Eyeglasses or spectacles are indispensable for most of us. They are the most common means to correct disorders related to the focus of the eye, such as presbyopia, myopia, hypermetropia, astigmatism etc.

Type and strength of spectacle lenses

Spectacles are made of lenses, the type and power of which depends on the degree of focusing error of the eye. Generally, there are two broad categories of lenses:
- A 'plus' lens is convex and focuses light inwards
- A 'minus' lens is concave and focuses light outwards

The power of a lens is measured in dioptres (D). The higher the dioptre count the stronger the lens, in either a plus or a minus direction.

Lenses can also be also categorized according to the errors they correct:

Single lenses

These lenses correct myopia, hypermetropia, presbyopia and astigmatism. Myopic patients need single concave lenses that are thicker at the edges and make objects look smaller. A single lens to correct hypermetropia or presbyopia is just the opposite; being thicker in the centre and making things look larger. The single lens spectacle prescribed for astigmatism has a straight line through the lens, which rotates if the lens is moved both clockwise and anti-clockwise.

Combination lenses

These are commonly known as bifocals and half-glasses and are used to correct presbyopia. These are a combination of lenses of two or more powers in two parts, in which the larger upper part is for far vision (minus lens) and a smaller, base part (plus lens) for close viewing.

The presbyope can have a half-glass, the lower half of which can correct vision for near objects and the upper half for far objects. Presbyopes are spared the trouble of keeping two separate sets of eyeglasses for near and far vision if they use bifocals. However, bifocals are not tolerated by everybody. Often there is confusion of focus, say for instance, when you look down at the stairs with the bifocals on.

Thus, evolved the concept of a trifocal. A trifocal consists of a third, intermediate part added to a lens (with a power between the far and near viewing sections). Such trifocals are suited to patients who need a middle distance vision. To prevent the image from 'jumping' between the various portions of the lens, it needs to be combined gradually for a range of viewing distances.

Prisms

Spectacles also often include prisms that bend all light in a single direction. They help to correct double vision. Prisms are fitted into eyeglasses either permanently or temporarily by simply sticking an adaptable plastic Fresnel prism onto a spectacle lens.

How to get the correct eyeglasses?

Professionals trained in eye care such as the optometrist, optician and ophthalmic medical practitioner can accurately determine your need for glasses and other optical requirements. They can even approximately determine the type and measure the strength of the spectacle lenses by just looking at and through the lens.

Patients can also clinically test for focusing errors by a mechanized process, technically called 'refraction.' The process involves a retinoscope that measures the power and determines the type of lens required by a patient. This is followed by trails with lens of various strengths to reach the optimal power suited to a patient.

Once the power of your lens has been determined and a prescription issued, you need to get hold of a pair of eyeglasses, unless you opt for contact lenses. Remember the following when you get your glasses:

• Those under 16 must get their glasses from a registered optician
• Those over 16 can get their glasses from a registered optician or any other supplier
• Unregistered suppliers cannot sell prescription spectacles to kids or to adults with their names in the blind or partially sighted records
• Adults can get reading glasses without a prescription at retail outlets, chemist shops and even gas stations.
• There are many concessions available as well. But do not compromise on an examination by a qualified practitioner, as this could lead to blindness.

What material to choose for your glasses?

Glass, as the name goes, is the conventional material for spectacle lenses. It is the still the most common choice since it is cost effective, has sound optical properties and is more resistant to pressure or heat and scratches. Its main disadvantages are its weight and breakability. It becomes a very heavy option when the lens has to be solid to correct large errors, especially if the lens of the eye has been clinically removed in a condition called aphakia. For these special cases, the glass may be made more durable.

Plastic lenses are the other option. They are lighter and hardy, but develop scratches easily. Modern resins have addressed most of these problems, but they are cost prohibitive for many. There is also the option of tinted lens, which darken in response to light. They generally act as sunshades but are also prescribed for clinical conditions like dilated pupils or people with albinism.