A video camera is one example of an image sensing apparatus. In the field of video cameras, the recording not only of moving pictures but of still pictures as well has recently become a matter of much interest. A video camera on which a strobe light can be mounted in order to record a high-quality still picture on a recording medium has been proposed. Such a video camera provides a greater number of photographic opportunities.
Such strobe lights include one having a main-light emission unit and a fill-in light emission unit for actuating an autofocus function prior to photography. In a strobe light of this kind, focusing is achieved by lighting the fill-in light emission unit and actuating the autofocus function, after which the main-light emission unit is lit and the still picture stored on the storage medium.
The main technique used for implementing the autofocus function in an ordinary video camera is TVAF. According to the TVAF technique, autofocusing is performed by detecting the sharpness of an image using a video signal obtained as a result of photoelectrically converting the image of an optical subject by an image sensing device or the like, and controlling the position of a focusing lens so as to maximize the detected value.
In general, the degree of sharpness is evaluated using the level (referred to a “focus evaluation value” below) of the high-frequency components of a video signal that has been extracted by a bandpass filter having a certain pass band. Specifically, when an ordinary subject image is photographed, the focus evaluation value increases with focusing, as illustrated in the characteristic diagram of FIG. 2. Accordingly, the point at which the focus evaluation value peaks is taken as being the in-focus position.
Control of the autofocus function in a video camera that is capable of still photography and possesses a strobe light will be described on the basis of the flowchart shown in FIG. 12.
Step S901 is a preparatory step at which the fill-in light emission unit is lit and the autofocus function actuated. This is followed by step S902, at which the focusing lens is driven in very small increments and the focus evaluation value is detected. Next, on the basis of the focus evaluation value thus obtained at step S902, it is determined at step S903 whether the camera is presently in focus as a result of finely driving the focusing lens.
If it is determined that the camera is not in the focused state, control proceeds to step S904. Here, in accordance with the result of finely driving the focusing lens, it is determined in which direction the in-focus point is located (i.e., if the focusing lens should be moved toward the camera side or subject side from its present position to obtain the peak of the focus evaluation value). If the direction in which the in-focus point is located cannot be determined, control returns to step S902. If the direction in which the in-focus point is located can be determined, control proceeds to step S905, at which an operation for moving the focusing lens in the determined direction is executed. (This is referred to as a “hill-climbing operation” owing to the shape of the graph shown in FIG. 2.)
Next, at step S906, it is determined whether the position of the focusing lens has exceeded the in-focus point, i.e., whether it has exceeded the peak of the focus evaluation value. Control returns to step S905 if it is determined that the peak of the focus evaluation value has not been exceeded or proceeds to step S907 if it is determined that the peak of the focus evaluation value has been exceeded. Step S907 calls for the focusing lens to be moved back in the direction of peak of the focus evaluation value. It is then determined at step S908 whether the focus evaluation value has reached the peak. Control returns to step S907 if it is determined that the focus evaluation value has not reached the peak or to step S902 if it is determined that the focus evaluation value has reached the peak.
By executing the processing of steps S907 and S908, the focusing lens can be controlled so as to be moved to the position at which the focus evaluation value is maximized. However, since there are instances where the subject being photographed changes owing to panning or the like during the operation for returning the focusing lens to the position at which the focus evaluation value attains its peak, there are occasions where it cannot be ascertained whether this position is the true peak.
Accordingly, once the focus evaluation value has arrived at the peak, processing returns to that from step S902 onward and the operation for finely driving the focusing lens is performed again in order to confirm that the present focus evaluation value is the true peak, i.e., to confirm that the camera is in the focused state.
If it is determined at step S903 that the camera is in the focused state, control proceeds to a still-picture capture routine from step S909 onward.
Movement of the focusing lens is halted at step S909, the fill-in light emission unit is turned off at step S910 and the main-light emission unit is turned on at step S911 to increase the illumination of the subject, in which state the still picture is captured. This processing is then exited.
The example of the prior art described above has certain shortcomings.
Specifically, in view of problems relating to durability and power consumption of the fill-in light emission unit, keeping the fill-in light emission unit lit over the entire period of the focusing operation is undesirable. In actuality, therefore, the fill-in light emission unit is turned on and off.
However, an AF (autofocus) evaluation value which prevails when the fill-in light emission unit is on differs from that which prevails when the fill-in light emission unit is off. Therefore, autofocus control cannot be performed accurately if the fill-in light emission unit is turned on and off at a random timing.
Control of a fill-in light emission according to the prior art is synchronized to the vertical synchronizing signal of the video signal to turn on the fill-in light emission unit based upon the pulses of the vertical synchronizing signal, as proposed in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 11-69224. FIG. 13 is a timing chart showing the timing at which the fill-in light emission unit is turned on and the charge accumulation timing of an image sensing device (CCD). As shown in FIG. 13, ON and OFF timings are synchronized to a vertical synchronizing signal and the duration of fill-in lighting is made a whole-number multiple of the vertical scanning interval (twice the vertical scanning interval in the example of FIG. 13). Fill-in light is emitted during this multiple of the vertical scanning interval. In the example depicted in FIG. 13, the fill-in light is turned off for one vertical scanning interval that follows the emission period. The focus evaluation values in respective ones of the three vertical scanning intervals are found, the values are averaged and the average value thus obtained is used as the focus evaluation value. If this is done, the average values of focus evaluation values obtained at the timings of focus-evaluation-value averaging intervals a, b and c will be the same. This makes it possible to perform stable autofocus control.
In the example of the prior art described above, the focus evaluation value of each vertical scanning interval can be obtained artificially by adding and averaging the focus evaluation values of a plurality (e.g., three) of successive vertical scanning intervals. However, the focus evaluation value thus obtained is a focus evaluation value of a past vertical scanning interval, i.e., a focus evaluation value which contains information that is old in terms of time. It is difficult to perform real-time autofocus control using this focus evaluation value.
Further, at least one of the plurality (e.g., three) of successive vertical scanning intervals is a vertical scanning interval in which the fill-in light is off. As a consequence, even if autofocus control is carried out every vertical scanning interval using a focus evaluation value obtained based solely upon a signal value acquired in each vertical scanning interval, a focus evaluation value obtained while the fill-in light is off will be unsuitable for use in autofocus control in a case where illumination of the subject is inadequate. This places a limitation upon control and results in a troublesome operation.
Further, the fill-in light emission unit is turned on at the same timing regardless of whether drive is performed at a high shutter speed in which the charge accumulation period of the image sensing device is, e.g., 1/250 of a second or 1/1000 of a second, or at an ordinary shutter speed in which the charge accumulation period of the image sensing device is 1/60 of a second.
FIG. 14 is a diagram illustrating the charge accumulation timing of an ordinary image sensing device (CCD) driven at a high shutter speed of 1/250 of a second, which shows CCD signal quantity and a vertical synchronizing signal.
As shown in FIG. 14, a signal that has accumulated over the first half of 1/60 of a second, which is the drive cycle of one field, is dumped at a timing Ta. A signal that has accumulated over a period of 1/250 of a second, which extends from Ta to Tb, is then read out as an image signal. Thus, in a case where drive is performed at a high shutter speed, turning on the fill-in light emission unit in a manner the same as that when drive is performed at the ordinary shutter speed of 1/60 of a second means that the emission of light from the fill-in light emission unit up to Ta has no effect and is a wasteful light emission. Power is thus consumed needlessly.
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