Method and apparatus for controlling exposure of camera

• US 5,774,747 A
• Filed: 04/04/1997
• Issued: 06/30/1998
• Est. Priority Date: 06/09/1994
• Status: Expired due to Fees

• Current Assignee: Fujifilm Corporation (Fujifilm Holdings Corporation)
• Original Assignee: Fuji Photo Film Co Limited (Fujifilm Holdings Corporation)
• Inventors: Nakagawa, Norifumi, Naruse, Mutsumi, Kurosu, Tomio, Kobayashi, Kouichi, Sakamoto, Tetsuo, Kohno, Takanori, Tongu, Teruyuki, Ishihara, Fumio
• Primary Examiner(s): Perkey, W. B.
• Application Number:
  US08/833,004
• Time in Patent Office: 452 Days
• Field of Search: 396/61, 396/161, 396/165, 396/166, 396/168, 396/187
• US Class Current: 396/61
• CPC Class Codes: G03B 7/09979 Multi-zone light measuring
• CPC Class Codes: G03B 7/28 Circuitry to measure or to take account of the object contrast

Abstract

An exposure control method and apparatus for a camera having a two-area divided brightness measurement system. A central light value is determined based on a central area brightness of a photographic scene, and a peripheral light value is determined based on a peripheral area brightness of the scene. After determining by comparison between the central and peripheral light values whether the scene is back-lighted or front-lighted, a correction coefficient specific to back-lighted scenes or that specific to front-lighted scenes is read from a memory. An exposure value suitable for a main subject is calculated according to the following equation:

Es=log.sub.2 {1-(1-α).sub.2.sup.LVb-LVa }-log.sub.2 α+LVa

wherein Es represents the exposure value and α represents the correction coefficient.

Claims

1. An exposure control method for a camera using a central photometric element for detecting a central light value from a central area of a scene to be photographed and a peripheral photometric element for detecting a peripheral light value from a peripheral area of said scene, said method comprising the steps of:

A. judging depending on a difference between said central and peripheral light values whether said scene is back-lighted or front-lighted;

B. calculating a central exposure value proper for said central area of said scene from said central and peripheral light values using a first equation and a first coefficient;

C. calculating a peripheral exposure value proper for an entire area of said scene from said central and peripheral light values using a second equation and a second coefficient;

D. selecting values of said first and second coefficients depending on a result of judging step A, among from values predetermined for front-lighted scenes and values predetermined for back-lighted scenes;

E. determining an effective exposure value based on said peripheral exposure value and a bias value added to said peripheral exposure value to bias said effective exposure value toward under-exposure side;

F. driving a shutter device in accordance with said effective exposure value so that an amount of ambient light contributing to film exposure is lacking for a proper exposure; and

G. projecting artificial light from a flash device in synchronism with the driving of said shutter device to compensate for lack of said ambient light, wherein percentage of said artificial light to be contributed to film exposure is determined depending on a difference between said effective exposure value and said central exposure value when said scene is judged to be back-lighted.

2. An exposure control method as claimed in claim 1, wherein percentage of artificial light to be contributed to film exposure is determined depending up a difference between said effective exposure value and said peripheral exposure value when said scene is judged to be front-lighted.

3. An exposure control method as claim in claim 2, wherein determining step E comprises the steps of:

comparing an exposure value calculating from said peripheral exposure value and said bias value with a predetermined limit exposure value below which hand-shake can occur; and

selecting said limit exposure value as said effective exposure value when said calculated exposure value is less than said limit exposure value.

4. An exposure control method as claimed in claim 3, wherein said bias value is set to zero when a main subject of said scene is disposed beyond a predetermined subject distance range.

5. An exposure control method as claimed in claim 4, wherein said bias value is variable depending on a density control signal which may be manually inputted.

6. An exposure control method as claimed in claim 2, wherein said shutter is a program shutter of which both aperture size and exposure time are able to be controlled, and said exposure control method further comprises the steps of:

determining a flash start time to start firing said flash device in relation to the aperture size of said program shutter, said flash start time being changed depending on subject distance;

determining a light control level based on said percentage of artificial light to be contributed to film exposure and said flash start time;

detecting artificial light projected from said flash device and reflected from a subject to be photographed to obtain an integral light amount from said flash start time;

comparing said integral light amount with said light control level; and

terminating flash firing when said integral light amount reaches said light control level.