Patent ID: 8223256

Claim:
A focus detection device, comprising: a light-receiving unit that receives a pair of images formed with a pair of light fluxes passing through an optical system and outputs a first signal string and a second signal string corresponding to the pair of images; a decision-making unit that determines a condition assumed as the light-receiving unit receives the pair of images; a selection unit that selects a correlation operation expression among a plurality of correlation operation expressions, to be used to calculate a correlation between the first signal string and the second signal string, in correspondence to the condition determined by the decision-making unit; a correlation calculation unit that calculates a correlation quantity indicating an extent of correlation between a plurality of first signals included in the first signal string and a plurality of second signals included in the second signal string by applying the correlation operation expression selected by the selection unit to the plurality of first signals and the plurality of second signals; a shift amount detection unit that detects a shift amount indicating an extent of shift between the pair of images based upon an extent of relative shift between the first signal string and the second signal string manifesting when the correlation quantity calculated by the correlation calculation unit assumes an extreme value; and a focus detection unit that detects a focusing condition for the optical system based upon the shift amount detected by the shift amount detection unit, wherein: the selection unit selects the correlation operation expression so as to reduce an extent to which signal intensity difference between the first signal string and the second signal string affects the correlation quantity indicating the extent of correlation between the plurality of first signals and the plurality of second signals.