A focus detecting apparatus includes a first image sensor configured to receive a light beam that has passed through an optical system and to output a first signal to be used for a focus detection by a phase difference detection method, a second image sensor configured to receive, by a masking device, a light beam narrower than that received by the first image sensor, which has passed through the optical system and to output a second signal to be used for the focus detection by the phase difference detection method, and a calculating circuit configured to calculate the focus detection by the phase difference detection method. In the focus detection calculation, a search range for an in-focus position using the first signal is wider than that using the second signal.