1. An image capturing device which captures an imaging target while varying a focus position and outputs at least one output image as a capturing result, the image capturing device comprising: a pre-capturing unit configured to capture the imaging target at each of different focus positions in a predetermined focus range and output, as a capturing result, a plurality of pre-captured images lower in resolution than the output image; a sharpness level computing unit configured to compute a sharpness level of each of the pre-captured images captured by the pre-capturing unit; a focus varying range determining unit configured to determine a focus position varying range within the predetermined focus range based on sharpness levels computed by the sharpness level computing unit, such that a signal-to-noise ratio of the output image is greater than or equal to a predetermined threshold; an image capturing unit configured to capture the imaging target by sweeping a lens and varying the focus position during exposure according to the focus position varying range determined by the focus varying range determining unit, and output the at least one output image as a capturing result; and an extended depth of field image deriving unit configured to derive an extended depth of field image having an extended depth of field, using the at least one output image outputted by the image capturing unit and at least one predetermined point spread function: wherein the focus varying range determining unit is configured to: divide each pre-captured image into regions; obtain, for each pre-captured image, an in-focus level that corresponds to a sharpness level of each of the regions; compare the in-focus levels between corresponding regions in respective pre-captured images; obtain, for each region, a focus position corresponding to a pre-captured image having a highest in-focus level among the in-focus levels of the regions obtained for each pre-captured image; and determine the focus position varying range such that the obtained focus positions are included.