Photography – Flash system responsive to focal length or distance
Reexamination Certificate
2003-10-07
2004-08-17
Perkey, W. B. (Department: 2851)
Photography
Flash system responsive to focal length or distance
C396S158000, C396S159000
Reexamination Certificate
active
06778769
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a camera having a flash emission device which emits flashing light and used to take a picture of a subject.
2. Description of the Related Art
Many cameras have a flash emission device which emits flashing light toward a subject. From such cameras, flashing light is emitted in synchronization with a picture-taking operation. When this flash photography is performed, the aperture to be set when a picture is taken is determined on the basis of the luminance of field, film speed information about a loaded film, etc., and the amount of flashing light is adjusted according to the aperture. Thus, the amount of light applied to the film is optimized by adjusting the amount of flashing light.
Methods of adjusting the amount of light include a method of adjusting the amount of light through the time period during which flashing light is emitted.
In a camera having a flash emission device using this method, however, a phenomenon occurs in which if the temperature of a certain portion decreases after adjustment of the amount of flashing light through the control of emission time, the amount of emitted light becomes smaller than a target amount, and in which if the temperature rises, the amount of emitted light becomes larger than the target amount. When this phenomenon occurs, there is a possibility of failure to obtain the desired exposure.
This is because the internal resistance of a capacitor provided in the flash emission device and used for emitting flashing light, etc., change when the temperature changes.
A camera has therefore been proposed in which temperature is detected and flashing light is emitted for an emission time period selected by considering the detected temperature information (see, for example, Patent Document 1).
In the proposed camera, a reference table in which predetermined emission time periods are set with respect to predetermined temperature ranges and amounts of flashing light determined according to distance information and film speed information. In this camera, an emission time period determined by considering temperature information as well as other kinds of information is obtained from the reference table and, therefore, the target amount of flashing light can be emitted with stability.
Patent Document 2 also proposes a related technique, which discloses to a focal-point-detecting preliminary lighting device for emitting auxiliary flashing light when detecting a distance to a subject. Specifically, this document discloses a technique for variably setting the amount of emitted light at the time of measurement by detecting a charging voltage during emission of flashing light for emitting auxiliary light toward the subject.
Also, there is a possibility of stoppage of flashing due to a reduction in voltage across a main capacitor when a focal plane shutter is made to travel. A camera is known in which such stoppage of flashing is prevented in such a manner that the luminous intensity of emitted flashing light is made constant over the exposure time and the amount of emitted light is uniformly applied to the entire film, although the amount of light becomes slightly short (see Patent Document 3).
[Patent Document 1]
Japanese Patent Laid-Open No. 2002-72311
[Patent Document 2]
Japanese Patent Laid-Open No. 7-92371
[Patent Document 3]
Japanese Patent Laid-Open No. 7-120813
In the camera proposed in Patent Document 1, emission of a target amount of flashing light may be stabilized with high accuracy in such a manner that the above-mentioned temperature ranges are further divided into narrower temperature ranges and different flashing times are set in a one-to-one relationship with the narrower temperature ranges. However, such subdividing of the temperature ranges is limited due to memory capacity.
Some cameras have a red-eye prevention mode which is provided for prevention or reduction of red-eye flash photography and in which a preliminary flash is generated immediately before taking a picture and a main flash is thereafter generated in synchronization with a picture-taking operation. When flash photography is performed by setting the red-eye prevention mode in this type of camera, the voltage across a main capacitor is reduced by generation of a preliminary flash. Therefore it is difficult to set a suitable amount of light for a main flash.
Patent Document 1 shown above discloses a technique for correcting the emission time according to the temperature inside the camera. However, this technique is irrelevant to correction of variation in the amount of light due to a reduction in the voltage across the main capacitor.
According to Patent Document 2 shown above, the amount of emitted auxiliary light is changed by detecting the voltage across a capacitor in a flash emission means. However, this is a technique for roughly changing the amount of emitted light based on comparison of the voltage across the capacitor with a plurality of threshold values. This technique cannot be used for flash photography.
According to Patent Document 3, the voltage across a main capacitor in a flash is also detected for correction, and the luminous intensity of emitted light is controlled so that emission of light from the flash is maintained during the emission time determined by the curtain speed and the shutter speed of a focal plane shutter, while a slight deficiency of the amount of exposure is allowed. A suitable amount of light during main light emission cannot be obtained by using this technique.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, an object of the present invention is to provide a camera devised in such a manner that flash emission time periods are set in correspondence with small differences in temperature to maintain with high accuracy a target amount of flashing light to be emitted.
Another object of the present invention is to provide a camera capable of optimizing the amount of emitted flashing light in a main flash even in a case where preliminary light emission and main light emission are continuously performed.
To achieve the above-described objects, according to a first aspect of the present invention, there is provided a camera which takes a picture of a subject, the camera including a flash emission device which emits flashing light in synchronization with a picture-taking operation, and which controls the amount of emitted light by selecting an emission time period, the flash emission device having a temperature sensor which detects temperature, a storage section which stores a computation expression by which an emission time period for obtaining a predetermined amount of emitted light is obtained by using temperature as a variable, and an emission control section which obtains an emission time period from the temperature obtained from the temperature sensor on the basis of the computation expression, and which controls the emission time period so that light is emitted only for the obtained emission time period.
In the camera according to the first aspect of the present invention, the target amount of emitted light is ensured by making the flash emission device emit light for the emission time period obtained by the computation expression using the temperature as a variable. That is, in the camera according to the first aspect of the present invention, constants of the computation expression can be stored in a memory. That is, in the camera according to the first aspect of the present invention, if constants of the computation expression are stored in a memory, flash emission time periods corresponding to small differences in temperature can be set by using the detected temperature as a variable. Therefore the camera according to the first aspect of the present invention is capable of maintaining the target amount of emitted flashing light with improved accuracy in comparison with the conventional camera using the same memory capacity.
Preferably, the computation expression is represented by an expression includ
Fuji Photo Film Co. , Ltd.
Perkey W. B.
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