Cornea is the eye's clear, protective outer layer and provides most of the eye's optical power. Cornea plays a major role in focusing vision. Its curvy shape helps in refracting light from an object in a way that it falls at the perfect spot on the retina thereby enabling sharpness of vision.
Lens is a transparent convex structure that can change shape, helping the eye to focus light accurately onto the retina. Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects. Thanks to this adjustment in the lens, known as accommodation, the eye can see sharply both near and far.
Retina is a thin layer of tissue lining the back of the eye. Contains light sensitive cells and nerve fibres. Light falling on the retina produces chemical changes in the cells which then send signals along the nerve fibres via the optic nerve to the brain.
Macula is the “yellow spot” in the small (approximately 1.5 mm in diameter) central area of the retina. Contains a very high concentration of photoreceptor cells that detect light and send signals to the brain, which interprets them as images. Macula processes sharp, clear, straight-ahead vision whereas the rest of the retina provides peripheral vision.
Normal eye (Emmetropic eye)
Vision occurs when light rays are bent or refracted as they pass through the cornea and the lens. The light is then focused directly on the retina. Macula is the area where the light information is concentrated on the retina. A person with normal vision can see objects clearly near and faraway.
Refractive errors (ametropies)
Farsightedness, sometimes referred to as "hypermetropia", is the result of the visual image being formed behind the retina, rather than directly on it. The eyeball of a farsighted person is shorter than normal (D<24mm). Farsighted persons generally can see distant objects clearly, but have difficulty focusing on objects that are up close. Farsightedness can be corrected with eyeglasses or contacts that use plus lenses.
occurs when the visual image is formed at a point in front of the retina, rather than directly on its surface. A nearsighted person sees near objects clearly, while objects in the distance are blurred. If you're nearsighted, the first number on your eyeglasses or contact lens prescription will be preceded by a minus sign.
Astigmatism is a common eye condition which usually occurs with nearsightedness or farsightedness. Astigmatism is most often caused by an ellipsoid cornea or lens rather than a normal, spherically shaped cornea or lens. In normal vision, the interaction of the cornea’s spherical shape and lens allows light to enter the eye and come to a single point of focus on the retina. With astigmatism, light enters the eye, refracts, and comes to multiple points of focus, each taking place at different locations in the eye. A single clear image cannot be formed on the retina and patients with this visual problem experience blurred vision. Cylindrical lenses are used to correct astigmatism.
Accommodation disorder – Presbyopia
Presbyopia is an age-related process. It is a gradual thickening and loss of flexibility of the natural lens inside the eye. With less flexibility, it gets difficult for the eyes to focus on close objects. Most people begin to notice the effects of presbyopia sometime after age 40, when they start holding books and newspapers at arm's length to be able to read them.
Due to presbyopia, the single pair of glasses or contact lenses people previously wore generally will no longer give them clear vision at all distances. They will need two prescriptions – one for distance vision (driving, watching TV) and one for reading or other near-vision tasks. This can be achieved with bifocal or progressive lenses as well as specialized Digital Free monofocals.
Other types of refractive errors (astigmatism, myopia or hyperopia) may exist at the same time as presbyopia.
Strabismus is a failure of the two eyes to maintain proper alignment and work together as a team. Strabismus can be corrected with prism lenses.