1. An image processing method for converting an input image into an image having a relatively narrow dynamic range, the image processing method comprising: a smoothing step for smoothing each of a plurality of images, into which the input image is segmented, to generate a plurality of smoothed images different from each other in the degree of smoothing, an edge intensity calculating step for calculating an edge intensity based on the plurality of smoothed images, a synthesizing step for synthesizing the plurality of smoothed images based on the calculated edge intensity, a coefficient calculating step for calculating a coefficient for use in the conversion of pixel values of the input image based on the synthesized smoothed image generated through the synthesis of the plurality of smoothed images, and a pixel value converting step for converting the pixel values of the input image based on the calculated coefficient.