Naked eye visual acuity of 4.0 is converted to decimal visual acuity of about 0.1, which is the level of visual acuity represented by the largest visual scale. 4.0 visual acuity is a relatively low visual acuity, the degree of myopia is about 300 degrees or more, if it is 500-600 degrees of myopia, then probably 4.0 or 0.1 visual acuity can not be achieved. For this vision is very affect the quality of daily vision or life safety, it is necessary to wear glasses to correct vision. It is usually necessary to wear glasses through optometry, especially in children, not only to wear them, but also to wear them all the time to improve vision and to control the growth of myopia.