When someone undergoes laser surgery to improve eyesight, how does it correct accommodation problems?
Why does it not help presbyopia?
Laser eye surgery works by altering the shape of the cornea. The cornea works together with the lens to focus rays of light onto the retina. The cornea accounts for two-thirds of the optical power of the eye (1) (i.e. the eyes capability to focus light), however unlike the lens is of fixed power. It is the lens that changes is shape by the action of suspensory ligaments and ciliary muscles in order to focus light by the correct amount depending on how far away the light originated from (and consequently the angle of incidence with the cornea). I'm sure that you are aware of all this, however for the benefit of future readers this is fully explained diagrammatically on this webpage.
Three forms of accommodation problems can be treated by laser surgery:
Hypermetropia (long-sight) is where light focusses behind the retina:
Laser treatment is used to make the cornea thicker, resulting in a greater degree of refraction of light and correction of the focus.
Myopia (short-sight) is where light focusses before the retina:
Laser treatment is used to make the cornea thinner, resulting in a reduced degree of refraction of light and correction of the focus.
Astigmatism is where the cornea is not the correct shape - it is closer to a rugby ball rather than the sweeping curve demonstrated in the first diagram. This results in multiple focal points therefore a blurred image.
Laser eye surgery is used to alter the shape of the cornea until it is more normal.
Presbyopia is an age related condition where the lens becomes more rigid and is less able to change its shape to accommodate the light.
As this is a problem with the lens rather than the cornea, it can not be treated with laser eye surgery. It must be corrected with glasses, as shown in the above diagram.
(1) Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990.