The human eye is in a relaxed state when it is looking away, and looking away allows the regulation to rest. The closer you look at the time you have to use regulation and convergence, the closer you look, the more effort it takes. Adolescents usually read and write at a distance of 35-40cm (two arms down, bend the elbow 80 degrees, natural chest), this time to use 2.9-3D regulation power (regulation power = the inverse of the visual distance, the unit is meters; 1D is equivalent to the refractive power of 100 degrees lens). When climbing down to read a book distance of only 10-15cm is to use 10-6.7D adjustment force, which is usually 2-3 times the adjustment used for sight distance, so that the eyes are not only easy to fatigue, but also multiply the chances of myopia formation.