Exposure control method and imaging apparatus

Abstract

A digital camera automatically changes its imaging sensitivity based on a proper exposure value that is calculated at a predetermined standard imaging sensitivity based on subject brightness measured from a photometry area of a scene to shoot. When the decided imaging sensitivity is higher than a predetermined sensitivity threshold value, the scene is judged to be a low brightness backlit scene that contains a higher brightness portion around a dark main subject, and a lower brightness portion around the higher brightness portion. A rear light judgment value is calculated as a difference between a central photometric value and a peripheral photometric value of a photometry area, and is compared with a predetermined rear light threshold value, to judge whether the scene is backlit or not. When the scene is judged to be a backlit scene or a low brightness backlit scene, the digital camera emits a flash light synchronously with an exposure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the present invention will be more apparent from the following detailed description of the preferred embodiments when read in connection with the accompanied drawings, wherein like reference numerals designate like or corresponding parts throughout the several views, and wherein:

FIGS. 1A and 1B are front and rear perspective views of a digital camera embodying an imaging apparatus of the present invention;

FIG. 2 is a block diagram illustrating the internal structure of the digital camera of FIG. 1;

FIG. 3 is a program chart for an automatic exposure control operation including a low brightness rear light judgment by an imaging sensitivity decided for the scene;

FIG. 4 is a flow chart illustrating the automatic exposure control operation including the low brightness rear light judgment by the imaging sensitivity;

FIG. 5 is an explanatory diagram illustrating a sequence for a flash shot;

FIG. 6 shows timing charts illustrating the timing of flashing for the flash shot;

FIG. 7 is a program chart for an automatic exposure control operation including a low brightness rear light judgment by a calculated proper exposure value;

FIG. 8 is a flow chart illustrating the automatic exposure control operation including the low brightness rear light judgment by the proper exposure value;

FIG. 9 is a program chart for an automatic exposure control operation including a low brightness rear light judgment using a different threshold value from that used for discriminating an ordinary backlit scene;

FIG. 10 is a flow chart illustrating the automatic exposure control operation according to the program chart of FIG. 9;

FIG. 11 is a program chart for a conventional automatic exposure control operation;

FIGS. 12A and 12B are explanatory diagrams illustrating a photometry area used for a rear light judgment;

FIGS. 13A and 13B are explanatory diagrams illustrating an example of a backlit scene and an example of a low brightness backlit scene, respectively; and

FIG. 14 is a program chart for an automatic exposure control operation in a conventional sensitivity changeable digital camera.

Claims

1. An exposure control method for a sensitivity changeable imaging apparatus with a flash device, comprising steps of: measuring subject brightness from a photometry area of a scene to shoot;calculating based on the measured subject brightness a proper exposure value at a predetermined standard imaging sensitivity;deciding based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity; andcomparing the decided imaging sensitivity with a predetermined sensitivity threshold value, to decide that said flash device is to emit a flash light when the decided imaging sensitivity is higher than the predetermined sensitivity threshold value.

2. An exposure control method as claimed in claim 1, wherein the scene to shoot is judged to be a low brightness backlit scene when the decided imaging sensitivity is higher than the predetermined sensitivity threshold value, wherein said low brightness backlit scene contains a higher brightness portion around a dark main subject, and a lower brightness portion around the higher brightness portion.

3. An exposure control method as claimed in claim 2, wherein the sensitivity threshold value is predetermined to be equal to the standard imaging sensitivity.

4. An exposure control method as claimed in claim 1, further comprising steps of: calculating a rear light judgment value from the measured subject brightness;judging whether the scene is a backlit scene or not by comparing the rear light judgment value with a predetermined rear light threshold value; anddeciding that said flash device is to emit a flash light when the scene is judged to be the backlit scene.

5. An exposure control method as claimed in claim 4, wherein said step of calculating the rear light judgment value comprises steps of: calculating a mean photometric value of a central zone of said photometry area and a mean photometric value of a peripheral zone of said photometry area; andsubtracting the mean photometric value of the central zone from the mean photometric value of the peripheral zone, to obtain a difference as the rear light judgment value.

6. An exposure control method as claimed in claim 1, wherein said step of deciding the shutter speed and the imaging sensitivity based on the proper exposure value comprises steps of: comparing the shutter speed decided according to the proper exposure value with a predetermined hand-shake warning threshold value; andchanging the imaging sensitivity to a value higher than the standard sensitivity and the shutter speed to a predetermined hand-shake limit shutter speed or a value higher than the hand-shake limit shutter speed, when the decided shutter speed is slower than the hand-shake warning threshold value.

7. An exposure control method as claimed in claim 6, wherein if the imaging sensitivity is changed to a maximum value and the decided shutter speed is slower than the hand-shake warning threshold value, the shutter speed is changed to the hand-shake limit value, and the flash device is decided to emit a flashlight.

8. An exposure control method as claimed in claim 7, wherein the hand-shake warning value is such that hand-shake is likely to occur at a shutter speed slower than the hand-shake warning value, and the hand-shake limit value is a slowest limit shutter speed at which the hand-shake does not normally occur.

9. An exposure control method for a sensitivity changeable imaging apparatus with a flash device, comprising steps of: measuring subject brightness from a photometry area of a scene to shoot;calculating based on the measured subject brightness a proper exposure value at a predetermined standard imaging sensitivity;deciding based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity; andcomparing the calculated proper exposure value with a predetermined threshold exposure value, to decide that said flash device is to emit a flash light when the calculated proper exposure value is lower than the predetermined threshold exposure value.

10. An exposure control method as claimed in claim 9, wherein the scene is judged to be a low brightness backlit scene when the calculated proper exposure value is lower than the predetermined threshold exposure value, wherein said low brightness backlit scene contains a higher brightness portion around a dark main subject, and a lower brightness portion around the higher brightness portion.

11. An exposure control method as claimed in claim 9, further comprising steps of: calculating a rear light judgment value from the measured subject brightness;judging whether the scene is a backlit scene or not by comparing the rear light judgment value with a predetermined rear light threshold value; anddeciding that said flash device is to emit a flash light when the scene is judged to be the backlit scene.

12. An exposure control method as claimed in claim 11, wherein said step of calculating the rear light judgment value comprises steps of: calculating a mean photometric value of a central zone of said photometry area and a mean photometric value of a peripheral zone of said photometry area; andsubtracting the mean photometric value of the central zone from the mean photometric value of the peripheral zone, to obtain a difference as the rear light judgment value.

13. An exposure control method as claimed in claim 9, wherein said step of deciding the shutter speed and the imaging sensitivity based on the proper exposure value comprises steps of: comparing the decided shutter speed with a predetermined hand-shake warning threshold value; andchanging the imaging sensitivity to a value higher than the standard sensitivity and the shutter speed to a predetermined hand-shake limit shutter speed or a value higher than the hand-shake limit shutter speed, when the decided shutter speed is slower than the hand-shake warning threshold value.

14. An exposure control method as claimed in claim 13, wherein if the imaging sensitivity is changed to a maximum value when the decided shutter speed is slower than the hand-shake warning threshold value, the shutter speed is changed to the hand-shake limit value, and the flash device is decided to emit a flash light.

15. An exposure control method as claimed in claim 14, wherein the hand-shake warning value is such that hand-shake is likely to occur at a shutter speed slower than the hand-shake warning value, and the hand-shake limit value is a slowest limit shutter speed at which the hand-shake does not normally occur.

16. An exposure control method for a sensitivity changeable imaging apparatus with a flash device, comprising steps of: measuring subject brightness from a photometry area of a scene to shoot;calculating a rear light judgment value from the measured subject brightness;calculating based on the measured subject brightness a proper exposure value at a predetermined standard sensitivity;deciding based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity;comparing the calculated proper exposure value with a predetermined threshold exposure value;comparing the rear light judgment value with a predetermined first rear light threshold value to judge whether the scene is backlit or not, when the calculated proper exposure value is not less than the predetermined threshold exposure value;comparing the rear light judgment value with a predetermined second rear light threshold value to judge whether the scene is backlit or not, when the calculated proper exposure value is less than the predetermined threshold exposure value; anddeciding that said flash device is to emit a flash light when the scene is judged to be a backlit scene.

17. An exposure control method as claimed in claim 16, wherein the scene is not judged to be a low brightness backlit scene when the calculated proper exposure value is not less than the threshold exposure value, wherein said low brightness backlit scene is a scene where there is a higher brightness portion around a dark main subject, and a lower brightness portion around the higher brightness portion.

18. An exposure control method as claimed in claim 17, wherein said step of calculating the rear light judgment value comprises steps of: calculating a mean photometric value of a central zone of said photometry area and a mean photometric value of a peripheral zone of said photometry area; andsubtracting the mean photometric value of the central zone from the mean photometric value of the peripheral zone, to obtain a difference as the rear light judgment value.

19. An exposure control method as claimed in claim 18, wherein the scene is not judged to be a backlit scene when the rear light judgment value is less than the compared rear light threshold value.

20. An exposure control method as claimed in claim 19, wherein the first rear light threshold value is Δ2EV, and the second rear light threshold value is Δ0.5EV.

21. An exposure control method as claimed in claim 16, wherein said step of deciding the shutter speed and the imaging sensitivity based on the proper exposure value comprises steps of: comparing the decided shutter speed with a predetermined hand-shake warning threshold value; andchanging the imaging sensitivity to a value higher than the standard sensitivity and the shutter speed to a predetermined hand-shake limit value or a value higher than the hand-shake limit value, when the decided shutter speed is slower than the hand-shake warning threshold value.

22. An exposure control method as claimed in claim 21, wherein if the imaging sensitivity is changed to a maximum value when the decided shutter speed is slower than the hand-shake warning threshold value, the shutter speed is changed to the hand-shake limit value, and the flash device is decided to emit a flash light.

23. An exposure control method as claimed in claim 22, wherein the hand-shake warning value is a shutter speed below which the hand-shake is likely to occur, and the hand-shake limit value is a shutter speed at which the hand-shake does not normally occur.

24. An imaging apparatus comprising: a flash device for emitting a flash light toward a subject;an imaging device for converting an optical image of the subject into electric image signal;a shutter speed changing device;a sensitivity changing device for changing imaging sensitivity of said imaging device by changing gain of amplification of the electric image signal; andan exposure control device that measures brightness of the subject from a photometry area, calculates based on the measured subject brightness a proper exposure value at a predetermined standard imaging sensitivity, decides based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity, compares the decided imaging sensitivity with a predetermined threshold value, and decides that said flash device is to emit a flash light when the decided imaging sensitivity is higher than the predetermined threshold value.

25. An imaging apparatus comprising: a flash device for emitting a flash light toward a subject;an imaging device for converting an optical image of the subject into electronic image signal;a shutter speed changing device;a sensitivity changing device for changing imaging sensitivity of said imaging device by changing gain of amplification of the electric image signal; andan exposure control device that measures brightness of the subject from a photometry area, calculates based on the measured subject brightness a proper exposure value at a predetermined standard imaging sensitivity, decides based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity, compares the calculated proper exposure value with a predetermined threshold value, and decides that said flash device is to emit a flash light when the calculated proper exposure value is less than the predetermined threshold value.

26. An imaging apparatus comprising: a flash device for emitting a flash light toward a subject;an imaging device for converting an optical image of the subject into electric image signal;a shutter speed changing device;a sensitivity changing device for changing imaging sensitivity of said imaging device by changing gain of amplification of the electric image signal; andan exposure control device that measures brightness of the subject from a photometry area, calculates based on the measured subject brightness a proper exposure value at a predetermined standard imaging sensitivity, decides based on the calculated proper exposure value at least a shutter speed and an imaging sensitivity, calculates a rear light judgment value by subtracting a mean photometric value of a central area of said photometry area from a mean photometric value of a peripheral area of said photometry area, compares the calculated proper exposure value with a predetermined threshold exposure value, to compare the rear light judgment value with a predetermined first rear light threshold value when the calculated proper exposure value is not less than the predetermined threshold exposure value, or with a predetermined second rear light threshold value when the calculated proper exposure value is less than the predetermined threshold exposure value, and decides that said flash device is to emit a flash light when the rear light judgment value is not less than the compared rear light threshold value.