragm value AvMINLMT2 thus obtained is compared with the first limit diaphragm value AvMINLMT at step S429 so that the larger value can be used as the fixed diaphragm value (minimum limit diaphragm value AvMINLMT). Namely, if the second minimum limit diaphragm value AvMINLMT2 is greater than the first limit diaphragm value AvMINLMT, the minimum limit diaphragm value AvMINLMT is set to be the second minimum limit diaphragm value AvMINLMT2 (step S431). When the second minimum limit diaphragm value AvMINLMT2 is greater than the first limit- diaphragm value AvMINLMT, the object distance is usually very small. Accordingly, the second minimum limit diaphragm value AvMINLMT2, which increases as the object distance decreases, is used as the minimum limit diaphragm value AvMINLMT.

For instance, if the open aperture (diaphragm value) of the zoom lens 2 is F4 (AvMIN=4) and the f ilm sensitivity is ISO 100 (Sv=5), when the object distance D is 1 m (Dv=O), the second minimum limit diaphragm value AvMINLMT2 is 6 (AvMINLMT2=6) which is larger than the calculated minimum limit diaphragm value AvMINLMT which is 4 1/2 (AvMINLMT=4 1/2). Consequently, the minimum limit diaphragm value AvMIN= is set at AvMINLMT2 which is equal to 6 so that the diaphragm is stopped- down by two steps. Thus, the' program diagram as shown In Fig. 12A is obtained.

Thereafter, the minimum limit diaphragm value AvMIN= is compared with the maximum limit diaphragm value AvMAXLMT at step S433. If the minimum limit diaphragm value AvMIN= is greater than the- maximum limit diaphragm value AvMAXLMT, the minimum limit diaphragm value AvMIN= is replaced with the maximum limit diaphragm value AvMAUMT2 (step S435).

After the limit AvMAX= are obtained, calculated. First, diaphragm values AvMIN= and the control shutter speed Tv is the minimum limit shutter speed TvMIN= is set at the camera-shake limit shutter speed Tvf from which 0.5 Tv is subtracted at step S437. Thereafter, the diaphragm value Av is set at the minimum limit diaphragm value AvMIN=. Then, the Tv calculation sub-routine is called to calculate the accordance with the diaphragm value S441).

Thereafter, the limit operation is carried out to restrict the control shutter speed Tv between the minimum limit shutter speed TvMIN= and the maximum shutter speed TvMAX (i.e., synchronization shutter speed Tv) at steps shutter speed Tv in Av (steps S439 and S443 through S449. Namely, if the control shutter speed Tv is less than the minimum limit shutter speed TvMINLMT, the control shutter speed Tv is set at the minimum limit shutter speed TvMIN= (steps S443, S447). Conversely, if the control shutter speed Tv is greater than the maximum shutter speed TvMAX, the control shutter speed Tv is set at the maximum shutter speed TvMAX, and the Tv-over flag is set at 111" to indicate that the optimum shutter speed Tv is a large value (steps S443, S445 and S449).

Thereafter, the Av calculation sub-routine (Fig. 11) is called to calculate the control diaphragm value Av in accordance with the control shutter speed Tv which is then subject to the limit operation to restrict the same between the maximum limit diaphragm value AvMAX= and the minimum limit diaphragm value AvMIN= (steps S453 through S459). Namely, if the control diaphragm value Av is greater than the maximum limit diaphragm value AvMAUMT, the control diaphragm value Av is set at value AvMAX= (steps S453, control diaphragm value Av is diaphragm value AvMINLMT, the set at the minimum diaphragm S459).

When the control shutter diaphragm value Av are set, the maximum limit diaphragm S455). Conversely, if the less than the minimum limit control diaphragm value Av is value AvMIN= (steps S457, speed Tv and the control the strobe program shift sub-routine (Fig. 9) is called to effect the shift operation of the control shutter speed Tv and the control diaphragm value Av. Thereafter, control is returned (step S461). - Tv, Av Calculating Operation:

The calculation of Tv and Av to restrict the same will be discussed below with reference to Figs. 10 and 11.

In the calculation of Tv, the Tv-under flag and the Tv-over f lag are both set at "0". Thereafter, the optimum control shutter speed Tv is obtained, based on the exposure value Lv and the control diaphragm value Av (steps S601, 9603). Thereafter, whether or not the optimum control shutter speed Tv thus obtained is between the minimum shutter speed TvMIN and the maximum shutter speed TvMAX is checked (steps S605 -and S607). If the optimum control shutter speed Tv is between the minimum shutter speed TvMIN and the maximum shutter speed TvMAX, control is returned.

If the optimum control shutter speed Tv is smaller than the minimum shutter speed TvMIN, the optimum control shutter speed Tv is set at the minimum shutter speed TvMIN, and the Tv-under flag is set at "1" to indicate that the optimum control shutter speed Tv is smaller than the minimum shutter speed TvMIN (steps S605 and S609). If the optimum control shutter speed Tv is greater than the maximum shutter speed TvMAX, the optimum control shutter speed Tv is replaced with the maximum shutter speed TvMAX, and the Tv- over flag is set at 111" to indicate that the optimum control shutter speed Tv is greater than the maximum shutter speed TvMAX (steps S605, S607 and S611).

Similar to the calculation of Tv, in the calculation of Av, the Av-under f lag and the AV-over f lag are both set at "0". Thereafter, the optimum control diaphragm value Av is obtained, based on the exposure value Lv and the control shutter speed Tv (steps S601, S603). Thereafter, whether or not the optimum control diaphragm value Av thus obtained is between the minimum diaphragm value AvMIN and the maximum diaphragm value AvMAX is checked (steps S655 and S657). If the optimum control diaphragm value Av is between the minimum diaphragm value AvMIN and the maximum diaphragm value AvMAX, control is returned.

If the optimum control diaphragm value Av is smaller than the minimum diaphragm value AvMIN, the optimum control diaphragm value Av is set at the minimum diaphragm value AvMIN, and the Av-under flag is set at "1" to indicate that the optimum control diaphragm value Av is less than the minimum diaphragm value AvMIN (steps S655 and S659). If the optimum control diaphragm value Av is greater than the maximum diaphragm value AvMAX, the optimum control diaphragm value Av is replaced with the maximum diaphragm value AvMAX, and the Av-over flag is set at 11111 to indicate that the optimum control diaphragm value Av is greater than the maximum diaphragm value AvMAX (steps S655, S657 and S661).

The control shutter speed Tv and the control diaphragm value Av thus adjusted can be practically controlled by the camera body 1 and the zoom lens 2.

Strobe P Shift Operation:

The following discussion will be addressed to the strobe program (P) shift operation, with reference to Fig. 9 and Figs. 12B and 12C.

With the present invention, the strobe P shift is effected in a wide range defined by the solid lines in the program diagrams shown in Figs. 12B and 12C. Note that the f low chart shown in Fig. 9 is called at step S461 in the strobe program sub-routine, mentioned above.

In the flow chart shown in Fig. 9, first, whether or not the amount of program shift is 0 is checked at step S501. If the program shift amount is 0, control is returned. Conversely, if the program shift amount is not 0, the following operations are carried out.

The exposure value Lv obtained In the photometering and calculating operation shown in Fig. 3 is set in the Lv memory. Thereafter, the exposure value Lv is replaced with the sum of the control shutter speed Tv and the control diaphragm value Av obtained in the strobe program operation mentioned above (steps S503 and S505).

Consequently, if the exposure value Lv obtained in the photometering and calculating operation is less than the lower limit exposure value Evl, the combination of the control shutter speed Tv and the control diaphragm value Av can be shifted along the lower limit exposure value Evl indicated by the solid line shown_ in Figs. 12B and 12C. Conversely, if the exposure value Lv is greater than the lower limit exposure value Evl and less than the upper limit exposure value Ev3, the combination of the control shutter speed Tv and the control diaphragm value Av can be shifted along the line of the exposure value Ev depending on the object brightness. Moreover, if the exposure value Lv is greater than the upper limit exposure value Ev3, the combination of the control shutter speed Tv and the control diaphragm value Av can be shifted along the line of the upper limit exposure value Ev3.

At step S507, the program shift direction bit is checked. If the program shift direction bit is 11011 (i.e., positive), the amount of the program shift is added to the control shutter speed Tv. If the control shutter speed Tv plus the program shift amount is greater than the maximum shutter speed TvMAX, the latter is set as the control shutter speed Tv (steps S507, S509 to S513).

If the program shift direction bit is 1,111 (i.e., negative) at step S507, the amount of the program shift is subtracted from the control shutter speed Tv. If the control shutter speed Tv minus the program shift amount is less than the minimum shutter speed TvMIN, the latter is set as the control shutter speed Tv (steps S507, S515 to S519).

Thereafter, based on the control shutter speed Tv thus obtained, the Av calculation sub-routine is called to obtain the control diaphragm value Av which can be controlled (step S521). Whether or not the control diaphragm value Av is an optimum value is checked (steps S523 and S525). If the Av-over flag or Av-under flag is set at "V', the control diaphragm value Av obtained at step S521 is not an optimum value, and the Tv calculation sub-routine is called to again calculate the optimum shutter speed Tv corresponding to the control diaphragm value Av (steps S523 and S527 or S523 and S527).

When the above-mentioned operations are completed, control proceeds to step S529 to replace the exposure value Lv which has been set in the Lv memory at step S503 with the exposure value Lv. Control is then returned.

As can be seen from the above discussion, in the strobe photographing mode, even if the exposure value Lv is - 33 less than the value which is determined by the minimum limit diaphragm value AvMIMMT and the minimum limit shutter speed TvMIMMT, the diaphragm value Av is shiftable between the minimum diaphragm value AvMIN and the maximum diaphragm value AvMAX, and the shutter speed Tv is shiftable between the minimum shutter speed TvMIN and the synchronization maximum shutter speed TvMAX, as indicated by the phantom lines in the program diagrams shown in Figs. 12B and 12C. Thus, the shutter speed and the diaphragm value are shiftable within the range defined by the phantom lines.

In the strobe program in a single lens reflex camera, of the present invention, since the minimum limit diaphragm value Is set in accordance with the film sensitivity Sv, which is weighted with a predetermined weight value above 0 and below 1 (e.g., 1/2), if a film having a high sensitivity is used, an object at a farther distance can be photographed at an optimum exposure value.

On the other hand, for an object at a close object distance, the diaphragm value is modified to increase the stopping-down operation as the object distance D decreases.

Accordingly, a close object can also be photographed at an optimum exposure value.

Furthermore, since the program shift can be effected even in the strobe photographing mode, the shutter speed - 34 Tv and the diaphragm value Av in combination can be set in a wide range so as to meet an extended photographing condition and photographer's intention.

Although the illustrated embodiment is applied to a single lens reflex camera which is provided with the external strobe 3 being detachably attached to the camera body, the present invention can be applied to a camera having a built-in strobe. Furthermore, the application of the present invention is not limited to a single lens reflex camera.

As can be understood from the above discussion, with the present invention, in the strobe photographing mode, the first minimum limit diaphragm value is set in accordance with the film sensitivity which is weighted with a predetermined weight value above 0 and below 1. Accordingly, if a film of high sensitivity i:s used, an object at a farther object distance can be taken at an optimum exposure value. On the other hand, for an object at a close object distance, since the second minimum limit diaphragm value is set such that the diaphragm value increases as the object distance decreases, using the above-mentioned equation (2), a closer object can be also photographed at an optimum exposure value.

Moreover, since the exposure control apparatus of the present invention includes a program shift means which modifies the shutter speed and the diaphragm value within the range defined by the minimum shutter speed, the strobe synchronization limit speed, the minimum diaphragni value and the maximum diaphragm value, without changing the calculated exposure value, when the calculated exposure value is greater than the lower limit exposure value which is determined by the minimum limit diaphragm value and the minimum limit shutter- speed and less than the upper limit exposure value which is determined by the maximum limit diaphragm value and the strobe synchronization limit speed, respectively, the combination of the shutter speed and the diaphragm value can be modified within a wider range, while retaining the optimum exposure value for the strobe photographing mode.

Furthermore, with the present invention, In the strobe photographing mode, when the calculated exposure value depending on the object brightness is below the lower limit exposure value which is determined by the minimum limit diaphragm value greater than the minimum diaphragm value of the taking lens and the minimum limit shutter speed smaller than the strobe synchronization limit shutter speed, the calculated exposure value is replaced with the lower limit exposure value, and the shutter speed and the diaphragm value are modified while maintaining the lower limit exposure value.

On the other hand, when the calculated exposure value is above the upper limit exposure value which is determined by the maximum limit diaphragm value less than maximum diaphragm value of the taking lens and the strobe synchronization limit speed, the calculated exposure value is replaced with the upper limit exposure value, and the shutter speed and the diaphragm value are modified while maintaining the upper limit exposurevalue.

Consequently, the program shift can be effected within the shutter speed and diaphragm value that can be controlled by the camera even in the strobe photographing mode so that a wide range of combined shutter speed and diaphragm value can be obtained.

37

Claims (3)

1. An exposure control apparatus for a camera including a photometering means, a strobe light emitting means, a strobe program mode, and a calculating means for calculating an exposure value, a diaphragm value and a shutter speed in accordance with a photometering measurement made by the photometering means, the apparatus comprising:- a means for replacing, in use, the exposure value calculated by the calculating means with a lower limit exposure value when the calculated exposure value is less than the lnwer limit exposure value in the strobe program mode, the lower limit exposure value being determined by a minimum limit diaphragm value which is greater than a minimum diaphragm value of an associated taking lens and a minimum limit shutter speed which is less than a strobe synchronization limit speed; and a program shift means for varying the shutter speed and the diaphragm value in the strobe program mode whilst retaining the lower limit exposure value when said replacing means replaces the calculated exposure means with the lower limit exposure value.

2. An exposure control apparatus according to claim 1 wherein said minimum limit shutter speed is substantially identical to a camera-shake limit shutter speed.

3. An exposure control apparatus for a camera including - 38 a photometering means, a strobe light-emitting means, a strobe program mode, and a calculating means for calculating an exposure value in accordance with a photometering measurement made by the photometering means, the apparatus comprising:a means for replacing, in use, the exposure value calculated by the calculating means with an upper limit exposure value when the calculated exposure value is greater than the upper limit exposure value in the strobe program mode, the upper limit exposure value being determined by a maximum limit diaphragm value which is less than a maximum diaphragm value of an associated taking lens and a strobe synchronization limit speed; and a program shift means for varying the shutter speed and the diaphragm value in the strobe program mode whilst retain-Ing the upper limit exposure value when said replacing means replaces the calculated exposure means with the upper limit exposure value.