1. A method for calculating a depth map from an original matrix image, comprising at least the steps of: calculating, by processing circuitry, a first matrix image corresponding to the original matrix image with a first specified resolution and in which a corresponding depth of field is similar to that of the original matrix image; calculating a second matrix image corresponding to the original matrix image with a second specified resolution, which comprises a number of pixels similar to that of the first matrix image and in which a corresponding depth of field is greater than that of the original matrix image; and outputting the depth map that results from implementing a depth from defocus (DFD) type three-dimensional reconstruction algorithm based on the first and the second matrix images, wherein each of the first specified resolution and the second specified resolution is lower than a resolution of the original matrix image.