1. An apparatus for capturing images, comprising: an image-capturing unit capturing an image light from an object and generating a first digital image; a night-scene-sensing unit sensing whether the first digital image is a night-scene image; an exposure-adjustment unit adjusting an optimized exposure time by comparing an edge level of the night-scene image with that of a pre-generated reference image if it is sensed that the input image is the night-scene image; and a controller controlling the image-capturing unit to generate a second digital image based on the adjusted exposure time.