wikipedia

Getting sufficient depth of field can be particularly challenging in macro photography, because depth of field is smaller (shallower) for objects nearer the camera, so if a small object fills the frame, it is often so close that its entire depth cannot be in focus at once. Depth of field is normally increased by stopping down aperture (using a larger f-number), but beyond a certain point, stopping down causes blurring due to diffraction, which counteracts the benefit of being in focus. It also reduces the luminosity of the image. Focus stacking allows the depth of field of images taken at the sharpest aperture to be effectively increased.