Abstract:
The present invention provides a photometer for making the photometry that can deal with various back light situations. For this purpose, a screen is two-dimensionally divided into plural areas, whereby one-dimensional projection data is calculated from the brightness data acquired from plural areas, and the maximum value of the calculated one-dimensional projection data is detected. Thus, the exposure compensation is made based on the detected maximum value.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a photometer useful for an image sensing device such as a still camera or a video camera, the image sensing device, a photometric method, a program and a recording medium. 
   BACKGROUND OF THE INVENTION 
   For a photometer or an exposure control unit for the camera, a technique for correct exposure has been put to practical use in which a photographing screen is divided into plural areas to acquire a plurality of brightness information for the field, and it is discriminated whether or not a back light is present on the basis of the plurality of brightness information in accordance with a predetermined algorithm. In Japanese Patent Laid-Open No. 6-225205, such conventional technique was described. According to Japanese Patent Laid-Open No. 6-225205, the photographing screen is divided into a plurality of blocks, and the photometric value data is obtained from cumulative data for each block and the maximum value within the screen. Also, an iris control is made by changing a photometric reference value depending on whether a central part of the screen is a back light state or a follow light state, enabling the correct iris operation to be made for photographing under the back light condition. In Japanese Patent Laid-Open No. 6-225205, a video camera is taken as an example, in which the term of iris operation is used, but has the same meaning as the exposure control for the still camera to make the correct exposure for an image sensing element or film. 
   A typical example of the condition called back light is a situation where the brightness is low in a central part and a central lower part of the screen and high in other upper part and left and right parts of the screen, as shown in  FIG. 3  of Japanese Patent Laid-Open No. 6-225205. In this situation, there is a high possibility of correct exposure by the conventional technique as described in Japanese Patent Laid-Open No. 6-225205. Examining a number of pictures taken under the back light condition, it does not always follow that the back light situation is the above situation where the brightness is low in a central part and a central lower part of the screen and high in other upper part and left and right parts of the screen. Thus, it has been found that there are various patterns, including a pattern where an upper part of the screen is only light and a pattern where a left part of the screen is only light, depending on the composition. For example,  FIG. 10A  shows one example of photographing composition, in which reference numeral  71  is a portrait area for the principal subject,  72  is a sky area,  73  is a ground area,  74  is a mountain area, and  75  to  77  are the tree areas. When this scene is in the back light, the sky area  72  among these areas has a high brightness, and other areas, particularly the portrait area  71  and the tree areas  75  to  77 , have the low brightness. In this scene, an upper-part of the screen is only light, and the left and right parts of the principal subject are dark. Therefore, if this scene is photographed using a photometer as described in Japanese Patent Laid-Open No. 6-225205, it is often difficult to discriminate a back light, irrespective of the back light scene. Thereby, the picture was taken without making exposure compensation and producing the flash in accordance with the back light, and often collapsed black in the portrait area for the principal subject due to underexposure. 
   SUMMARY OF THE INVENTION 
   The present invention has been achieved in the background as described above, and it is an object of the invention to provide a photometer to make the exposure control for an image sensing device, the photometer comprising a photometric sensor being two-dimensionally divided into plural areas within a screen, in which one-dimensional data is calculated from the outputs of the photometric sensor, the maximum value of the one-dimensional data is detected, an exposure compensation value is calculated in accordance with the detected maximum value of the one-dimensional data, an average brightness value of the subject is calculated over the screen, and the exposure compensation value and the average brightness value are added to acquire a correct brightness value of the subject. 
   In a back light scene where only a part of the screen is light, the back light is adequately detected to make a correct exposure, thereby realizing the exposure control for the camera. 
   Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
       FIG. 1  is a cross-sectional view showing an arrangement of optical members for a camera and an interchangeable lens according to the present invention; 
       FIG. 2  is a view showing a constitutional example of a focus detecting sensor; 
       FIG. 3  is a view showing a constitutional example of a photometric sensor; 
       FIG. 4  is a view exemplifying focus detected positions on a photographing screen; 
       FIG. 5  is a block diagram showing a constitutional example of the electrical circuits for the camera and the interchangeable lens; 
       FIG. 6  is a flowchart showing the operation of control means for the camera; 
       FIG. 7  is a flowchart showing the operation of the control means for the camera in conjunction with  FIG. 6 ; 
       FIG. 8  is a view showing the relationship between two-dimensional brightness data and one-dimensional projection data; 
       FIG. 9  is an explanatory graph representing the high brightness correction value; 
       FIGS. 10A and 10B  are views showing a photographing screen and a table exemplifying the brightness data; 
       FIG. 11  is an explanatory graph representing the flash producing condition; and 
       FIGS. 12A and 12B  are views showing a constitutional example of a photometric sensor according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1  is a cross-sectional view showing an arrangement of optical members for a camera according to the present invention. 
   In  FIG. 1 , the camera is a so-called single-lens reflex type in which the lens is interchangeable, in which reference numeral  10  is a camera main body and  30  is an interchangeable lens. In the camera main body  10 , reference numeral  11  is an optical axis of a photographing lens,  12  is a film face,  13  is a semitransparent main mirror, and  14  is a first reflecting mirror, in which the main mirror  13  and the first reflecting mirror  14  are sprung up together at the time of photographing. Reference numeral  15  is a paraxial image formation face conjugate with the film face  12  by the first reflecting mirror  14 ,  16  is a second reflecting mirror,  17  is an infrared cut filter,  18  is a diaphragm having two openings,  19  is a secondary imaging lens, and  20  is a focus detecting sensor. The focus detecting sensor  20  is a photoelectric conversion element of area storage type such as a CMOS or a CCD, which comprises two pairs of photo sensors  20 A and  20 B having multiply divided areas, corresponding to two openings of the diaphragm  18 , as shown in FIG.  2 . In % addition to the photo sensors  20 A and  20 B, the peripheral circuits for signal storage and signal processing are composed of integrated circuits on the same chip. A mechanism from the first reflecting mirror  14  to the focus detecting sensor  20  enables the focus detection to be made based on an image drift at any position within the photographing screen, as described in detail in Japanese Patent Laid-Open No. 9-184965. 
   Reference numeral  21  is a focusing screen having diffusivity,  22  is a pentaprism,  23  is an ocular lens,  24  is a third reflecting mirror,  25  is a condensing lens, and  26  is a photometric sensor for acquiring the information regarding the brightness of the subject. The photometric sensor  26  consists of a photoelectric conversion element such as a silicon photodiode, in which a plurality of photo sensors are arranged like a lattice to contain the entire photographing screen in the field of view, as shown in FIG.  3 . In this example, the field of view for receiving the light is divided into 7 columns×5 rows=35. The 35 divided photocells are referred to as PD 11  to PD 57 . Besides the photo sensors, the peripheral circuits for signal amplification and signal processing are composed of integrated circuits on the same chip. 
     FIG. 4  is a view showing the positional relation of correspondence between the focus detected position by focus detecting means such as the focus detecting sensor  20  and the 35 divided photo sensors of the photometric sensor  26  within the photographing screen. In this example, there are nine focus detected positions S 01  to S 23  within the photographing screen, in which the focus detected position S 01  corresponds to the photocell PD 23  of the photometric sensor  26  to detect the focal point. Furthermore, the focus detecting position S 02  corresponds to the photocell PD 24  of the photometric sensor  26  to detect the focal point, the focus detecting position S 03  corresponds to the photocell PD 25  of the photometric sensor  26  to detect the focal point, and the focus detecting position S 23  corresponds to the photocell PD 45  of the photometric sensor  26  to detect the focal point, as shown in FIG.  4 . 
   Reference numeral  27  is a mount portion for mounting the photographing lens, and  28  is a contact portion for communicating the information with the photographing lens. In the interchangeable lens  30 , reference numeral  31  is a diaphragm,  32  is a contact portion for communicating the information with the camera main body,  33  is a mount portion to be mounted on the camera, and  34  to  36  are optical lenses making up the photographing lens. 
     FIG. 5  is a block diagram showing a constitutional example of the electrical circuits for the camera main body  10  and its interchangeable lens  30  according to the invention. In the camera main body  10 , reference numeral  41  is control means with a one-chip microcomputer comprising ALU, ROM, RAM, an A/D converter and a serial communication port internally to make the overall control of the camera mechanism. A specific sequence of the control means  41  will be described later. The focus detecting sensor  20  and the photometric sensor  26  are the same as those shown in FIG.  1 . The output signals from the focus detecting sensor  20  and the photometric sensor  26  are connected to an input terminal of the A/D converter in the control means  41 . 
   Reference numeral  42  is a shutter connected to an output terminal of the control means  41  and controlled. Reference numeral  43  is a first motor driver connected to the output terminal of the control means  41  and controlled, in which the first motor driver drives a first motor  44  for feeding the film or moving the main mirror  13 . 
   Reference numeral  45  is a sensor for detecting the attitude of the camera, its output signal being connected to the input terminal of the control means  41 . The control means  41  inputs the information of an attitude detecting sensor  45  to acquire the information as to whether the camera is held in the transverse or longitudinal position for photographing. Reference numeral  46  is an AF light source for emitting an infrared ray when the focus detection is made by the focusing sensor  20  under the condition of low illumination, in which the AF light source emits light in accordance with an output signal of the control means  41 . Reference numeral  47  is flash means for producing the flash at the photographing time when the brightness of the subject is insufficient, in which flash means is excited in accordance with an output signal of the control means  41 . Reference numeral  48  is a display for displaying the number of photographing sheets, the date and time, and the photographing information. The display is composed of a liquid crystal panel, and each indicator lamp is lighted in accordance with an output signal of the control means  41 . Reference numeral  49  is various kinds of switches, including a release button. Reference numeral  28  is a contact portion as shown in  FIG. 1  to which an input/output signal of the serial communication port in the control means  41  is connected. 
   In the interchangeable lens  30 , reference numeral  51  is lens control means with a one-chip microcomputer comprising ALU, ROM, RAM and the serial communication port internally. Reference numeral  52  is a second motor driver for driving a second motor  53 , which is connected to an output terminal of the lens control means  51  and controlled to make the focus adjustment. Reference numeral  54  is a third motor driver for driving a third motor  55 , which is connected to the output terminal of the lens control means  51  and controlled to regulate the diaphragm  31 , as shown in FIG.  1 . Reference numeral  56  is a distance encoder for acquiring the information regarding an extension amount of focusing lens, namely a subject distance, the distance encoder being connected to an input terminal of the lens control means  51 . Reference numeral  57  is a zoom encoder for acquiring the focal length information in photographing, when the interchangeable lens  30  is zoom lens, the zoom encoder being connected to the input terminal of the lens control means  51 . Reference numeral  32  is the contact portion as shown in  FIG. 1 , to which an input/output signal of the lens control means  51  at the serial communication port is connected. 
   When the interchangeable lens  30  is mounted on the camera main body  10 , the contact portions  28  and  32  are connected so that the lens control means  51  is enabled to make data communication with the control means  41  of the camera main body. The optical information intrinsic to the lens that is necessary for the control means  41  of the camera main body to make the focus detection or exposure operation, and the information regarding the subject distance or focal length based on the distance encoder  56  or the zoom encoder  57  are passed through the data communication from the lens control means  51  to the control means  41  of the camera main body. Also, the focusing information or diaphragm information that is obtained as a result of the focus detection or exposure operation made by the control means  41  of the camera main body is output through the data communication from the control means  41  of the camera main body to the lens control means  51 , so that the lens control means  51  may control the second motor driver  52  in accordance with the focusing information and control the third motor driver  54  in accordance with the diaphragm information. 
   Referring to the flowcharts of FIG.  6  and beyond, a specific operation sequence of the control means  41  in the camera main body according to the invention will be described below. If a power switch, not shown, is turned on, the control means  41  becomes operable. If a first stroke switch of the release button, not shown, is turned on, the operation starts at step ( 101 ) in FIG.  6 .
     ( 101 ) Start signal accumulation by issuing a control signal to the focus detecting sensor  20 .   ( 102 ) Wait for the focus detecting sensor  20  to end the signal accumulation.   ( 103 ) Read the signal accumulated in the focus detecting sensor  20  and make the A/D conversion. Moreover, various kinds of necessary data correction including the shading are performed for read digital data.   ( 104 ) Input the lens information necessary for focus detection from the lens control means  51 , and calculate the focus state in each part of the photographing screen based on the input lens information and digital data obtained from the focus detecting sensor  20 . An area for focusing within the screen is decided in accordance with the calculated focus state in each part within the photographing screen by a method as described in Japanese Patent Laid-Open No. 11-190816. The lens movement amount of moving the lens into focus is calculated in accordance with the focus state in the decided area.   ( 105 ) Output the calculated lens movement amount to the lens control means  51 . Accordingly, the lens control means  51  sends a signal to the second motor driver  52  for driving the second motor  53  to adjust the focusing lens. Thereby, the photographing lens is placed in a focused state for the subject. After the focused state, the information regarding the subject distance from the distance encoder  56  is input by the lens control means  51  to acquire the distance information to the subject.   ( 106 ) Start the signal accumulation by sending a control signal to the photometric sensor  26 .   ( 107 ) Wait for the photometric sensor  26  to end the signal accumulation.   ( 108 ) Read the signal of each photocell PD 11  to PD 57  accumulated in the photometric sensor  26  and make the A/D conversion.   ( 109 ) Make the exposure operation. The brightness of the subject is obtained by arithmetical operation, so that the shutter speed and the stop value for correct exposure are decided. Also, a discrimination whether or not the scene is the back light scene is made according to a predetermined algorithm. Also, a discrimination whether or not to produce the flash is made. The details of the operation contents will be described later with reference to the flowchart of FIG.  7 .   ( 110 ) Wait for the second stroke switch of the shutter button to be turned on. If it is not turned on, the procedure returns to step ( 101 ). However, if it is turned on, the procedure proceeds to step ( 111 ).   ( 111 ) Spring up the main mirror  13  and the first reflecting mirror  14  by sending a control signal to the first motor driver for driving the first motor  44 .   ( 112 ) Forward the diaphragm value information calculated at step ( 109 ) to the lens control means  51 . Based on this information, the lens control means  51  outputs a control signal to the third motor driver  54  for driving the third motor  55  to move the diaphragm  31 . Thereby, the photographing lens becomes in a stopped-down state.   ( 113 ) Control the shutter  42  according to the shutter speed calculated at step ( 109 ) to expose the film. By an exposure operation as will be described in detail later, if it is determined that the scene is to be photographed employing the flash means  47 , the flash means  47  is excited.   ( 114 ) Open the diaphragm by forwarding the information to the lens control means  51 . Based on this information, the lens control means  51  sends a control signal to the third motor driver  54  for driving the third motor  55  to move the diaphragm  31 . Thereby, the photographing lens becomes in a stop open state.   ( 115 ) Spring back the main mirror  13  and the reflecting mirror  14  down by sending a control signal to the first motor driver for driving the first motor  44 .   ( 116 ) Wind up the film by sending a control signal to the first motor driver for driving the first motor  44 .   

   Thus, a series of photographing sequence is completed. 
   Referring now to the flowchart of  FIG. 7 , the detailed contents of the exposure operation at step ( 109 ) will be described.
     ( 151 ) Input the lens information necessary for exposure operation from the lens control means  51  and correct the digital brightness data obtained from the photocells PD 11  to PD 57  of the photometric sensor  26  at step ( 108 ). The brightness data corresponding to photocells PD 11  to PD 57  after correction are referred to as ED 11  to ED 57 , respectively. Moreover, the attitude information of the camera is acquired by inputting the information of the attitude detecting sensor  45 .   ( 152 ) Calculate the projection data Y 1  to Y 5  and X 1  to X 7  based on the corrected brightness data ED 11  to ED 57  of the photocells. When the projection data Y 1  to Y 5  are calculated, the calculation range of projection data is changed according to the information of the area where the focus detection is made at step ( 104 ). Referring now to  FIG. 8 , a specific calculation method will be described. Generally, a method for transforming the two dimensional array of m rows×n columns to one dimensional array with data added or added and averaged in the row or column direction is called a projection or a projective transformation from two dimensions to one dimension. Also, one dimensional array data resulted from the addition in the row or column direction is called a projected image or projection data. In this embodiment, the projection data Y 1  to Y 5  and X 1  to X 7  are calculated from the two dimensional brightness information ED 11  to ED 57  according to the array of photocells PD 11  to PD 57  in the photometric sensor  26 . For the projection data X 1  to X 7 , the calculation method is fixed, irrespective of the focus detecting position information.
 
 X   1 =( ED   11 + ED   21 + ED   31 + ED   41 + ED   51 )+5 
 
 X   2 =( ED   12 + ED   22 + ED   32 + ED   42 + ED   52 )+5 
 
 X   3 =( ED   13 + ED   23 + ED   33 + ED   43 + ED   53 )+5 
 
 X   4 =( ED   14 + ED   24 + ED   34 + ED   44 + ED   54 )+5 
 
 X   5 =( ED   15 + ED   25 + ED   35 + ED   45 + ED   55 )+5 
 
 X   6 =( ED   16 + ED   26 + ED   36 + ED   46 + ED   56 )+5 
 
 X   7 =( ED   17 + ED   27 + ED   37 + ED   47 + ED   57 )+5 
   

   For the projection data Y 1  to Y 5 , the calculation data range is changed depending on the focus detecting position information in the following way. Since the focus detecting position is fundamentally considered to be the position of the principal subject, the exposure operation is made by weighting the 5×5 area including the position of principal subject as the principal photometric area with higher value or feature. 
   When the focus detecting position is S 01 , S 11  or. S 21  in  FIG. 4 ,
 
 Y   1 =( ED   11 + ED   12 + ED   13 + ED   14 + ED   15 )+5 
 
 Y   2 =( ED   21 + ED   22 + ED   23 + ED   24 + ED   25 )+5 
 
  Y   3 =( ED   31 + ED   32 + ED   33 + ED   34 + ED   35 )+5
 
 Y   4 =( ED   41 + ED   42 + ED   43 + ED   44 + ED   45 )+5 
 
 Y   5 =( ED   51 + ED   52 + ED   53 + ED   54 + ED   55 )+5 
 
   When the focus detecting position is S 02 , S 12  or S 22  in  FIG. 4 ,
 
 Y   1 =( ED   12 + ED   13 + ED   14 + ED   15   +ED   16 )+5 
 
 Y   2 =( ED   22 + ED   23 + ED   24 + ED   25 + ED   26 )+5 
 
 Y   3 =( ED   32 + ED   33 + ED   34 + ED   35 + ED   36 )+5 
 
 Y   4 =( ED   42 + ED   43 + ED   44 + ED   45 + ED   46 )+5 
 
 Y   5 =( ED   52 + ED   53 + ED   54 + ED   55 + ED   56 )+5 
 
   When the focus detecting position is S 03 , S 13  or S 23  in  FIG. 4 ,
 
 Y   1 =( ED   13 + ED   14 + ED   15 + ED   16 + ED   17 )+5 
 
 Y   2 =( ED   23 + ED   24 + ED   25 + ED   26 + ED   27 )+5 
 
 Y   3 =( ED   33 + ED   34 + ED   35 + ED   36 + ED   37 )+5 
 
 Y   4 =( ED   43 + ED   44 + ED   45 + ED   46 + ED   47 )+5 
 
 Y   5 =( ED   53 + ED   54 + ED   55 + ED   56 + ED   57 )+5 
     ( 153 ) Calculate the average brightness value Ea over the entire screen in which the area farther away from, the focus detecting position is weighted with lower value by emphasizing the focus detecting position according to the information of focus detecting position.   

   When the focus detecting position is S 01  in  FIG. 4 ,
 
 Ea ={( X   1 + X   2 + X   3 + X   4 + X   5 )×5+( X   6 + X   7 )×2.5 +ED   23 ×5}+35 
 
   When the focus detecting position is S 02  in  FIG. 4 ,
 
 Ea= {( X   2 + X   3 + X   4 + X   5 + X   6 )×5+( X   1 + X   7 )×2.5 +ED   24 ×5}+35 
 
   When the focus detecting position is S 03  in  FIG. 4 ,
 
 Ea ={( X   3 + X   4 + X   5 + X   6 + X   7 )×5+( X   1 + X   2 )×2.5 +ED   25 ×5}+35 
 
   When the focus detecting position is S 11  in  FIG. 4 ,
 
 Ea ={( X   1 + X   2 + X   3 + X   4 + X   5 )×5+( X   6 + X   7 )×2.5 +ED   33 ×5}+35 
 
   When the focus detecting position is S 12  in  FIG. 4 ,
 
 Ea={ ( X   2 + X   3 + X   4 + X   5 + X   6 )×5+( X   1 + X   7 )×2.5 +ED   34 ×5}+35 
 
   When the focus detecting position is S 13  in  FIG. 4 ,
 
 Ea ={( X   3 + X   4 + X   5 + X   6 + X   7 )×5+( X   1 + X   2 )×2.5 +ED   35 ×5}+35 
 
   When the focus detecting position is S 21  in  FIG. 4 ,
 
 Ea ={( X   1 + X   2 + X   3 + X   4 + X   5 )×5+( X   6 + X   7 )×2.5 +ED   43 ×5}+35 
 
   When the focus detecting position is S 22  in  FIG. 4 ,
 
 Ea= {( X   2 + X   3 + X   4 + X   5 + X   6 )×5+( X   1 + X   7 )×2.5 +ED   44 ×5}+35 
 
   When the focus detecting position is S 23  in  FIG. 4 ,
 
 Ea ={( X   3 + X   4 + X   5 + X   6 + X   7 )×5+( X   1 + X   2 )×2.5 +ED   45 ×5}+35 
     ( 154 ) Detect the maximum value among the projection data Y 1  to Y 5  and X 1  to X 7 . The maximum value is denoted as Eh. When Eh is greater than a predetermined value, the high brightness correction value γ is calculated. The relationship between the maximum value Eh and the high brightness correction value γ is indicated by a line  81  in FIG.  9 . The line  81  in  FIG. 9  represents a graph for calculating the high brightness correction value γ, when Eh is greater than 9 in the Bv value. For example, γ=1.5 for Eh=12, and γ=2 for Eh=14. In the case of Eh=9, γ=0. If the high brightness correction value γ is calculated, the average brightness value Ea calculated at previous step is corrected for high brightness to obtain Ea(γ).
 
 Ea (γ)= Ea−γ 
   

   When the maximum value among the projection data Y 1  to Y 5  and X 1  to X 7  satisfies the following condition, the value indicated by the line  81  of  FIG. 9  that is multiplied by 0.5 is defined as γ. 
   The condition includes when the focus detecting position is S 01 , S 11  or S 21  in FIG.  4  and the maximum value among the projection data is X 6  or X 7 , when the focus detecting position is S 02 , S 12  or  522  in FIG.  4  and the maximum value among the projection data is X 1  or X 7 , or when the focus detecting position is S 03 , S 13  or S 23  in FIG.  4  and the maximum value among the projection data is X 1  or X 2 . Under this condition, since the maximum value detecting area has a lower weight, when the average brightness value Ea is calculated at step ( 153 ), it is necessary that the high brightness correction value γ is reduced. 
   By making the high brightness correction in this manner, when a high brightness area in the back light sky is included in the screen, it is possible to correct for the influence of brightness value in the high brightness area. One example will be taken in connection with  FIGS. 10A and 10B . The brightness information ED 11  to ED 57  corresponding to a scene of FIG.  10 A and the calculated projection data Y 1  to Y 5  and X 1  to X 7  are represented as numerical values in  FIG. 10B  in accordance with the array of FIG.  8 . In this scene, since it is appropriate to make the focus detection near the face of the portrait  71  as the principal subject, the focus detecting position is S 11 , whereby Ea=8.7. Since the maximum brightness among the projection data is 10.8 at Y 1 , Eh=10.8, and γ=0.9 results from FIG.  9 . 
   Accordingly, Ea(γ)=Ea−γ=7.8. If the exposure of the camera is decided in accordance with Ea(γ), the exposure value suitable for photographing the portrait, trees and mountains is obtained by removing the influence of the sky portion on the upper part of the screen.
     ( 155 ) Calculate a gradient value ΔE of brightness in the top and bottom direction within the photographing screen on the basis of the calculated projection data, camera attitude data and focus detecting position information.   

   When the camera attitude is in the transverse normal position, photocells PD 11  to PD 17  of the photometric sensor  26  are on the top side of the screen, and photocells PD 51  to PD 57  are on the bottom side of the screen. Hence, the gradient value ΔE of brightness is calculated in accordance with the following expression.
 
 E ={( Y   1 + Y   2 )/2−( Y   4 + Y   5 )/2}/3 
 
   When the camera attitude is in the longitudinal position, photocells PD 11  to PD 51  of the photometric sensor  26  are on the top side of the screen, and photocells PD 17  to PD 57  are on the bottom side of the screen, the gradient value ΔE of brightness is calculated on the basis of the focus detecting position information. When the focus detecting position is S 01 , S 11  or S 21  in  FIG. 4 ,
 
Δ E ={( X   1 + X   2 )/2−( X   4 + X   5 )/2}/3 
 
   When the focus detecting position is S 02 , S 12  or S 22  in  FIG. 4 ,
 
 ΔE ={( X   2 + X   3 )/2−( X   5 + X   6 )/2}/3 
 
   When the focus detecting position is S 03 , S 13  or S 23  in  FIG. 4 ,
 
Δ E ={( X   3 + X   4 )/2−( X   6 + X   7 )/2}/3 
 
   When the camera attitude is in the longitudinal position, photocells PD 17  to PD 57  of the photometric sensor  26  are on the top side of the screen, and photocells PD 11  to PD 51  are on the bottom side of the screen, the gradient value ΔE of brightness is calculated on the basis of the focus detecting position information in accordance with the following expression. 
   When the focus detecting position is S 01 , S 11  or S 21  in  FIG. 4 ,
 
Δ E ={( X   4 + X   5 )/2−( X   1   +X   2 )/2}/3 
 
   When the focus detecting position is S 02 , S 12  or S 22  in  FIG. 4 ,
 
Δ E ={( X   5 + X   6 )/2−( X   2 + X   3 )/2}/3 
 
   When the focus detecting position is S 03 , S 13  or S 23  in  FIG. 4 ,
 
 ΔE ={( X   6 + X   7 )/2−( X   3 + X   4 )/2}/3 
 
   The meaning of the calculated gradient value ΔE of brightness is the numerical value simply indicating the average change rate of brightness in the top and bottom direction for each row (column) in the 5×5 area near the focus detecting position within the photographing screen.
     ( 156 ) Calculate a deviation γEs between the calculated average brightness value Ea(γ) corrected for high brightness and the brightness value Es at the focus detecting position.
 
Δ Es=Ea (γ)− Es  
   

   Herein, Es is equal to ED 23  (when the focus detecting position is S 01 ), ED 24  (when the focus detecting position is S 02 ), ED 25  (when the focus detecting position is S 03 ), ED 33  (when the focus detecting position is S 11 ), ED 34  (when the focus detecting position is S 12 ), ED 35  (when the focus detecting position is S 13 ), ED 43  (when the focus detecting position is S 21 ), ED 44  (when the focus detecting position is S 22 ), or ED 45  (when the focus detecting position is S 23 ).
     ( 157 ) Discriminate whether or not the flash means  47  should be employed to photograph the scene. The condition for employing the flash means  47  to photograph the scene is either the first condition that the calculated average brightness value Ea(γ) corrected for high brightness is lower than a predetermined brightness (e.g., below 5 in Bv value) or the second condition that the calculated gradient value ΔE of brightness and the deviation ΔEs are positive with their relation being outside a line  82  as represented in  FIG. 11  (hatched portion). When any of the gradient value ΔE of brightness and the deviation ΔEs is negative, the flash is not produced.   

   Also, when it is obvious that the distance to the subject is far and the excellent photographed results are not obtained with the light quantity of the provided flash means  47  as a result of focusing the photographing lens at step ( 105 ), the flash means  47  may not be employed, even if the first or second condition is satisfied. Also, when the camera is not in a photographing mode for automatically employing the flash means, it is unnecessary to perform this step. When it is discriminated that the flash means  47  should be employed to photograph the scene, the flash means  47  is prepared to produce the flash.
     ( 158 ) Calculate the exposure compensation value α when the predetermined condition is satisfied.   

   When Ea(γ)&gt;Es and the flash means is not employed as the condition,
 
α={ Es−Ea(γ)}× 0.5 
 
   When Ea(γ)&lt;Es and Ea(γ)&lt;0 as the condition,
 
α={ Es−Ea (γ)}×0.25 
 
Under other conditions, α=0.
     ( 159 ) Calculate the subject brightness value for exposure control as Ee=Ea(γ)+α. In this expression, α={Es−Ea(γ)}×0.5, whereby Ee=(Ea(γ)+Es)×0.5. Also, Ea(γ)=Ea−γ, whereby Ee=(Ea−γ+Es)×0.5.   

   The optimal exposure control factors, namely the shutter speed and the stop value, are decided on the basis of the subject brightness value Ee and the presence or absence of employing the flash means  47  that is discriminated at step ( 157 ). 
   The above is the detailed description for the exposure operation. Though in this embodiment, the photometric sensor is divided into 5×7 or 35 photocells, and the number of focus detecting positions is  9 , the invention is not limited to this case. 
   The description of the first embodiment is thus completed. 
   [Second Embodiment] 
   In the first embodiment, the photocells of the photometric sensor  26  are arranged in the two-dimensional array of m rows×n columns. However, other than the photometric sensor  26  as above, a photometer having the same back light detecting effect can be realized as follows. 
     FIG. 12A  is a view showing the photocells of a second photometric sensor  626 , instead of the photometric sensor  26  in the first embodiment. As shown in  FIG. 12A , the second photometric sensor  626  are divided into 29 photocells P 01  to P 45  arranged like a swage block, each photocell having a hexagonal shape. 
   Also,  FIG. 12B  is a view showing three focus detecting positions S 31  to S 33  in the second embodiment, in which the focus detecting position S 31  corresponds to the photocell P 23  of the photometric sensor  626  to make the focus detection, the focus detecting position S 32  corresponds to the photocell P 24  of the photometric sensor  626  to make the focus detection, and the focus detecting position S 33  corresponds to the photocell P 25  of the photometric sensor  626  to make the focus detection. Other constitution is not different from the first embodiment as shown in  FIG. 1  or  5 . Also, the overall flowchart as shown in  FIG. 6  in the first embodiment is employed in the first embodiment. 
   A part of the exposure operation as shown in  FIG. 7  is different from the first embodiment, and will be described below.
     ( 151 ) Input the lens information necessary for exposure operation from the lens control means  51  and correct the digital brightness data obtained from the photocells P 01  to P 45  of the second photometric sensor  626  at step ( 108 ). The brightness data corresponding to photocells P 01  to P 45  after correction are referred to as E 01  to E 45 , respectively. Moreover, the attitude information of the camera is acquired by inputting the information of the attitude detecting sensor  45 .   ( 152 ) Calculate the one-dimensional data Y 1  to Y 5  and X 1  to X 7  based on the corrected brightness data E 01  to E 45  of the photocells. When the one-dimensional data Y 1  to Y 5  are calculated, the calculation range of one-dimensional data is changed according to the information of the area where the focus detection is made at step ( 104 ). A specific calculation method will be described.   

   For the one-dimensional data X 1  to X 7 , the calculation method is fixed, irrespective of the focus detecting position information.
 
 X   1 =( E   21 ×2+ E   11 + E   31 )+4 
 
 X   2 =( E   01 ×2+ E   22 ×2+ E   41 ×2+ E   11 + E   12 + E   31 + E   32 )+10 
 
 X   3 =( E   02 ×2+ E   23 ×2+ E   42 ×2+ E   12 + E   13 + E   32 + E   33 )+10 
 
 X   4 =( E   03 ×2+ E   24 ×2+ E   43 ×2+ E   13 + E   14 + E   33 + E   34 )+10 
 
 X   5 =( E   04 ×2+ E   25 ×2+ E   44 ×2+ E   14 + E   15 + E   34 + E   35 )+10 
 
 X   6 =( E   05 ×2+ E   26 ×2+ E   45 ×2+ E   15 + E   16 + E   35 + E   36 )+10 
 
 X   7 =( E   27 ×2+ E   16 + E   36 )+4 
 
   For the one-dimensional data Y 1  to Y 5 , the calculation data range is changed depending on the focus detecting position information in the following way. Since the focus detecting position is fundamentally considered to be the position of the principal subject, the exposure operation is made by weighting the 5×5 area including the position of principal subject as the principal photometric area with higher value or feature. 
   When the focus detecting position is S 31  in  FIG. 12B ,
 
 Y   1 =( E   01 + E   02 + E   03 + E   04 + E   05 )+5 
 
 Y   2 =( E   11 + E   12 + E   13 + E   14 + E   15 )+5 
 
 Y   3 =( E   21 + E   22 + E   23 + E   24 + E   25 )+5 
 
Y 4 =(E 31 +E 32 +E 33 +E 34 +E 35 )+5 
 
 Y   5 =( E   41 + E   42 + E   43 + E   44 + E   45 )+5 
 
   When the focus detecting position is S 32  in  FIG. 12B ,
 
 Y   1 =( E   01 + E   02 + E   03 + E   04 + E   05 )+5 
 
 Y   2 =( E   11 ×0.5 +E   12 + E   13 + E   14 + E   15 + E   16 ×0.5)+5 
 
 Y   3 =( E   22 + E   23 + E   24 + E   25 + E   26 )+5 
 
 Y   4 =( E   31 ×0.5 +E   32 + E   33 + E   34 + E   35 + E   36 ×0.5)+5 
 
 Y   5 =( E   41 + E   42 + E   43 + E   44 + E   45 )+5 
 
   When the focus detecting position is S 33  in  FIG. 12B ,
 
 Y   1 =( E   01 + E   02 + E   03 + E   04 + E   05 )+5 
 
 Y   2 =( E   12 + E   13 + E   14 + E   15 + E   16 )+5 
 
 Y   3 =( E   23 + E   24 + E   25 + E   26 + E   27 )+5 
 
 Y   4 =( E   32 + E   33 + E   34 + E   35 + E   36 )+5 
 
 Y   5 =( E   41 + E   42 + E   43 + E   44 + E   45 )+5 
     ( 153 ) Calculate the average brightness value Ea over the entire screen in which the area farther away from the focus detecting position is weighted with lower value by emphasizing the focus detecting position according to the information of focus detecting position.   

   When the focus detecting position is S 31  in  FIG. 12B ,
 
 Ea={X   1 ×2+( X   2 + X   3 + X   4 + X   5 )×5 +X   6 ×2.5+ X   7 + E   23 ×3.5}+29 
 
   When the focus detecting position is S 32  in  FIG. 12B ,
 
 Ea={ ( X   2 + X   3 + X   4 + X   5 + X   6 )×5 +X   1 + X   7 + E   24 ×2}+29 
 
   When the focus detecting position is S 33  in  FIG. 12B ,
 
 Ea ={( X   3 + X   4 + X   5 + X   6 )×5+ X   7 ×2+ X   2 ×2.5+ X   1 + E   25 ×3.5}+29 
     ( 154 ) Detect the maximum value among the one-dimensional data Y 1  to Y 5  and X 1  to X 7 . The maximum value is denoted as Eh. When Eh is greater than a predetermined value, the high brightness correction value γ is calculated. The relationship between the maximum value Eh and the high brightness correction value γ is indicated by the line  81  in  FIG. 9  as in the first embodiment. If the high brightness correction value γ is calculated, the average brightness value Ea calculated at previous step is corrected for high brightness to obtain Ea(γ).
 
 Ea (γ)=Ea−γ
   

   When the maximum value among the one-dimensional data Y 1  to Y 5  and X 1  to X 7  satisfies the following condition, the value indicated by the line  81  of  FIG. 9  that is multiplied by 0.5 is defined as y. 
   The condition includes when the focus detecting position is S 31  in FIG.  12 B and the maximum value among the one-dimensional data is X 6  or X 7 , when the focus detecting position is S 32  in FIG.  12 B and the maximum value among the one-dimensional data is X 1  or X 7 , or when the focus detecting position is S 33  in FIG.  12 B and the maximum value among the one-dimensional data is X 1  or X 2 . In the case where the maximum value detecting area has the lower weight in calculating the average brightness value Ea at step ( 153 ) as in the first embodiment, the high brightness correction value γ is reduced.
     ( 155 ) Calculate a gradient value ΔE of brightness in the top and bottom direction within the photographing screen on the basis of the calculated projection data, camera attitude data and focus detecting position information.   

   When the camera attitude is in the transverse normal position, the gradient value ΔE of brightness is calculated in accordance with the following expression.
 
Δ E ={( Y   1 + Y   2 )/2−( Y   4 + Y   5 )/2}/3 
 
   When the camera attitude is in the longitudinal position, photocell P 21  of the photometric sensor  626  is on the top side of the screen, and the photocell P 27  is on the bottom side of the screen, the gradient value ΔE of brightness is calculated on the basis of the focus detecting position information. 
   When the focus detecting position is S 31  in  FIG. 12B ,
 
 ΔE={ ( X   1 + X   2 )/2−( X   4 + X   5 )/2}/3 
 
   When the focus detecting position is S 32  in  FIG. 12B ,
 
Δ E ={( X   2 + X   3 )/2−( X   5 + X   6 )/2}/3 
 
   When the focus detecting position is S 33  in  FIG. 12B ,
 
 ΔE ={( X   3 + X   4 )/2−( X   6 + X   7 )/2}/3 
 
   When the camera attitude is in the longitudinal position, the photocell P 27  of the photometric sensor  626  is on the top side of the screen, and the photocell P 21  is on the bottom side of the screen, the gradient value ΔE of brightness is calculated on the basis of the focus detecting position information in accordance with the following expression. 
   When the focus detecting position is S 31  in  FIG. 12B ,
 
Δ E ={( X   4 + X   5 )/2−( X   1 + X   2 )/2}/3 
 
   When the focus detecting position is S 32  in  FIG. 12B ,
 
 ΔE ={( X   5 + X   6 )/2−( X   2 + X   3 )/2}/3 
 
   When the focus detecting position is S 33  in  FIG. 12B ,
 
Δ E ={( X   6 + X   7 )/2−( X   3 + X   4 )/2}/3 
     ( 156 ) Calculate a deviation ΔEs between the calculated average brightness value Ea(γ) corrected for high brightness and the brightness value Es at the focus detecting position.
 
Δ Es=Ea (γ)− Es  
   

   Herein, Es is equal to E 23  (when the focus detecting position is S 31 ), E 24  (when the focus detecting position is S 32 ), or E 25  (when the focus detecting position is S 33 ).
     ( 157 ) Discriminate whether or not the flash means  47  should be employed to photograph the scene. The condition for employing the flash means  47  to photograph the scene is either the first condition that the calculated average brightness value Ea(γ) corrected for high brightness is lower than a predetermined brightness (e.g., below 5 in Bv value) or the second condition that the calculated gradient value ΔE of brightness and the deviation ΔEs are positive with their relation being outside the line  82  as represented in  FIG. 11  (hatched portion). When any of the gradient value ΔE of brightness and the deviation ΔEs is negative, the flash is not produced.   

   Also, when it is obvious that the distance to the subject is far and the excellent photographed results are not obtained with the light quantity of the provided flash means  47  as a result of focusing the photographing lens at step ( 105 ), the flash means  47  may not be employed, even if the first or second condition is satisfied. Also, when the camera is not in the photographing mode for automatically employing the flash means, it is unnecessary to perform this step. When it is discriminated that the flash means  47  should be employed to photograph the scene, the flash means  47  is prepared to produce the flash.
     ( 158 ) Calculate the exposure compensation value a when a predetermined condition is satisfied.   

   When Ea(γ)&gt;Es and the flash means is not employed as the condition,
 
α={ Es−Ea (γ)}×0.5. 
 
   When Ea(γ)&lt;Es and Ea(γ)&lt;0 as the condition,
 
α={ Es−Ea (γ)}×0.25 
 
Under other conditions, α=0.
     ( 159 ) Calculate the subject brightness value for exposure control as Ee=Ea(γ)+α. The optimal exposure control factors, namely the shutter speed and the stop value, are decided on the basis of the subject brightness value Ee and the presence or absence of employing the flash means  47  that is discriminated at step ( 157 ).   

   The description of the second embodiment is thus completed. 
   Though in the above embodiments the camera for reproducing images on the photographic film is employed, this invention is also applicable to a so-called video camera or electronic still camera in which the photoelectric conversion element such as CCD converts the image information in the field into electric signal which is then output or processed. When this invention is applied to the video camera or electronic still camera, the photometric sensor may not be specifically provided, in which the brightness information of the subject is acquired from the photoelectric conversion element for image pickup, and the acquired brightness information is converted into one-dimensional brightness data to discriminate the back light. 
   Further, in transforming two-dimensional brightness information into one-dimensional brightness information, it is well known that a method for reading out an output signal of the photoelectric conversion sensor having a plurality of photocells arranged in two dimensions and enabling the microcomputer to make the arithmetical operation on the output signal by software as described in the embodiments, and a method employing a processing circuit integrated on the same chip as the photoelectric conversion sensor to make the conversion by hardware are provided. 
   The present invention is not limited to the above embodiments and various changes and modifications can be made within the spirit and scope of the present invention. Therefore, to apprise the public of the scope of the present invention, the following claims are made.