Are plastic eyeglasses lenses injurious to the eyes?

A popular misconception in India. Whether the lenses are made from plastic, glass, compound resins or quartz, it matters little as long as they are optically perfect.

Optically speaking, moulded plastic lenses are excellent as improved designs, which decrease optical aberrations in the periphery of the lens, can be made, which is not possible with glass lenses.

What are the advantages of plastic spectacle lenses over glass lenses?

There are specific indications:

(a) When the power of a lens is high, especially plus, the glasses become every heavy and slip down the noes. The weight causes a constant ache. Plastics because of their extreme lightness are the answer.

(b) Since plastic lenses do not fog due to changes in temperature, a worker who goes in and out of in air-conditioned or refrigerated room would find them extremely useful.

(c) For those with low vision, who need a combination of lenses, plastic lenses are the answer.

What are the disadvantages of plastic lenses?

(a) They scratch easily. Hence always wash the grit and dirt off before wiping with a soft cloth.

(b) Heat distorts their optics and therefore they cannot be left in unduly hot places (the dashboard of a car, for instance).

What is meant by "getting used to glasses"?

When you get glasses for the first time or when there is a change in your prescription, you can expect a change in your visual world.

Certainly, the vision will be sharper and clearer, but for a transient period of time, some visual effects will be noticeable. Objects will be larger or smaller, closer or further away and there will be distortion in your perception of depth or spatial (in space) orientation.

If you are near-sighted and change glasses to a higher number the world will look clearer but smaller. Since the brain thinks of smaller objects as further away, you will perceive an object as further away than it actually is and pour tea over the cup into the saucer. In other words you will overshoot.

If you are far-sighted and change glasses to a higher power, the opposite will occur. Now objects are actually further than what you see. When you reach for a door knob, you may hold only air.

Both these changes are minor disturbances and go away in a few days as the brain reorients itself to a new spatial perception.

The problems however are amplified in a three-dimensional way for the astigmat. Objects are tilted and curved, the floor slants away or tilts to one side. Again reorientation is rapid.

There is also, the problem created by large lenses. The large spectacle lens increases the distortion markedly. The weight causes it to slip down the nose, and since the optical axis will no longer match the eye, poor vision, a distorted view, and headaches will follow. Hence the simple rule is that the higher the power, the smaller should be your spectacle lenses and the frame that carries them. Centring of the lenses is critical and great care should be taken to match them precisely.

Dustin Flores

Copyrighted material; do not reprint without permission.

 

View all articles by Dustin Flores