1. A method for detecting a focus condition of an imaging optical system comprising the steps of: receiving at least a part of an image formed by an imaging optical system by means of at least one light receiving element array comprising a plurality of light receiving elements which produce element array output signals; processing the element array output signals of said at least one light receiving element array in real time by means of at least one processing device including at least one surface acoustic wave device to produce a device output signal; and detecting the focus condition of said imaging optical system from said device output signal.