Presbyopia

Presbyopia is a reduction in the focusing power of the eye with age.  It results in a decrease in clarity of vision at near distances (but does not affect distance vision).   It is generally defined functionally - e.g. when a person can no longer read a newspaper at arm’s length, and buys reading spectacles, they are considered to be presbyopic.

The eye is like a camera, in that it has an optical system (the tear film, cornea, aqueous humour, lens, and vitreous humours) and a light-responsive ‘film’ (the retina).  The image of an object is in focus on the retina when the optical power and the length of the eye are equal.  [See emmetropia]

The length of the eye is fixed, but the power of the eye can be altered by a process called accommodation during which a muscle within the eyeball (the ciliary muscle) contracts, causing a change in shape of the lens, and a consequent change in optical power of the lens.  When changing from distance vision to near vision the eye is required to increase its power to bring the image into focus - in the same way that the focus of a camera needs to be changed to maintain a clear picture when changing between objects which are at different distances.

The maximum increase in power that can be achieved decreases with age.  This happens because the lens hardens (NOT because of changes to the muscle as is often stated).  Although there is a common belief that this change occurs around the age of 40, in fact it starts at a very young age - probably in childhood.  Presbyopia is when the person can no longer accommodate sufficiently to achieve clear vision, and is functionally-defined.


The diagram shows the change in near point of accommodation (the nearest that you can see clearly) with age, and it is apparent that, although the near point recedes well before a person is in their forties, it is only at that age that it will affect functions such as the reading of a book or newspaper.

Functionally presbyopia results in either, or both, blurred near vision and symptoms such as eyestrain and headaches.  Its effects are more apparent at low light levels (when the pupil of the eye is larger).
 

A hypermetrope will use accommodation to overcome their hypermetropia, and so as they age they suffer problems at near distances before a normal person (as part of their accommodative ability is used up overcoming their refractive error).  Eventually, once they lose all of their accommodation, they will need an optical correction for both distance vision and near vision.