1. Field of the Invention
The present invention relates to an image pickup apparatus.
2. Description of the Related Art
Generally, a ranging accuracy of a phase-difference auto-focus (AF) (hereinafter, appropriately called as ‘AF’) is determined by a center of gravity Fno which can be captured by an AF sensor and a distance between pixels for focal-point detection (hereinafter, ‘focal-point detection pixels’).
As a conventional example of an image pickup apparatus in which, an element in which pixels for image pickup (hereinafter, ‘image-pickup pixels’) and the focal-point detection pixels are arranged in rows two-dimensionally, a structure disclosed in Japanese Patent No. 3592147 Specification has hitherto been known. In the conventional image pickup apparatus, there are mainly two ranging methods which are as follow.    (1) Contrast method    (2) Phase-difference method.
(1) In the contrast method, a so-called mountain-climbing method in which, a contrast value is evaluated while carrying out wobbling by changing a focusing state has been adopted. Therefore, focusing time is long.
(2) Moreover, in video photography, an image at the time of wobbling is also recorded. Therefore, an image quality is degraded. Moreover, in the phase-difference method in (2), basically, an amount of defocus (hereinafter, ‘defocus amount’) can be detected by ranging once. Therefore, focusing time is short.
Generally, the ranging accuracy of the phase-difference method changes according to characteristics of an object. For example, for an object with a low illuminance and low contrast, the ranging accuracy is degraded. In a conventional measure, an operation to check the reliability of the defocus amount which has been calculated by correlation matching, ranging to verify has been carried out.
For example, when a low-contrast object is captured, the ranging accuracy is degraded, and as a result of a first ranging, focusing is rearward of the best image-plane position. Moreover, by a second ranging, it is possible to focus at the best image-plane position.
At this time, when it is a still photography, it is desirable to expose in a state of being focused at the best image-plane position, and to acquire the final image. Whereas, in video photography, during the ranging operation, an image is focused at a position rearward of the best image-plane position in the first ranging or in other words, at a position on an image-plane side. Furthermore, in the second ranging, even a step of focusing at the best image-plane position is also recorded. Therefore, the image quality is degraded. This state is same as the wobbling of the contrast AF being recorded.