1. Field of the Invention
The present invention relates to an imaging technique and in particular, to an imaging technique performing focusing.
2. Description of the Related Art
Digital video cameras, or digital camcorders, which form an image of a subject on a semiconductor imaging device, such as a two-dimensional charge-coupled device (CCD) image sensor, by means of an optical imaging system, convert the image into an electrical signal, and record image data of moving images obtained on a record medium, such as a semiconductor memory, a magnetic disk, or a magnetic tape, and digital still cameras capable of recording moving images in addition to still images are becoming popular. For current digital video cameras and digital still cameras, all important tasks in image capturing, such as exposure settings, focusing, and the like, are automated, so that even users who are inexperienced at camera operations seldom fail to capture images properly.
CCD image sensors, as mentioned above, have the electronic shutter capability that allows a user to freely set a shutter speed. Therefore, a user can adjust the exposure time in accordance with the motion of a subject or illumination conditions. For example, for capturing a moving image of a subject in dark conditions, due to the low illumination, a user selects a low speed setting so as to make an electronic shutter speed lower in order to have an increased exposure. This shutter speed is slower than a normal shutter speed of 1/60 second in one field of a video signal, for example, 1/30 sec. or 1/15 sec. However, slow shutter speeds cause image capturing to be susceptible to the effects of camera shaking, and therefore, an image of a subject is blurred. To prevent this, a method is proposed in which an image capturing operation with an exposure time that does not cause motion blurring is performed multiple times and the obtained images are combined while their displacements are corrected, thereby realizing a resulting image with a long exposure time (see, for example, Japanese Patent No. 3110797).
Japanese Patent No. 3110797 discloses an apparatus for combining captured images. The apparatus includes a detecting unit for detecting information regarding motion between a plurality of images and an image moving unit for transforming the positions of the plurality of images in a plane coordinate system in accordance with the detected information. With the application of this technique, in a case when a moving image of a subject in dark conditions requiring a low-speed shutter setting for sufficient exposure is to be captured, an exposure time that does not cause an image to be affected by camera shaking, for example, a normal speed of 1/60 sec. in one field of a video signal is set, instead of a normal low-speed shutter setting of 1/30 sec., 1/15 sec., or a slower time, an image capturing operation is performed multiple times in units of 1/60 sec. for a period of time equal to an exposure period of a low-speed shutter, and the plurality of images obtained are combined, so that moving images that have exposure amounts equal to that of a low-speed shutter and that are not affected by camera shaking can be acquired in units of the same time period as an exposure time of the low-speed shutter.
However, the method for setting an exposure time that does not cause an image to be affected by camera shaking, instead of selecting a low-speed shutter setting, for capturing an image multiple times in a period of time equal to an exposure period of a low-speed shutter, and for combining the plurality of images captured in order to acquire moving images that do not suffer the effects of camera shaking and that have the same exposure as that of the low-speed shutter setting, as described above, significantly affects a focus performance in an automatic focusing unit. Specifically, a normal focusing unit for processing moving images performs focusing by a method (so-called “television autofocus (TV-AF) method”) for extracting a definition signal for focusing from a signal obtained from a signal output from an imaging device every image capturing in accordance with a focusing state of the lens unit and controlling a lens unit using the definition signal such that, for example, the amount of definition signal is maximized. However, when a moving image of a subject in dark conditions, due to the low illumination, is captured, an amplitude of a signal output from the imaging device at intervals of multiple image capturing operations is too small, and as a result, an amplitude of a definition signal for focusing obtained from the imaging device is too small. In other words, the obtained signal is inadequate to realize a sufficient focus performance (low-contrast conditions).
FIG. 4 shows the amount of signal stored in the imaging device, the amount of signal output from the imaging device, and the amplitude of the definition signal obtained from the output from the imaging device in this case. In FIG. 4, chart (3-1) represents the relationship between the amount of signal stored in the imaging device and an exposure period. Reference numerals e2, o3, e3, . . . individually represent repeat numbers of even or odd fields in the exposure period in the imaging device. Since a resulting moving image is not affected by camera shaking and has an exposure equal to that of a low-speed shutter setting due to processing of an image combining unit disposed at a subsequent stage even when an image of a subject in the dark is captured, an exposure period, T, is set to the order of 1/60 sec., and thus a signal is read every 1/60 sec., as shown in chart (3-2). Combining signals of four reading operations realizes an image (composite image) with an exposure equal to an exposure time of 1/15 sec. of a low-speed shutter setting. However, the amount of definition signal extracted is small in accordance with the amount of signal stored in the imaging device, as shown in chart (3-3), and therefore, the extracted definition signal is inadequate to realize a sufficient focus performance.
Charts (3-4), (3-5), and (3-6) represent the amount of signal stored in the imaging device, the amount of signal output from the imaging device, and the amplitude of the definition signal obtained from the output from the imaging device, respectively, when a normal low-speed shutter setting of 1/15 sec. is selected as an exposure period. In this case, although the amplitude of the definition signal is large, the output from the imaging device, i.e., the definition signal has many remaining components leading to poor contrast because of a long exposure. This causes degradation in focus performance.
If, for example, image combining processing, as described above, is not performed, in a case when an image of a subject in the dark is captured, an amplitude of the definition signal can be amplified by increasing the gain of a signal that is output from the imaging device every a normal speed of 1/60 sec. in one field of a video signal and that has a small amplitude, thus realizing an image with contrast, albeit with much noise. However, since this method amplifies the amplitude of a signal output from the imaging device and increases an apparent exposure, a high-quality moving image with less noise is not acquired.