Correlation phase difference type focus detecting interpolation

Abstract

A pair of optical systems of the same characteristics are disposed spaced apart by a base line length in the direction perpendicular to an optical axis, to form a focal plane, on which a standard photosensor and a reference photosensor, each having a plurality of photosensor elements, are disposed. An image of an object is focused onto the standard and reference photosensors, to generate a standard optical signal and a reference optical signal which are compared with each other, while changing the phase of the reference optical signal relative to the phase of the standard optical signal to calculate correlation factors. A distance to the object is detected from a phase having an extreme value of the calculated correlation factors. If the calculated correlation factors are asymmetrical relative to the extreme value, one correlation factor change by a unit phase shift before the extreme value is subtracted from the other correlation factor change by the unit phase shift after the extreme value. The subtraction result is divided by a constant. The division result is added to the correlation factors at sampling points adjacent the extreme value on the side of the other correlation factor change. The addition results are used as the corrected correlation factors, to perform an interpolation calculation to obtain a phase having an extreme value of the corrected correlation factors.