In conventional image sensing apparatuses, to obtain proper exposures, many brightness adjusting techniques of emitting auxiliary light to an object in advance and controlling, for example, the amount of auxiliary light emitted or the emission time have been proposed.
A typical brightness adjusting technique in a conventional image sensing apparatus will be described with reference to FIG. 2.
Referring to FIG. 2, reference numerals 150 and 152 denote imaging optical systems for forming an object image on an imaging plane.
Reference numeral 151 denotes a stop interposed between the imaging optical systems 150 and 152. This stop is generally disposed in an afocal (parallel light) range.
Reference numeral 161 denotes a photoelectric conversion element for converting the amount of light imaged into a quantity of electricity or charge. As this element, for example, an image sensing element such as a CCD (Charge-Coupled Device) is used.
Reference numeral 162 denotes a camera signal processing circuit for converting a sensed signal into, for example, a standard video signal on the basis of the quantity of electricity obtained from the photoelectric conversion element 161.
Reference numeral 13 denotes a recording device for recording the video signal having undergone signal processing in the camera signal processing circuit 162 as a photographing signal.
Reference numeral 1 denotes a detection means for generating a detection signal used for exposure control by performing detection processing such as integral processing for the luminance component of the video signal having undergone the signal processing in the camera signal processing circuit 162.
Reference numeral 2 denotes a predetermined reference value serving as a reference for exposure control, which is a voltage or charge generating means.
Reference numeral 3 denotes a comparing means for comparing the detection signal with the reference value 2 and outputting a signal corresponding to the comparison result.
Reference numeral 4 denotes a flash control means for controlling the amount of light emitted on the basis of the comparison result obtained by the comparing means 3.
Reference numeral 5 denotes a flash unit 5 for emitting auxiliary light; and 6, an object.
The operation of the above prior art will be described next with reference to the flow chart of FIG. 3.
The steps in this flow chart will be sequentially described below.
Step S1: This step is the beginning of the flow, which starts in synchronism with, for example, the operation of an imaging start switch.
Step S2: It is checked whether the flash unit 5 is completely charged. If the flash unit is not completely charged, the flow waits until charging is completed.
Step S3: Preliminary emission is performed in a predetermined light amount by using the flash unit 5.
Step S4: At the same time the flash unit 5 emits light, a sensed image is converted into a captured image signal.
Step S5: The detection means 1 is used to generate a detection signal used for exposure control from the sensed signal.
Step S6: The flash control means 4 determines an exposure level from the detection signal. If the exposure is proper, the flow advances to step S6. If an underexposure is determined, the flow advances to step S7. If an overexposure is determined, the flow advances to step S8.
Step S7: Preparation for emission is done in the same light amount as that in the preceding emission, and the flow advances to step S10.
Step S8: Preparation for emission is done by increasing the amount of light emitted in accordance with the preceding underexposure. The flow then advances to step S10.
Step S9: Preparation for emission is done by decreasing the amount of light emitted in accordance with the preceding overexposure. The flow then advances to step S10.
Step S10: It is checked whether the flash unit 5 is completely charted. If the flash unit 5 is not completely charged, the flow waits until changing is completed.
Step S11: At the same time the flash unit 5 emits light, a sensed image is converted into an image signal by the camera signal processing circuit 162, and the signal is recorded on the recording device 13.
Step S12: This flow is terminated.
With the above operations, proper exposure can be performed in photographing operation with emission of auxiliary light.
As described above, preliminary emission of the flash unit is performed in a predetermined light amount, and main emission is determined by the flash control means 4 on the basis of the level difference between the detection signal obtained in the preliminary emission and the reference value 2.
According to the arrangement of the conventional image sensing apparatus, however, exposure control is performed with reference to the detection output obtained upon preliminary emission of auxiliary light. According to this exposure adjustment, although it depends on the scheme used by the detection means and its characteristics, in the case of average light measurement which is a general detection technique using an integrator for averaging the overall brightness of a frame, if a main object occupies a high proportion of an imaging range, a proper exposure state can be obtained. In contrast to this, if the main object occupies a low proportion of the imaging range, or the background is located far away from the object or blackish, the above integral detection output is greatly influenced by the background, and a proper exposure state may not be obtained for the object.
Consider the object condition shown in FIG. 4. Referring to FIG. 4, reference numeral 6 denotes a main object to be photographed by a photographer; 21, a tree in the distant background; and 22, the photographing direction of the photographer. Assume that the object is in an illumination condition that requires auxiliary light.
FIGS. 5b to 5d show the images sensed by the image sensing apparatus in the object condition shown in FIG. 4.
FIGS. 5a and 5b show the sensed image of the object 6 as a single person, whereas FIGS. 5c and 5d indicate the sensed image of the object 6 including two persons exhibiting nearly equal reflectances and located at the same distance from the image sensing apparatus.
FIGS. 5b and 5d schematically show the degrees of illumination on the objects at the time of emission of the flash unit in brightness. As an object is located closer to the photographer (the image sensing apparatus including the flash unit), the object becomes brighter; reflected light from the tree 6 in the background or an object in the more distance background becomes very dark.
Consider an exposure level based on a comparison between the images shown in FIGS. 5b and 5d. Assuming that integral detection is performed by the detection means in the prior art described above, the detection output in FIG. 5b is smaller than that in FIG. 5d. This is because the main object in FIG. 5d at the short distance occupies a higher proportion of a detection area 25.
In controlling the main emission amount with reference to preliminary emission, therefore, although the object distances in FIGS. 5b and 5d are the same, since the detection output at the time of preliminary emission of auxiliary light in FIG. 5b becomes smaller than that in FIG. 5d, the emission amounts obtained from the above results at the time of main emission have the relationship represented by “amount in FIG. 5b>amount in FIG. 5d”. 
As described above, if a detection scheme like the above integral detection scheme is used, control is performed to set the sum total of luminance signals in the detection area 25 to a predetermined level. If, therefore, an object occupies a low proportion of the overall angle of view as shown in FIGS. 5a and 5b, and the background occupies a high proportion, since the output from the detection means based on integral detection becomes small, the flash control means 4 determines an underexposure and controls the amount of auxiliary light emitted. As a consequence, proper brightness control is not performed on the object, and an overexposure occurs.
In general, such a phenomenon is influenced by the position of an object, the brightness of a background, the proportion occupied by the object, and the like.
Note that the detection area 25 is a frame indicating the entire imaging area (detection area).