Visual acuity is primarily a function of the central macular sulcus and refers to the ability of the eye to distinguish the minimum distance between two object points, as measured by the angle of view, the smaller the angle of view, the better the visual acuity. Visual acuity testing is extremely important in the ophthalmology clinic, especially in the process of optometry, and as a patient it is also important to understand the relevant knowledge that affects visual acuity testing. Because vision is a process in which the eye receives physical stimulation and the brain performs psychological cognition, and visual acuity is one of the comprehensive indicators of the entire visual function test, the visual acuity is related to the anatomical, optical, physiological and psychological factors of the entire eye: 1. Refractive error Patients with refractive error are generally affected by refractive error because light cannot be accurately focused on the retina, which is the most common reason affecting the visual acuity test This is the most common cause of vision test results. The number, location and distribution of photoreceptors on the retina determine the individual’s minimum angle of view and, therefore, the person’s best visual acuity. The transparency of the refractive interstitium also affects the focus of light on the retina, which affects visual acuity. 3, pupil size A small pupil can reduce aberration, increase depth of field and improve visual acuity. The normal pupil diameter is 2-4mm, but a pupil that is smaller than 2mm will produce diffraction and reduce visual acuity. 4, lighting Vision testing environment should have a standard unified lighting system, such as brightness is too strong, pupil narrowing, can improve visual acuity due to increased depth of field, but also because the pupil is too narrow to reduce the brightness of light and reduce visual acuity, affecting the results of vision testing. 5, contrast is the relationship between the depth of the visual marker itself and the depth of the background color of the visual marker, the contrast of the visual marker will directly affect the results of visual acuity testing. Contrast sensitivity is related to age, and various eye diseases can affect contrast visual acuity, such as early cataract, early glaucoma, amblyopia, etc. 6, eye position When the eye is skewed, it will cause the visual marker imaging can not fall on the macular central area, thus affecting the accuracy of the test results. 7.Crowding phenomenon The patient’s judgment of a single visual marker is less than that of multiple rows and rows of visual marks, and the visual acuity is reduced because of the action of the surrounding contour, which is especially obvious in people with amblyopia and macular degeneration, so it is better to use a specially designed visual acuity table when testing the visual acuity of amblyopic patients. 8. Time factor Some patients (especially amblyopic patients) are slow to respond, and their visual acuity tests are only accurate if they are provided with time for the patient to make an appropriate response. In addition to eye disease, the gender and age of the person being tested will also affect the results of the vision test. For example, vision testing of adult women should take into account factors such as menstruation and menopause, and excessive mood swings can lead to a difference of one to two lines in vision measurement. When the person being tested is under stress, care should be taken to relax to ensure the accuracy of the test. If the patient has dry eyes, care should be taken to increase the number of blinks during the test. When testing children, one eye should be covered when both eyes are tested separately, and the accompanying parent should assist with the covering. This will prevent the reliability of the visual acuity test from being affected by the pediatric fear caused by the examiner going for the covering. It is best for patients to go to the same hospital for follow-up examinations, and to use the same illumination, distance, and visual scales as for the initial examination, and to conduct a comprehensive analysis of near and far vision in order to obtain accurate vision test results.