Astigmatism is a kind of refractive abnormality of the eye, which is related to the curvature of the cornea. In some people, the cornea of the eye is curved in a certain angle area, while in other angle areas it is flat. The cause of astigmatism is that the unevenness of the thickness of the cornea or the uneven curvature of the cornea makes the refractive rates of the corneal meridians inconsistent, so that the light passing through these meridians cannot be concentrated in the same focal point, and the light cannot be accurately focused on the retina A clear image is formed on the surface, which is called astigmatism.
So how to refraction for astigmatism?
1. Crossed cylindrical mirror method
First measure the axis of the near astigmatism, make the cross-cylinder handle coincide with the axis of the far astigmatism, flip the cross-cylinder handle, and use the "forward ten retreat five" method on the clearer side. The handle rotates to the red side to make it clear at the same time Or blur as the end point; secondly, determine the near-use astigmatism, the red dot of the cross cylinder coincides with the astigmatism axis, flip the cross cylinder, add -0.25D when the red dots coincide, and decrease -0.25D when the white dots coincide.
2. Astigmatism dial method
Remove the astigmatism for the distance, and according to the 30-degree rule, multiply the clearest scale of the astigmatism dial for the near-use astigmatism axis by 30. On this basis, increase or decrease the astigmatism power to make the astigmatism meter clear.