What is accommodation?
Accommodation is the ability of the eye to change the focal length of the lens by changing the curvature of the eye lens. Accommodation allows the eye to automatically adjust focus from seeing things at a distance and "tune" it to seeing nearer objects.
Why does the eye need to accommodate?
The ciliary muscles, responsible for accommodation of the eye, are usually at rest. When at rest, parallel light rays that form distant objects converge onto the retina, giving you a sharp and clear view of the object.
If the eye were to remain in such a state of rest and an object placed nearer to it, the light rays would converge behind the retina. Since the sharp image is behind the retina, our brain will only detect a blurry image of the closer object.
Therefore, in order to bring that image of the closer object back into focus, the eye performs the process of accommodation.
How does the eye accommodate?
The cornea (the transparent front part of the eye covering the iris and pupil), provides only 2/3 of the refractive power and the lens, 1/3 of it. However, it is our eye that changes the curvature of the lens during the accommodation process and not the cornea. The curvature of the cornea cannot be changed. It is the changing of the thickness of the lens which results in a change of its focal length.
In a normal resting state, the ciliary muscles are usually relaxed, causing the elastic lens to become thick. If the aqueous humour (spaced between the lens and the cornea) and vitreous humour (spaced between the lens and the retina) push outward on the sclerotic coat (the outermost membrane surrounding the eye), the ligaments become tensed. This causes the lens to pull itself into a thin shape, resulting in a short focal length.
When we grow old, our lens will turn hard. Our accommodation ability will decrease and it will get more and more difficult to focus. This defect is called presbyopia.
Relation to Laser Eye Surgery
Refractive surgery is used to correct eyesight and restore near-perfect vision. LASIK is a surgical technique that corrects a person's vision, reducing the need for spectacles or contact lenses. Known as Laser Assisted in Situ Keratomileusis or LASIK, the surgery involves changing the shape of the cornea.
Refractive surgery can also be used to treat presbyopia. Monovision or laser blended vision is used to provide the patient with good vision for both near and distance. It is a very effective way of combating the loss of accommodation in later life.