The ciliary muscles control the curvature of the lens in the eye and hence can alter the effective focal length of the system. When the muscles are fully relaxed, the focal length is maximum. When the muscles are strained, the curvature of the lens increases and the focal length decreases. For a clear vision, the image must be formed on the retina. The image – distance is, therefore, fixed for clear vision and it equals the distance of the retina from the eye – lens. It is about 2.5 cm for a grown – up person. If we apply the lens formula to eye, the magnitudes of the object – distance, the image – distance and the effective focal length satisfy.

$\frac{1}{{\mathrm{v}}_{\mathrm{o}}}-\frac{1}{(-{\mathrm{u}}_0)}=\frac{1}{\text{f}}$

Here, v₀ is fixed, hence by changing f, the eye can be focused on objects placed at different distances u₀. For an average grown – up person, u₀ can be varied from around 25 cm or less to infinity for clear vision i.e. image focused on retina. Nearsightedness or Myopia. A person suffering from this defect cannot see distant objects clearly. This is because f_max is less than the distance from the lens to the retina and the parallel rays coming from the distant object focus short of the retina. The ciliary muscles are fully relaxed in this case and any strain in it can only further decrease the focal length which is of no help to see distant objects.

The remedy of myopia is quite simple. The rays should be made a bit divergent before entering the eye so that they may focus a little later. Thus, a divergent lens should be given to a myopic person to enable him/her to see distant objects clearly.