Note: The information provided here is not a substitute for professional care. If you are having any problems with your eyes, you should see your ophthalmologist or optometrist for diagnosis and treatment.
It is estimated that 25% of the world population is myopic.The following information may help you to understand some of the vision problems.

The Eye is a magnificent piece of engineering: it takes external light rays through the lens system and focuses them on the Retina, where the optical nerve connects to the brain where the vision is processed.

A simplified Eye Cross Section

Figure 1

The main parts of the eye lens system are the Cornea and the Lens. The cornea, optically, is a fixed PLANO POWER bent lens. The Lens itself, optically, is a variable radii BI-CONVEX flat lens.

For distance vision, the eye is in a fixed focus mode: the Lens is at a rest position, that is, is in a fixed position with maximum radii. The light rays coming through the cornea, are bent toward the Lens, where they are bent (or refracted) again toward the back of the eye and focused on the Retina

For close vision, the eye is in an auto focus mode. The change in the Lens radii is called Accommodation. If the Lens would not accommodate, the light rays would be focused behind the retina and the vision would be blurred, as shown in below.

Figure 2

When the Lens accommodates, that is, it changes its spherical radii, the focal point moves to the correct point on the retina. Muscles in the Lens change its shape, making it of stronger power by decreasing the radii of both curvatures. The figure below shows the accommodated Lens for close vision. Notice the difference in the Lens shape before and after accommodation.

Figure 3

As we get older, the Lens of the eye loses its flexibility and the muscles that control its shape become weaker: the eye auto-focus system for close distance vision is degraded. Objects seen at short distance focus behind the retina, causing blurred vision. Figure 2 above shows this condition.

This condition is called PRESBYOPIA (Greek for old eyes), and refers to the eye's loss of ability to focus on close objects by changing the Lens shape.
Presbyopia is not an eye disease, it is caused by the natural process of aging, and as we all know, it can't be prevented. The first sign of Presbyopia is difficulty reading fine print at normal reading distance, especially in the absence of good reading light, the vision is blurred. That is why presbyopic persons tend to hold reading material at arm's length.

Presbyopia may seem to occur suddenly, but it actually progresses gradually from a young age and it begins to be noticeable when one reaches their forties. Unfortunately, it does not stop at the forties, it keeps on progressing.

To correct this condition, your optometrist or ophthalmologist can prescribe reading glasses, bifocals, trifocals or contact lenses. Since presbyopia may be more advanced in one eye than the other, the reading glasses may have lenses of different power.

Reading Glasses are simple "Plus Lenses", which move the focal point of light from the back of the retina forward directly to the retina. Mild presbyopia is corrected with low power lenses (1.0 or 1.5 diopter). Stronger presbyopia needs higher power lenses, 3.0 or higher. Since presbyopia progresses with time, stronger lenses are required as we age. At 45 we may need 1.5 diopter lenses, but at 60, we will need stronger lenses.

If your eye doctor determines that your presbyopia is the same for both eyes and you do not have any other vision problems, he/she may suggest that off the shelf reading glasses may be OK for you.

Figure 4

Notice that in the above figure, the Lens has not changed its shape (radii).

Your eyes are precious. Periodic check-ups with an optometrist or ophthalmologist will ensure your vision health. Below there is information about other eye problems.

Hyperopia or Farsightedness, like Presbyopia, also moves the focal point of light behind the retina for close vision. This is not an age-induced condition, but it is caused by an abnormality on the eye shape (cornea, lens or eyeball). People with this condition have trouble with close vision, but may also have problems with distance vision. This condition is also corrected with Plus Lenses. Figure 4 above also applies to the Farsightedness condition.

Myopia or Nearsightedness, unlike Farsightedness, moves the focal point of light in front of the retina instead of directly on it. Like Hyperopia, this is not an age-induced condition, but it is also caused by an abnormality on the eye shape (cornea, lens or eyeball). People with this condition may have good close vision, but have difficulty for distance vision. This condition may be corrected with Minus Lenses: surgery and other procedures are available for adults who desire to see without glasses. Consult with your eye doctor on how to best treat this condition.

Figure 5

Astigmatism is a condition that causes light to focus on more than one point on the retina, resulting in blurred, close and distance vision and often occurs along with Hyperopia or Myopia. This is not an age-induced condition, but it is also caused by an abnormality on the eye shape (cornea, lens or eyeball): a normal eye Lens-System is spherical, but an Astigmatic one is not, having a more elliptical shape. You may remember from your high school geometry that an ellipsis has two focal points: the light rays entering the eye are focused in more than one point on the retina causing the blurred vision. This condition may be corrected with Toroidal lenses or surgery. Consult with your eye doctor on how to best treat this condition.

Figure 6

Since Astigmatism often occurs along with Hyperopia or Myopia, the lenses to correct the vision may be a combination of Toroidal, Minus or Plus lenses.

Surgical Procedures for vision correction.

As we discussed before, the mayor vision problem can be corrected with optical lenses. In the XX century, great advances have been made in surgery for vision correction. The following is a brief informational non-technical description of some of the procedures. Although there are other surgical procedures, here we describe some of them. For more details, consult with your eye doctor.

RK (Radial Keratotomy).
This procedure was discovered accidentally in Russia in the early 1970's. One young myopic boy got in an accident while wearing his eyeglasses. A minute sliver of glass cut the boy's cornea and a few days later, his myopic vision improved. Dr Svyatoslav Fyodorov, his doctor, realized that the cuts had flattened the cornea, somehow correcting the boy's myopia. He developed the technology to correct myopia by making very small precise and small radial cuts in the cornea. Radial Keratotomy (RK) became successful in Russia and came to the US in the late 1970s. With the advent of Laser surgery, it is now seldom performed. If you remember, myopic vision is caused mainly because the cornea, (and maybe the lens), reflects the light rays in front of the retina.
The incisions are made in a radiating pattern, like the spokes on a bicycle wheel. By varying the number, length, depth, and location of these incisions, different amounts of nearsightedness can be corrected. By flattening the cornea with the radial cuts, the cornea's radius is increased.

Astigmatic Keratotomy (AK)
Astigmatic keratotomy is a variation of RK, used to treat astigmatism. AK uses arc-shaped incisions in the cornea, whereas RK uses radial incisions, like the spokes of a wheel.

Laser Surgery
The four procedures described below use the computer controlled Excimer Laser to reshape the front of the cornea for Myopia, Farsightedness and Stigmatism.
In general, for Myopia, the cornea is reshaped to have a larger radius: for Farsightedness, the cornea is reshaped to have a smaller radius an for Astigmatism, the cornea is reshaped with an appropriate Toroidal shape.

PRK (PhotoRefractive Keratectomy)
The top thin membrane on the surface of the cornea is removed. Laser energy is then applied to reshape the cornea with the appropriate contour. A contact lens is worn for 5-7 days while the soft material regenerates. Only one eye is usually treated at a time; the other eye is usually treated in 2-4 weeks.

LASIK (laser in-situ keratomileusis).
This is the most common type of treatment. A flap is made in the cornea using an instrument known as a microkeratome. The flap is folded back 180 degrees on one side ("hinge"). The laser energy is then applied under the flap to the cornea to reshape it with the appropriate contour. After the cornea has been re-shaped, the hinged Flap is returned to its original position on the cornea. LASIK allows for the most rapid visual recovery. Both eyes can be treated at the same time.

IntraLASIK (all-laser LASIK).
This procedure is similar to LASIK, the only difference is how the Flap is made. Here, the Flap is created with the Intralase laser. The Intralase laser beam, computer controlled, passes through the cornea and creates a very precise cut, similar to the one created by the microkeratome. The vision is then corrected using the Excimer laser as in the LASIK procedure. The visual recovery is slower than with standard LASIK, though both eyes can be treated at the same time. People with thin corneas are good candidates for this procedure.

LASEK (laser epithelial keratomileusis).
This procedure is similar to the LASIK. The major difference is that the Flap is done on almost exclusively on the top material covering the cornea.

And don't forget, periodic check-ups with an eye doctor will ensure your vision health.