The difference of 400 degrees between dilated and non-dilated pupils is often caused by pseudomyopia in ophthalmology clinics. The occurrence of pseudomyopia is often caused by ciliary muscle spasm after excessive eye use, and pupil dilation is to point to the eyes of the local drops of tropicamide and other ciliary muscle paralyzing agents, accordingly, the spasm of the ciliary muscle can be relieved, then, because of excessive eye use caused by pseudomyopia will mostly be relieved. Therefore, there may be a difference in the degree of myopia between dilated and non-dilated pupils, which is mainly manifested as a different degree of myopia reduction after dilated pupils. The difference in myopia after pupil dilation is often the true refractive state of the eye. The exact cause of the difference needs to be clarified by the patient’s doctor.