Years of research and hard work were dedicated to develop cameras and multi-featured objective lenses. Initially, one to three lens objective cameras were used for simple shots, whereas today, objectives with seven to ten lenses are being used to take better photographs from a snow drop falling onto a flower to a buzzing bee resting on a flower. I wonder to what extent human beings are aware of the pair of eyes that has been bestowed upon them by God, and its ability to see different colors and shapes both near and far. Unfortunately, as people who often understand the true value of things once they are lost, we understand the blessing of being able to see near after the age of forty when we cannot read the newspaper without glasses and when we cannot put a thread through a needle.
So why is it that we can still see far after the age of forty but fail to see near? In order to understand this we need to examine the structure of the eye and its functions.
The structure of the eye and the ability to see
The exterior part of the eye is made up of a translucent layer (cornea) at the front and a white protective layer (sclera) behind it. The vascular layer of the eye (uvea) is located in the middle of the sclera. The most inner part of the eye is made up of the retina, the light-sensitive layer of tissue responsible for converting light rays into electrical signals. The hole located in the center of the iris, the colored part of the eye, is called the pupil. Behind the pupil is the crystalline lens. For a clear vision, lights reflected from objects need to be focused on the central part of the retina (fovea). While cameras have lens systems to focus the image on the film, it is the cornea and crystalline lens that are responsible for the same function in the eye.
Refraction power of cornea is constant and around 43 diopters. The refraction power of the eye lens when resting is around 20 diopters. Light rays coming from outside refracts at a set ratio and manages to focus on the retina. The light rays coming at the retina are then coded into electrical signals. Afterwards these signals are routed towards related regions of the brain via optic nerves. Most of the stimuli relayed by the optical nerve arrive at the visual center of the brain (occipital cortex). These coded electrical signals then become vision when they reach the optical lobe of the brain.
The function of the lens and accommodation
The refraction power of both the cornea and the lens (43+20+63 diopters) is sufficient to focus an image on the retina when looking at objects farther than 6 meters. Extra refraction power is needed for closer distances in order to focus images on the retina. Mobile lens systems enable this job to take place in camera objectives. Since refraction power of the cornea in human eye does not change, this additional task of refraction is set to be provided by the ocular lens. It is built as a flexible structure without any blood vessels. Aqueous humor (lens fluid) which is secreted by the ciliary body is responsible for lens nourishment, removal of waste products and toning of the eye since the lens does not contain any blood vessels. This internal fluid has low oxygen concentration therefore the lens is made to derive its energy supply mostly from anaerobic metabolism.
The iris is positioned in a suitable place where it can change the shape of the internal lens behind the pupil. The lens in this special place is suspended into position via zonule of zinn ligaments attached to the eye as a ciliary body. The ciliary body contains ciliary muscles where zinn ligaments are attached. Only 0.5 mm of space exists between the lens and the ciliary body. Zinn ligaments are tight when ciliary muscles are resting and this enables a flatter configuration of the lens. Upon contraction of ciliary muscles, zinn ligaments become relaxed and the diameter of the lens decreases along with an increase in its thickness. Thicker lens becomes more globular and this increases its refractive power, thus enabling vision of the closer distances. This increase in refractive power of the lens in order to see closer objects is called “accommodation.” If the stimuli of the ciliary muscles expire, ciliary muscles then relax making zinn ligaments tighter, reducing thickness of the lens, making it flatter and therefore less refractive. This reshapes it to focus on distant objects for a clearer vision.
Accommodation mechanisms and loss of accommodation during aging
The vision blurs temporarily when one takes an immediate shift from staring at an object in the distance to another object nearby. As soon as this blurry image reaches the occipital cortex, stimuli generated here arrives first at the Edinger-Westphal nucleus via special nerve tracks and then to the ciliary muscles of the eye. In a very short time, this blurry vision is corrected and becomes clearer without us even noticing with optimal increase of refraction in the internal lens. In a time as short as 0.35 seconds, for thousands of times in a day, this mechanism is set to function in such a perfect manner to spur those thoughtful minds into reflection and wonder.
Accommodation ability is at its highest point in children and this feature of the eye decreases with age. Refractive power of the lens can increase up to 34 diopters with a 14 diopters accommodation power along with 20 diopters of resting refraction during childhood. This way, children can clearly see objects as close as 7 centimeters. Accommodation power decreases with age. It reduces to 4-8 diopters after the age of 40 and 2-3 diopters around the age of 50. It is widely accepted that refractive power disappears entirely after the age of 60.
In the advanced stages of aging, the eye lens loses its transparency, becomes cloudy as it develops cataract. Eye lens in this poor transparent stage is removed via cataract surgery, to be replaced with an artificial lens to carry out the refracting task. Unfortunately today, technology is still unable to produce an artificial lens that is capable of all the tasks that a human eye can perform. Artificial internal eye lenses that are used in surgeries today cannot carry out accommodation functions. Majority of these lenses can only focus on one point at a near or far distance. Newly developed multifocal lenses can utilize various mechanisms to see both near and far distances yet they are not in any position to replace the human lens completely.
Ocular motions when looking near and far
Thanks to ocular movements, we do not have to move our head constantly while looking around. The eye movement involving both eyes in which each eye moves in the same direction is referred to as version type movements. Another movement type is called vergence, and this is when both eyes move in opposite directions. Vergence type movements are a type of ocular motility coded in a special center part of the brain. It is called convergence because the eyes get closer to each other when looking at closer distances, and called divergence when both eyes focus on the same spot by directing away from each other. If eyes only moved in the same direction without this convergence mechanism, both eyes would not be able to focus on closer points and would not be able to develop three dimensional visions (depth perception).
In addition to accommodation and convergence, when we look closer, our pupils get smaller (Miosis). Light rays coming from outside objects get improved focus on the retina via this constriction of the pupils. This way, a clearer image is provided.
When we look closer, accommodation, convergence and miosis all happen at the same time in a synchronized manner to provide a clear vision. The details of these complicated chains of events have yet to be understood.
The fineness of refractive power of the eye with a single lens, accommodation ability and sensitive balances of ocular motility is only a few of the blessings of the eye granted to humankind. The ability to see near being at its peak during young ages when learning is most active is another dimension to this miracle. These wisdom-filled capacities given to the eye makes one ponder upon the importance of the eye for survival, in addition to being a reminder to those with an open mind and heart to gaze upon the natural world and contemplate upon the Almighty.