To select a correlation operation expression suitable for a condition when receiving a pair of luminous fluxes for focus detection.

A focus detection device includes: a light-receiving unit that outputs a first data string and a second data string corresponding to a pair of images by a pair of luminous fluxes passing through an optical system; a decision-making unit that determines a condition assumed as the light-receiving unit receives the pair of images; a correlation calculation unit that calculates a correlation quantity between the first/second data strings by the specific correlation operation expression of a plurality of correlation operation expressions; selection units (S300 to S390) that select the correlation operation expression reducing the degree of the influence of the difference between the first/second data strings on the correlation quantity, among the plurality of correlation operation expressions, according to the determined condition, as the specific correlation operation expression; a shift amount detection unit that detects a shift amount between the pair of images, based on a relative shift amount between the first/second data strings whose correlation quantities are an extreme value; and a focus detection unit that detects the focusing condition of the optical system, based on the detected shift amount.

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