1. A digital image processing apparatus, comprising: a digital signal processor (DSP) configured to: perform a secondary differentiation operation on a digital image to generate secondary differentiation values; select secondary differentiation values that are greater in absolute value than a first predetermined constant, so as to generate selected values; determine a focus region of the image based on the spatial distribution of selected values, wherein a focus region is a region in which the selected values have magnitude greater than a second predetermined constant; and display a combination of the selected values on the image.