Automatic focusing device having a settable focus detection area

DETAILED DESCRIPTION This invention is directed to the solution of the above stated problems of the prior art.
It is also a first object of the invention to provide an improvement over the methods disclosed in the prior applications mentioned in the foregoing and to further the degrees of accuracy and reliability of them.
It is a second object of the invention to provide an automatic focusing device which is capable of continuing its focusing action on a main object by tracing the image of the main object on an image sensing plane even when the object's image moves.
To attain this object, an automatic focusing device arranged as a preferred embodiment of the invention comprises: area setting means for setting, on an image sensing plane, a tracing inner area and a tracing outer area surrounding the inner area in such a way as to have the positions of both the areas movable; computing means for computing information on a difference between levels of a given focused-degree-indicating component of a video signal obtained at each of varied positions of the tracing inner and outer areas moved within the image sensing plane; determining means for determining the location of an object within the image sensing plane from the difference information computed by the computing means; and focus detecting area shifting means for shifting a focus detecting area to the object's location within the image sensing plane determined by the determining means.
When a main object to be photographed is determined to have moved in the direction of increasing the difference in the given component between the tracing inner area and the tracing outer area, i.
e.
a luminance component or the high frequency component on the high band side thereof, the focus detecting area is shifted to the new position of the main object.
It is a third object of this invention to provide an automatic focusing device which is capable of continuing its focusing action on main objects of varied sized by tracing changes occurring in size of the main objects without being affected by objects other than the main objects