1. Field of the Invention
The present invention relates to the automatic focus adjustment of a lens unit and an imaging apparatus that is mountable to the lens unit.
2. Description of the Related Art
In recent years, AF (auto focus) devices for cameras that determine an AF evaluation value by detecting the sharpness of an image from an imaging signal and shift a focus lens to a position where the AF evaluation value is the highest to thereby perform focus adjustment have been prevailing. Hereinafter, the above method is referred to as the “TVAF method”. As an AF evaluation value, the high frequency component level of an image signal extracted by a band pass filter with a predetermined band is typically used. When an object is captured, an AF evaluation value increases as the focus lens is being focused and is maximized at an in-focus point as shown in FIG. 2A. In other words, in the example, the degree of focusing decreases with distance from the in-focus position. FIG. 2B shows an operation (hereinafter referred to as “wobbling operation”) for determining a focusing direction based on the change in the AF evaluation value obtained when the focus lens is driven at micro intervals. In the wobbling operation, the influence of the movement of the lens on a capturing screen is not noticeable, and thus, the wobbling operation is used particularly for capturing moving images. On the other hand, the drive amount of the focus lens and the image plane shift amount are not always the same as shown in FIG. 2C, the ratio (sensitivity) between both amounts is different for each lens unit and may vary depending on the positions of the focus lens and the zoom lens.
Japanese Patent Laid-Open No. 2008-242442 discloses an automatic focus adjustment device for adapting the AF method of this type to a video camera having an interchangeable lens unit. The wobbling operation is enabled by an interchangeable lens system by passing a wobbling operation signal to the lens unit and thus causing the lens unit to perform wobbling operation control.
However, in the case of controlling the focus lens by use of a conventional interchangeable lens system, it is difficult for a camera body to change the movement of the focus lens when the improvement in wobbling operation is desired in the future. Also, it is an undesirable necessity that different drive commands be prepared when it is desired that a camera realize the drive control of the focus lens for each lens unit, resulting in a complication of control. As described above, the image plane shift amount relative to the drive amount of the focus lens is different for each lens unit and may also vary depending on a focus lens position and a zoom lens position. For this reason, even if the control unit provided in the camera body of the interchangeable lens system can provide an instruction about a desired image plane shift amount to the lens unit, the control unit is unaware of the actual drive amount of the focus lens relative to the image plane shift amount. In other words, it is difficult for the control unit of the camera body to acquire detailed information about lens configuration, lens specification, and the like in order to provide an instruction about a specific drive amount of the focus lens to the lens unit.