An image capturing method including following steps is provided. A preview image is captured and compressed into a compressed preview image according to a preview quantization table. An image complexity is judged by determining the amount of high frequency components the preview image has according to a set of factors consisting of the preview quantization table, a resolution of the preview image and a size of the compressed preview image. The more the amount of high frequency components the preview image has, the higher the image complexity. A to-be-captured image is captured. An initial quantization table is determined according to a set of factors consisting of the image complexity, a resolution of the to-be-captured image and an image compression target. The to-be-captured image is compressed into an output image according to the initial quantization table.