1. An image correction apparatus for correcting gray levels representing pixels in an image, comprising: a controller that computes a correction range for the image by separately calculating for the image a correction range for correctively increasing the gray levels and a correction range for correctively decreasing the gray levels; and wherein each pixel in the image is represented by one of 2 m gray levels ranging from 0 to 2 m −1 where a bit depth of each pixel is m bits; and the controller calculates a statistical count of each of the 2 m gray levels representing a plurality of pixels in the image, calculates a first total by accumulating the statistical counts starting from gray level 0 and a second total by accumulating the statistical counts starting from gray level 2 m −1, calculates a value for determining a correction range for increasing the gray level and calculates a value for determining a correction range for decreasing the gray level, calculates a correction range that decreases the gray level based on the value at which the first total exceeds the value for determining the correction range increasing the gray level, and calculates a correction range that increases the gray level based on the value at which the second total exceeds the value for determining the correction range decreasing the gray level.