This has something to do with our blind spot and pupil structure. When looking at a painting, the pupil structure makes the picture look big in the middle and small on both sides, resulting in visual differences, so the picture will be dynamic. Pupil is a small round hole in the center of iris in animals or people's eyes, which is the passage for light to enter the eyes. The expansion and contraction of smooth muscle on iris can reduce or enlarge the aperture of pupil and control the amount of light entering pupil.