Automatic focusing apparatus

摘要

An image pickup element picks up an image formed by an optical system. A drive mechanism performs focus adjustment of the optical system on the basis of a signal output from the image pickup element. A storage unit stores MTF ratios corresponding to a plurality of spatial previously frequencies at each of two positions near a focal plane of the optical system in accordance with a defocus amount of the optical system, and to a predetermined value of characteristic values of the optical system. A converter converts the MTF ratio values stored in the storage unit in accordance with the characteristic value of the optical system. A first calculating unit calculates a ratio of the plurality of spatial frequency components corresponding to respective focusing states on the basis of a signal output from the image pickup element. A second calculating unit compares the calculated ratio of the spatial frequency components with the MTF ratio converted by the converter to calculate the defocus amount of the optical system. The drive mechanism drives the optical system in the axial direction in accordance with the defocus amount output from the second calculating unit and at least one of the characteristic values of the optical system.