As methods for realizing Extended Depth of Field (hereinafter, referred to as “EDOF”) in an image pickup device, various methods have been proposed. According to one example of such proposed methods, a focus sweep motion of moving a focus lens or an image pickup element during an exposure time duration is performed, images focused uniformly are convoluted in a depth direction (i.e., blur is uniformized at each depth), and image recovery processing is performed by use of a blur pattern obtained in advance by measurement or simulation. Thus, an EDOF image is obtained (Non-Patent Document 1). This method is referred to as Flexible DOF (hereinafter, referred to as “F-DOF”).
F-DOF is known as a method for providing a high quality image, and provides a high EDOF effect. Since the off-axis property thereof depends on the lens characteristics themselves, the performance can be improved easily. However, even if the focus position is moved during exposure, the same subject needs to be convoluted at the same image position. Therefore, F-DOF needs to be performed under an optical condition that an image-side telecentric lens is used. One of the fields of application of the EDOF technology is microscope. When image pickup is performed by a microscope, it is possible to take time for shooting because the target of shooting is a still object. Thus, a Focal Stack system have been used for many years. According to this system, a plurality of images with different focus positions are shot, and an area which appears to be focused is extracted from each image and these areas are synthesized. Thus, an EDOF image is obtained. Since such a work is labor- and time-consuming, technologies of using an F-DOF system in combination with the Focal Stack system have been proposed (Patent Documents 1 through 4). In the case where F-DOF is used for a microscope, a sample as a subject or a lens barrel is moved during exposure. In the case where image recovery processing is to be performed after exposure, the subject or the lens barrel is moved such that the blur of the image is always uniform. It is known that this method is reasonable because when the manner of movement is controlled appropriately, an image recovery processing method using a single blur pattern can be applied (Patent Document 5). For this purpose, the image pickup element is moved at a constant speed when the image pickup element is to be moved. When the focus lens is to be moved, the focus lens needs to be displaced by a distance corresponding to the movement of the image pickup element at a constant speed (Non-Patent Document 1). It is known that the pattern of movement may be from a far-side focus end position to a near-side focus end position, or vice versa.
An example thereof is shown in FIG. 12. FIGS. 12(a) and (b) respectively show an exposure state and a read state of the image pickup element. The horizontal axis represents the time. FIG. 12(c) shows the displacement of the focus lens. The horizontal axis represents the time, and the vertical axis represents the focus position. In FIGS. 12(a) and (b), the hatched area represents the timing for exposing the image pickup element and the timing for reading data. The image pickup element is exposed in synchronization with an operation of displacing the focus position from a near-side focus position to a far-side focus position. Thus, an image is obtained in which subjects located at various positions in one, same scene are convoluted at one, same image position in a focused state. Hereinafter, such displacement of the focus position will be referred to as a “sweep pattern”, and an image obtained by a sweep pattern will be referred to as a “sweep image”. Another example is shown in FIG. 12(d). In this example, a sweep image is obtained by displacing the focus position between the near-side focus position and the far-side focus position in a reciprocating manner during the exposure time duration of the image pickup element. Even with such a sweep pattern, as long as the focus lens is displaced at a constant speed in straight line areas, the exposure time duration is uniform at the focus positions. Therefore, a sweep image equivalent to that described with reference to FIG. 12(c) can be obtained.
This technology is applicable to usual digital still cameras and digital video cameras. Recently, digital still cameras and digital video cameras are required to have performance which allows shooting to be done more easily and with less failures. The EDOF technology is expected to provide an effect of shooting an all-in-focus image, namely, an effect of decreasing focusing errors. When an EDOF technology is used for a digital still camera or a digital video camera, the F-DOF system is considered to be preferable because of the following reasons. With the F-DOF system, a high quality is provided, a splendid EDOF effect is provided, the EDOF range can be freely changed, a usual auto-focus mechanism can be applied (no specific optical system needs to be prepared), it is easy to switch the EDOF shooting and the usual shooting to each other, and the like.