Our eyes are very subtle and complex structures but basically they work like a camera. Light enters through the front clear part, the cornea, travels through the lens, which sits behind the colored part (iris), and continues on to the back inside surface (retina). In a 20/20 eye, it focuses on the retina, which is analogous to a camera film. The brain’s vision center functions like a dark room. It “develops” the images into intelligible scenes and objects, providing us with names.
The Main Eye Structures for Vision
- The Cornea
Good vision relies on the cornea being absolutely clear and transparent. Allowing the light through is one of the cornea’s functions, and another is bending (refracting) that light. Every clear, curved structure refracts light – that is what a lens is, whether it’s in a camera, a telescope, a pair of glasses, or the eye. Each person’s cornea is curved and shaped in an individual way and bends light differently.
- In a myopic (nearsighted) eye, it is curved too steeply and refracts the light coming from far objects too sharply for it to focus on the retina. That gives blurry far vision.
- In a hyperopic (farsighted) eye it is too flat and doesn’t refract the light coming from close objects enough. That gives blurry near vision.
- In an astigmatic eye it is more oval than round, which gives it two curvatures, each refracting light differently, and that gives blurry vision at all distances.
The lens is suspended behind the iris inside a membrane (pocket). Being another clear, transparent structure, it also refracts incoming light. The cornea does about 60% of the refraction and the lens about 40%. So incoming light passes through the lens to the retina in a bent formation which will focus somewhere – on the retina in a 20/20 eye.
If the cornea has refracted the light too much (myopia), the added lens refraction will focus it in front of the retina. If the cornea has not refracted it enough (hyperopia), the lens will refract it more, but this won’t be enough, and it will focus behind the retina.
Both in front of the lens and behind it are areas filled with fluid. For light to pass through, these fluids must also be clear.
The lens provides an important service for clear vision by changing its own curvature. When we focus on a far object, the lens automatically becomes flatter so as to bend incoming light less. When we focus on a near object, it becomes steeper to bend light more. This is called accommodation, and is done by tiny muscles which control the lens curvature.
Retinal cells (photoreceptor cells) are very light-sensitive and there are two main types:
- Cones – clustered mostly in the center of the retina. They give us color identification and our central vision in bright light, and a human eye has about 6 million of them.
- Rods – more dense towards the retina’s periphery. They provide low-light vision but not much color perception, and a human eye contains about 120 million.
Nocturnal animals have far more rods than humans do. Retinal cells receive information brought in by the light rays as a tiny upside-down image of what you are looking at. They convert this light information into neural data so that the optic nerve can carry it to the brain.
This is a large nerve that connects to the retina and exits the back of the eye, traveling through to the back of the brain (occipital lobe). The small area where it leaves the retina is a blind spot. When the brain’s vision center receives the information, it interprets it and finds names for it. The retina is really an extension of the brain via this large nerve.
The aging eye becomes less able to focus clearly on near objects. This happens to everyone at some point, usually around the age of 40 or a bit more. Presbyopia seems like hyperopia, since near vision is blurry in both cases, but it has a different cause. The lens becomes less able to accommodate to differing distances – whether because it becomes stiffer, or because the muscles controlling it become weaker, or for some other reason not entirely understood.
Reading glasses become necessary. If this interferes with your lifestyle too much, Dr. Huynh can do a surgery which is also done to treat cataracts (light blockages in the lens). The procedure removes the lens and replaces it with an intraocular lens (IOL). Since accommodation is lost along with the lens, the IOLs are designed to mimic that lost ability using different technologies.
A LASIK procedure corrects vision by changing the corneal curvature. PRK also does this. Laser vision correction has become more and more refined since it was first approved by the Food and Drug Administration (FDA) in 1995. Custom LASIK includes a very sophisticated diagnosis of exactly how each of your eyes varies from perfection. No two human eyes are the same, and your LASIK treatment is based on the very detailed 3-D maps created during diagnosis for each eye.
Many defects and diseases can affect the eyes, and Dr. Huynh can diagnose and treat them all. Examples are glaucoma and macular degeneration. But putting eye disease aside, there is no need any more for a person to live with defective vision. If you would like to know more about your eyes and vision, and whether laser vision correction could be a good option for you, please contact our San Diego office for a personal consultation.