Presbyopia is characterized by an age-related progressive loss of the ability to see things that are near. Presbyopia usually begins in a patient’s 40s. This affects close-up tasks such as reading or working at the computer. Most people become Presbyopic around the age of 40, even if they have never had a vision problem previously. People who were previously wearing eyeglasses or contact lenses for their Myopia or Hyperopia will also start to notice that their near vision blurs when they wear their usual eyeglasses or contact lenses. Presbyopia typically continues to worsen until around the age of 60.
Presbyopia predominantly arises from a stiffening and weakening of the eye's Crystalline Lens with age. When a person is looking at something up close, light rays enter the eye at a high angle. In a younger person, the eye's Crystalline Lens is flexible and strong, rapidly changing shape to bend those high-angle light rays so that they converge precisely where they are supposed to: on the surface of the Retina (producing a clear image). However, as we age, the Crystalline Lens stiffens and weakens, and is unable to sufficiently bend the high-angle incoming light rays. As a result, the incoming light rays converge at a point behind the Retina, thereby creating a blurred image for near vision. A patient with Presbyopia is prescribed a ‘relative plus’ powered corrective lens (plus relative to their pre-Presbyopia prescription), which increases the angle of the bend, thereby moving the convergence of the light rays forward to the Retinal surface.
For a person who, prior to developing Presbyopia, already has Myopia or Hyperopia due to the length of the eye, Bifocal or Multifocal lenses will be prescribed, containing both an area of ‘minus’ power (to correct the Myopia) or ‘plus’ power (to correct the Hyperopia) as well as an area of ‘relative plus’ power to correct the Presbyopia arising from age.
Sources: Definition – American Optometric Association website ©2013 American Optometric Association; Grosvenor. Primary Care Optometry 5th edition. Chapter 1. pages 4-5. Videos/video links – courtesy of geteyesmart.org; eye health information from the American Academy of Ophthalmology.