lens
Meaning of lens: Most of the eye's focusing power comes from the cornea; the lens supplies the adjustable part, and that adjustment is what age removes.
Definition of lens
The lens of the eye is a clear, flexible disc sitting just behind the iris that fine-tunes the focus of light onto the retina. It has no blood supply and no nerves, and its transparency depends on proteins packed in precise order. Altering its curvature shifts focus between distant and near objects. The word also names any curved piece of glass that bends light.
How does the eye change focus?
Light is bent twice on its way in. The cornea at the front does most of the bending, and its power is fixed. The lens supplies the remainder, and its power is adjustable. A ring of muscle surrounds the lens and is joined to it by fine fibers. When that muscle contracts, the fibers slacken and the elastic lens is free to become rounder, bending light more strongly for a near object. When the muscle relaxes, the fibers pull taut and the lens flattens for distance. The whole change happens in a fraction of a second and is called accommodation. Focused light then falls on the retina, the light-sensing layer at the back.
Why does near vision fade with age?
The lens keeps growing throughout life, laying new fibers over old ones without discarding anything, so it thickens and gradually stiffens. A stiff lens cannot round up on demand, and near objects blur while distance vision is untouched. This is presbyopia, it happens to everyone, and it is entirely separate from long-sightedness, which comes from the eyeball being short relative to its focusing power.
What goes wrong with the lens itself?
| Problem | What has changed | Effect on vision |
|---|---|---|
| Presbyopia | Lens stiffens with age | Near focus lost, distance unchanged |
| Cataract | Lens proteins clump and scatter light | Dimming, glare, faded color, gradual blur |
| Dislocation | Supporting fibers give way after injury or in some inherited conditions | Sudden marked distortion of vision |
Because the lens has no blood supply, it depends entirely on the fluid around it, and that is why conditions and medicines that alter that fluid, along with ultraviolet exposure, smoking and diabetes, all affect how early a cataract forms. A clouded lens can be removed and replaced with a clear artificial one, which is possible precisely because the structure carries no nerves or vessels of its own.
Used in a sentence
The eye examination noted early clouding of the lens consistent with a developing cataract.
Good to know
Clouding of the eye's lens, called a cataract, is a change in the lens itself rather than a problem of the retina or optic nerve, which also blur vision but by an entirely different mechanism.
Word origin
Where “lens” comes from
From Latin lens, meaning lentil, a comparison drawn from the similar rounded shape of the seed.