The lens is an important part of an extremely complex human visual system. The eye is made up of several clear structures that light must traverse on its journey to the retina (at the back of the eye) to be perceived as a clear image. To reach the retina, light must first pass through the air, the tear layer of the eye, the cornea, the aqueous fluid, the crystalline lens, and the vitreous fluid of the eye, with each structure contributing its own effect on the refraction of light.
The cumulative bending (refraction) of light after passing through all of these structures ideally delivers the light in perfect focus onto the retina. If light is abnormally bent too much or not enough, the resulting image on the retina is blurry. When the eye’s natural and unaided bending of light results in a blurry image, it is referred to as a “refractive error” of the eye. The purpose of eyeglasses, contact lenses, and laser eye surgery are to correct any error of refraction that interferes with the proper focusing of light onto the retina.
The Crystalline Lens
Each of us is born with a naturally clear lens inside the eye, simply called the “physiological lens” or the “crystalline lens.” About the width of a pencil eraser, this lens further refracts (or bends) light after the light’s passage through the cornea on the journey to its ultimate destination, the retina. The physiological lens resides directly behind the iris and is mostly covered by the iris. Not much of the lens is visible to the naked eye of an observer. It is an adaptable and flexible clear tissue with the purpose of making “on the fly” focusing adjustments to keep focused images on the retina.
How It Works
The outside edges of the lens are attached to a muscle inside the eye called the ciliary muscle, which exerts force to change the shape of the lens. The ciliary muscle instantaneously alters our focus by either contracting to put the lens under tension or by relaxing to let the lens rest in its normal state. This process is called accommodation. The pulling and releasing of the lens causes the lens shape to change, allowing our eyes to focus on images at different distances. Like manually focusing a camera by rotating the lenses, this method of focusing happens due to the ciliary muscle tightening or releasing its grip on it.
The Aging Lens
Like our skin, the lens continually generates new cells as the old ones die off. New skin cells push up from the deepest skin layers as the outer, older skin cells die and flake off our body. However, inside the eye, the lens does not have the same option of flaking off old cells, since it has no place to discard the dead cells internally. Instead, old lens cells are pushed inward toward the center of the lens as the new cells generate from the outside-in. Over many decades, this process of compacting cells inward causes the lens to become less flexible when the ciliary muscle attempts to act on it. This is the reason that most people around age forty begin having trouble focusing at images up close. At this age, the lens inside the eye can’t readily adjust and focus as easily as it once did.
Experiencing Changes in Your Vision?
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