A visual acuity of 5.1 is considered normal, but the patient’s vision should not be inferred from the patient’s vision, and the patient will need to undergo an optometric examination to determine the degree of myopia. Myopia can lead to vision loss, and the higher the degree of myopia, the more significant the loss of vision. However, there is no one-to-one quantitative relationship between visual acuity and refractive error, so it is not possible to speculate on the number of diopters based on a patient’s visual acuity of 5.1. If the patient wants to clarify the exact refractive error, the patient needs to go to the hospital for an optometry examination. Especially for children under 12 years of age, partial pupil dilation is needed, and then again for optometry, by paralyzing the ciliary body, avoiding the interference of false refractive error caused by ciliary muscle spasm, and detecting the true number of degrees of the eyes. It is recommended that patients who need to clarify the number of diopters should consult a doctor in time to assess their condition and follow the doctor’s instructions for treatment.