Contact lenses have been a boon for people with vision disabilities. They became popular in the United States in the early 1970s, and since then, have caught the fancy of people worldwide. Today, there are prescription contact lenses, as well as contact lenses for cosmetic purposes. There are a number of contact lenses available with different designs for various vision disabilities.

Soft Contact Lens Designs

There is a particular soft contact lens design available to suit a particular vision problem. It does not matter whether you are nearsighted or farsighted, you are suffering from astigmatism, or you need a bifocal prescription for your presbyopia, your ophthalmologist can recommend the best contact lens design that fits with your specific need.

Spherical Single Vision Contact Lenses ? These are the most common and popular type of soft contact lenses available. They take care of your nearsightedness, and farsightedness (myopia, and hyperopia). Such spherical single vision contact lenses have the same power all along its curvature. The power of contact lenses is expressed in units known as diopters (D).

As the power in all the meridians of the spherical contact lenses is the same, it is expressed by a single number to describe the lens. Soft spherical single vision contact lenses are made in the power increments of 0.25 D. For example: if your current contact lens power is + 1.75 D, the next stronger power lens available will be + 2.00 D.

The power of the contact lenses that correct nearsightedness is preceded by the minus (-) sign, whereas in the case of farsightedness, the power is preceded by the plus (+) sign. The soft spherical single vision contact lenses that have the power of ? 1.50 D correct 1.50 diopters of nearsightedness. The power is at times, denoted as ? 2.50 sph, or plainly ? 2.50.

Soft Toric Single Vision Lenses ? These soft contact lenses are used for the correction of astigmatism, in combination with either farsightedness, and/or with nearsightedness. Astigmatism, quite a common condition, is caused by irregularities in the cornea or lens of the eye. Usually a hereditary condition, in astigmatism, the normal spherical shape of the cornea, or the lens, of the eye is more oblong.

To correct the irregularly shaped cornea of the eye, these soft contact lenses have different powers in different lens meridians. The soft contact lenses usually rotate in the eye. In case of the spherical single vision contact lenses, it does not matter, as the power is the same all around its meridian. In the case of soft toric single vision lenses for astigmatism, the lens has to be kept from rotating in the eye, as there are different powers for different meridians.

This is essential for a consistent clear vision, as the power of the lens must remain in front of the intended meridian. This is done by 'ballasting' ? a special system of weighting and different thicknesses in different lens meridians. This keeps the lens from rotating.

Two numbers describe the powers of the soft toric single vision lenses - a sphere power (sph), and a cylinder power (cyl). The sphere powers can be preceded by either, a minus (-) sign, or a plus (+) sign, whereas the cylinder power is always in minus (-). In a prescription, the sphere power is always written first, followed by the cylinder power.

The prescriptions for toric soft lens include a third number, known as the 'cylinder axis'. This power is always preceded by 'x' and is also the last in the prescription. This axis denotes the location of the flattest meridian of the eye ? the meridian that is for the sphere power ? and its angle is measured in degrees.

Soft Bifocal Or Multi-focal Lenses ? These soft contact lenses have been designed to correct presbyopia, in combination with farsightedness, and/or with nearsightedness. Affecting adults over the age of 40, presbyopia is the normal age-related loss of near focusing ability.

The spherical power in the soft bifocal or multi-focal lenses correct nearsightedness or farsightedness. In addition, there are one or more magnifying powers for close-up vision. This additional magnifying power, called the 'add power' is for the correction of presbyopia. The 'add powers', just as the sphere and cylinder powers in soft contact lens prescriptions, are measured in diopters, and always preceded by the plus (+) sign. It has a range from +1.00 to +3.00 D.

The fit of the lenses and the size of the wearer's pupils are some of the factors on which the performance of the bifocal and multi-focal soft contact lenses depends. People who use bifocal or multi-focal contact lenses have a slightly reduced visual acuity as compared to when they use a normal single vision contact lenses for distance vision, and a pair of glasses over their contact lenses for near vision. Still, many are ready to overlook this slightly less clear vision for the ability to read without the need for reading glasses.

There is an alternate way by which presbyopia is corrected by using soft single vision lenses ? either spherical or toric. This technique, called monovision, has soft lenses prescribed in such a way that one eye is optimized for distance vision, and the other eye is optimized for near vision.

Contact lens wearers, uncomfortable with bifocal or multi-focal lenses, find monovision single vision lenses better as far as vision clarity is concerned. The major benefit of using single vision lenses in place of bifocal or multi-focal lenses is the affordability. The cost of single vision lenses is significantly lower than that of bifocal or multi-focal lenses.

Contact Lens Designs for Keratoconus

Keratoconus - abnormal cone-shaped protrusion of the cornea of the eye ? can be corrected by many different contact lens designs. The ophthalmologist evaluates the requirements of each individual to prescribe a lens that offers the best combination of visual acuity, comfort and corneal health. Each design has its own unique characteristics, and no one design suits all types of keratoconus.

Rigid lens designs, usually spherical, achieve the best visual results. Aspheric lens designs are used in cases of small to moderate nipple cones, where it aligns with the more normal peripheral cornea.