Our eyes have an automatic focusing system which adjusts the lens inside the eyes in order to see things clearly at all distances. When looking far away, up close and back again, the eyes are able to change focus at a rapid pace to keep the objects clear in our vision at close and long distance. The eye focuses light in a similar way to magnifying glass when it is used to concentrate the sun’s rays onto a piece of paper.
The distance from the magnifying lens to the piece of paper is the focal length. The bending of light mostly takes place as it enters the cornea and the aqueous fluid. This bending occurs due to the curve of the cornea as well as the change in refractive index as light moves from air into the cornea and the aqueous fluid between the cornea and the iris. Behind the aqueous fluid is the crystalline lens system which consists of a soft and pliable convex lens. The ciliary muscle is a circular ring of muscle that attaches all the way around this lens. This ciliary muscle can change the shape of the crystalline lens by stretching it at the edges. It is attached to the lens by ligament fibers that can be tight or loose. When looking at a near object, autonomic nervous system regulates the ciliary muscle to contract, allowing the lens to become more rounded at the central surface in order to focus the light rays and see a near object clearly. This causes the lens to form a more spherical shape achieving a higher level of refractive power. On the contrary, relaxation of the ciliary muscle can flatten the lens, resulting in an increased long range focus for far-away objects.
Presbyopia is a condition associated with an advancing age that results in progressively worsening ability to focus clearly on close objects. Presbyopia often affects people aged above 40 years old. Regarding aging, the two main reasons for this to happen are a reduction of the ciliary muscle functionality and a hardening of the crystalline lens. As a result, the shape of the lens fails to be properly adjusted and the high level of refractive power for seeing close objects does bot happen, leading to blurred vision. However, presbyopia mainly affects an ability to focus on nearby objects, whereas ability to see far-away objects is not usually impaired. Despite presbyopia is a natural change in the eyes as people age, it often is a significant problem that largely interferes with daily life and activities.
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