1. Field of the Invention
The present invention relates to an auto focus system having an AF frame auto-tracking function, and more particularly, to an auto focus system having an AF frame auto-tracking function that allows an AF area (AF frame), which is the target range of an object brought into focus by the auto focus (AF), to follow a predetermined object moved on an imaging screen.
2. Description of the Related Art
Broadcasting or commercial television camera systems having an auto focus function for auto focusing on a desired object have been known. This type of camera system generally uses contrast-type AF. In the contrast-type AF, the camera system detects the level of the contrast of the captured image, and controls the focus of an imaging optical system such that the contrast becomes the maximum (the largest).
In general, the target range of an object that is brought into focus by AF is not the entire screen of the captured image, but is limited to a portion of the screen, which is called an AF area. Focus control is performed such that the contrast of the captured image (object image) in the AF area becomes the maximum, thereby focusing on the object in the AF area.
In the specification, a frame indicating the outline of the AF area is referred to as an AF frame.
JP-A-2006-258944 (corresponding to US-A-2006/210260) discloses an auto focus system having an AF frame auto-tracking function that allows the AF frame to automatically follow a desired object moved on the screen of the captured image such that the object is continuously in focus. The auto-tracking of the AF frame is performed by detecting the image of an object to be tracked from the captured image and moving the AF frame to the detected position. As a method of detecting the image of an object to be tracked, JP-A-2006-258944 discloses a method of storing the image of the object to be tracked as a reference pattern and detecting an image matched with the reference pattern from the captured image using a pattern matching process.
In addition, the following method has been proposed: when the object to be tracked is a person's face, a known face detecting process is performed to detect a face image as the image of the object to be tracked.
However, during the tracking of the face image detected by the face detecting process, when the face is turned 30 degrees or more from the front side in the vertical or horizontal direction, the object may not be recognized as the face image, and tracking may be unavailable.
When the pattern matching process is performed to track a predetermined object, it takes a long time to perform the process, and it may be difficult to track an object being moved at a high speed.