Where parallel light rays from 5 meters away enter the eye through the refractive system of the eye (cornea, atrial fluid, lens, vitreous) and, without any adjustment of the eye, focus on the retina to form a clear image, this is orthoptic vision. If, for some reason (such as a change in the length of the eye or a change in the refractive power of a certain part of the eye), the focal point formed by parallel light rays entering the eye from 5 meters away is not on the retina, but in front of or behind the retina, and some cannot form a focal point at all, but form focal lines in front of or behind the retina, these are refractive errors, which include myopia, hyperopia, astigmatism, etc. Nearsightedness In the case of not using the adjustment power, after the parallel light enters the eye, it is not focused on the retina, but in front of the retina, so that the eye sees the scene is blurred, when the scene gradually moves closer to the eye, the focal point of the object image formed in front of the retina also gradually moves backward, and falls exactly on the retina, the object image becomes the clearest, which is myopia. Myopia is mainly characterized by poor distance vision and a tendency to squint at things. When reading and writing, the eyes need to be very close to the book, and a slight change in reading distance can cause blurred vision and visual fatigue. Nearsighted children are also prone to developing exotropia. Farsightedness When the eye is not adjusted, parallel light rays enter the eye and are focused behind the retina, so the eye sees blurred objects. The main manifestation of farsightedness is visual impairment, which is different from nearsightedness in that nearsightedness is blurred and clear, while farsightedness is not clear at all. Children with hyperopia also show the illusion of “nearsightedness”, which is mainly due to the fact that highly hyperopic children also like to read and write close to their eyes, making parents believe that they are nearsighted. Children with hyperopia can often develop internal strabismus. Astigmatism Astigmatism is a condition in which parallel rays of light entering the eye do not come together into a focal point and tend to form two focal lines, and the distance between the two focal lines determines the degree of astigmatism. The distance between the two focal lines determines the degree of astigmatism. With the help of adjustment or moving the target distance, a clear image cannot be formed, and only with suitable astigmatic glasses can we see clearly. Refractive parallax The refractive error of the two eyes is called refractive parallax, refractive parallax makes the image formed in the macula of the left and right eyes unequal in size and different in clarity, such as a large difference in refractive parallax, so that the imaging size is too disparate, resulting in the difficulty or inability to fuse the image of the two eyes. In order to eliminate mutual interference, the visual cortex center of the brain can only actively inhibit the refractive power of the larger eye, and finally, amblyopia and disuse exotropia are bound to form. It is generally believed that if the refractive difference between the two eyes does not exceed 250-300 degrees, it is still possible to obtain good binocular monocular function through correction.