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 25 cm for a grown-up person.
There are mainly three common refractive defects of vision :
- A person suffering from this defect cannot see distant objects clearly.
- This is because fmax 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 and the image is formed in front of the retina.
- The ciliary muscles are fully relaxed in this case.
- This defect may arise due to(i) the excessive curvature of the eye lens, or (ii) elongation of the eyeball.
- Remedy – 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.
- A person suffering from farsightedness cannot clearly see objects close to the eye.The least distance for clear vision is appreciably larger than 25 cm and the person has to keep the object inconveniently away from the eye.
- This defect arises either because (i) the focal length of the eye lens is too long, or (ii) the eyeball has become too small.
- The ciliary muscles even in their most strained position are not able to reduce the focal length to an appropriate value.
- This can be achieved by putting a converging lens in front of the eye.
- The power of accommodation of the eye usually decreases with ageing. It is difficult to see nearby objects comfortably and distinctly without corrective eye-glasses.
- It arises due to the gradual weakening of the ciliary muscles and diminishing flexibility of the eye lens.
- Sometimes, a person may suffer from both myopia and hypermetropia. Such people often require bifocal lenses. A common type of bifocal lenses consists of both concave and convex lenses.
- The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.