For presbyopia and hyperopia, a converging lens provides the extra curvature necessary to bring the near point closer to the eye while for myopia a diverging lens provides the curvature necessary to send the far point to infinity.
12.
The optical power of corrective lenses is measured in diopters, a value equal to the reciprocal of the focal length measured in metres; with a positive focal length corresponding to a converging lens and a negative focal length corresponding to a diverging lens.
13.
When this is achieved, the screen is replaced with a mirror and the diverging lens is inserted between the converging lens and the light source at such a distance to the mirror that the light returning through the diverging and converging lenses produces a sharp image on top of the luminous object.
14.
One special application is to determine the focal length of a diverging lens : A light source is placed at twice the focal length of a converging lens on one side and a screen at the same distance on the other side so that the image of the light source is the sharpest possible.
