The size of the eye is not the size of the lid, but the size of the pupil. The refractive system of the human eye produces a certain degree of aberration, making the image falling on the retina unclear. In addition to controlling the intensity of the light entering the eye, the pupil also controls the aberration of the refractive system. In real life, the refractive system of the human eye has more or less aberrations, and a large pupil will make the aberrations of the eye visible and manifest as a loss of visual acuity. Most of the time, this occurs in dim light conditions. Under pathological conditions, the iris atrophy and defects lead to pupil distortion, dilatation, and displacement, and the total aberration of the enlarged refractive system is revealed again, resulting in a significant decrease in the quality of the eye’s imaging, both during the day and in the dark. Therefore, the pupil plays a decisive role in controlling the aberration of the eye. The two diagrams below show the pupil in its normal state at about 2-3 mm in size. After dilatation with drugs it is about 8 mm. How to understand the effect of phase aberration on imaging quality. The following diagrams show the aberrations of the refractive system without an iris, with a large pupil, and with a small pupil: 1) without an iris, with a large pupil, the aberrations of the refractive system are fully revealed; 2) with a large pupil, the aberrations of the refractive system are more pronounced; 3) with a small pupil, the aberrations of the refractive system are less revealed.