Astigmatism is not common to everyone, but it is a common phenomenon, everyone may have more or less astigmatism, generally within 50 degrees is an acceptable range does not need to be corrected. For example, orthokeratology 5.0 may also have myopic astigmatism or hyperopic astigmatism, but only astigmatism of more than 50 degrees is obviously affecting the human eye, vision loss, it is necessary to correct. The cornea and axis of the human eye are not exactly the same, and it is impossible to achieve an ideal refractive state, no one is absolutely free of astigmatism, more or less, and it can be caused by the surrounding environment, ageing, or the development of the eye and lens.