US Patent 7873267 Focus detection device, focusing state detection method and imaging apparatus

A focus detection device comprises: focus detection elements including light-receiving elements that generate a pair of signals corresponding to a pair of light fluxes each passing through an imaging optical system that includes a lens and an aperture stop; and a focus detector including a processor that determines a focus adjustment state of the imaging optical system with an f-number slower than a full-aperture of the imaging optical system when a specific condition is fulfilled.