A device and a method for obtaining a clear image, the method is executed by a digital signal processor (DSP) chip or a microprocessor. Through merging clear parts of two images with different focal lengths, a single clear image is obtained. The image is divided into a plurality of blocks, and then edge detection is processed to obtain an edge image. Blocks having more complete edge information are selected as clear blocks. Then, the clear blocks are further merged into a single clear image. Once the images are processed with the method, a depth of field of the image can be adjusted, without adding hardware elements of a digital camera such as a variable diaphragm.