Calculation of focal length and power of correcting lens in myopia: Let x be the distance of the actual far point from the eye (distance from the concave lens placed near the eye).∴ Object distance or far point of the eye, u = – ∞Image formed, v = – x,Focal length, f =?Using lens formula,               1f = 1v-1u      = 1-x-1-∞       = -1x+0      = -1x. ∴   Required focal length, f = -x Required power, P = 1f =-1x Since the power is negative, this implies that the correcting lens is a concave lens.