Before you understand what presbyopia is, understand the following basic concepts first:
1. The eye is like a camera. The camera has two main parts. One is the lens system at the front part which focuses the light/image on the film of the camera (or an electronic sensor in modern cameras) which itself is the second part and lies at the back of the camera. The eye has a similar refractive system (cornea and lens) anteriorly which is analogous to the lens/lenses of the camera. The eye has retina posteriorly which is analogous to the film or electronic sensor of the camera. In simpler words, cornea and lens of the eye focus the light on the retina where image is formed.
2. Cornea which is the most anterior part of the eye is the main medium where the light rays falling on the eye converge. For all practical purposes, the converging power of cornea remains unchanged. This is attributed to almost constant shape of cornea.
3. The lens lies behind the cornea and iris. It is different from cornea in the ability to vary its shape because of its elasticity. It can become more convex (rounder/more curved) to have more light bending power. It can also become less convex (flatter/less curved) to have less light bending power.
The ability of the lens to become more spherical to increase the power of the optical system of the eye is called as ‘accommodation’. The lens is attached peripherally to a circular tissue called ciliary body with zonules. A circular muscle in the ciliary body known as ‘ciliary muscle’, when contracts, causes relaxation of zonules and allows the lens to change to its more spherical shape.
4. By its ability to vary its converging power, the lens essentially helps in fine tuning the focusing of light (on the retina) already refracted by the cornea.
5. The variation in the focusing power of the lens depends on the distance of the object (what the eye is looking at) from the eye. If the object is at some distance, the lens reduces its convexity (becomes flatter) to an extent so as to focus the incoming light at the retina. This results into formation of a clear image at the retina. If on the other hand, the object is close to the eye, the lens acquires more convex shape to an extent where a clear image is formed at the retina.
It is important to mention here that the light rays coming from distance are parallel and therefore less amount of convergence is required for them to form clear image at the retina whereas the light rays coming from a near object diverge and more amount of convergence is required for the light rays to form a clear image at the retina.
Coming back to the main question, presbyopia is a condition in which the lens gradually loses its elasticity (as the lens becomes harder and harder with age) and thereby its ability to change its shape to become more convex. This results into inability of the presbyopic person to see objects close to eyes clearly. In this condition, the light rays cannot be focused on the retina because the lens cannot become more convex for the rays to converge on the retina. The image forms behind the retina instead and appears blurred.
Presbyopia usually appears around the age of 40 years. It is rare before the age of 35 because the lens is quite elastic before that age. Presbyopia can easily be corrected with glasses with plus lenses. These plus lenses are used to compensate the loss of power of the natural lens of the eye proportionate to the loss of elasticity.
Presbyopia is bilaterally equal because almost same changes occur in both the eyes. Therefore the plus lenses for both the eyes are almost always equal.
The loss of elasticity of the lens continues till the age of approximately 60 years of age after which it becomes stable. The power of the corrective lenses in the reading glasses also gradually increases till the age of 60 years after which it remains almost the same.
Presbyopia is an age related condition. It happens to everybody sooner or later. The only exceptions are high myopic (short sighted) persons – their inability to see distant objects clearly and ability to see near objects clearly compensates for any presbyopia.