1. Field of the Invention
The present invention relates to a focal point adjustment device, a camera system, and a focal point adjustment method for an imaging device for performing focal point adjustment of a photographic lens, based on a focal point detection result.
2. Description of Related Art
Since a main subject is not always at the center of a photographic screen, a plurality of range-finding areas is provided, and focal point adjustment of a photographic lens is performed, based on a range-finding result (defocus direction and defocus amount) from each of the range-finding areas. In this case, the defocus amount obtained from each of the range-finding areas has a larger error as the distance from the optical axis center becomes larger, because of the aberration of the photographic lens or the like. Accordingly, it is proposed to calculate a correction amount for the correction of the defocus amount depending on the distance from the optical axis center to each of the range-finding areas, to perform the correction using this correction amount on the defocus amount obtained from each of the range-finding areas, and thereby to calculate an accurate defocus amount (refer to Japanese Patent Publication No. 3236799 (in the following, referred to as “Patent Literature 1”)).
Recently, there is known a camera mounting a camera-shake preventing mechanism for correcting camera shake by moving an image sensor in a direction in which the camera shake is compensated. When the camera-shake preventing mechanism is operated in the method of obtaining a correction amount to correct the defocus amount depending on the distance from the optical axis center to each of the range-finding areas in the image sensor, the camera incorporating this camera-shake preventing mechanism cannot calculate an accurate defocus amount.
This point will be explained by the use of FIG. 7. In a camera shown in FIG. 7, a plurality of range-finding areas 103 is arranged on an image sensor 21, and the plurality of range-finding areas 103 is arranged symmetrically centering a screen center 101 of the image sensor 21. In an example shown in FIG. 7, the range-finding areas 103 are arranged at three positions in the first row, five positions in each of the second to fourth rows, and three positions in the fifth row. The image sensor 21 is moved by a shift amount (Xs, Ys) by a camera-shake preventing mechanism to prevent the camera shake. Accordingly, the screen center 101 is moved by the shift amount (Xs, Ys), and a range-finding area 103a at the screen center and also a range-finding area 103b at a position apart from the optical axis center 101 by a distance d are moved by the shift amount (Xs, Ys).
In this manner, in the case of moving the image sensor (referred to as imager shift method) for preventing the camera shake, the positional relationship between the optical axis center and the screen center is shifted and the distance from the optical axis center to range-finding area is changed. That is, the distance d shown in FIG. 7 is changed. Accordingly, the accuracy of the correction amount for the defocus amount cannot be secured.
Further, in the conventional correction method, the correction amount of the defocus amount is obtained only for the distance d from the screen center. However, a specific condition of an interchangeable lens, for example, a zoom lens position or a focus lens position, is not taken into consideration, and therefore the correction amount for the defocus amount cannot be obtained accurately.