A simple way to describe a bifocal is to say that it has two prescriptions combined into one lens. A trifocal has three prescriptions combined into one lens; a progressive bifocal has a “continuous” number.

That old kite flyer, Benjamin Franklin, came up with the idea for the bifocal in the 18th century. He found it very bothersome to switch back and forth from his distance glasses to his reading glasses all day long. His clever solution was to cut each set of lenses into half moons (lenses were perfectly round in those days), and put them together in one frame. The top half was for his distance prescription and the bottom half his reading lenses.

No matter what form or shape modern bifocals have, they are essentially two pairs of glasses put together. As in Franklin’s case, the most common reason for bifocals is presbyopia—when the eyes can no longer easily focus at near small objects. If you wear only reading glasses distance vision is blurred and they have to be removed to see far away. If you also need glasses for distance seeing, then you are in Ben’s predicament. By placing the reading prescription in only the lower part of the lens, there is little interference with normal distance seeing.

There are many variations in design of the modern bifocal lens. The particular one chosen for you must depend on your prescription, occupation, life style, etc. The reading segment can be any size, from very small to very large; its top edge can be flat, round or oval; it can be positioned high or low in the frame. The possible combinations can run into the hundreds.

What are some of the factors in choosing a bifocal? How you use them is the main consideration. Here are two extreme examples: A symphony musician must read the music at about twenty-four inches and also occasionally glance at the conductor thirty feet away for the tempo. This person needs a bifocal with a large reading area and just a small distance portion at the top. On the other hand, a golfer wants a large distance portion with just a tiny reading spot to write on the score card. The reading position must be very low in the frame so it’s out of the line of sight when the golfer addresses the ball.

Each patient must have the bifocals designed for his or her particular use. Many times it’s impossible to design an “all purpose” bifocal for all activities. This is quite obvious when you consider that the musician could also be an avid golfer. You can’t expect to use the same bifocals for all occasions, any more than you would wear a suit of armor on the golf course.

The prescription strength of the reading portion can be made, within reason, for the distance at which you will use it. A carpenter working at arm’s length wants to see clearly at about twenty-four inches; someone working a sewing machine would prefer to see clearly at about fifteen inches. That doesn’t mean you are limited to exactly twenty-four and fifteen inches respectively. Depending on your age, there is a latitude of clear vision both closer and behind these distances.

Certain occupations require rather unusual lenses. For instance, a pharmacist has to be able to read labels on shelves well above eye level as well as normal reading material. A double bifocal, with a reading segment at the top and bottom serves this purpose. An electrician doing overhead wiring would also benefit from this arrangement.

Trifocals are three pairs of glasses in one. Why would anyone need three different prescriptions? As you get older and the eye’s focusing flexibility dwindles, you may not be able to see clearly at all distances with a bifocal. The distance prescription at the top of the lens will let you see distinctly from about four feet all the way out to the stars; the reading segment will let you see up close. This leaves an area from about twenty inches to three feet which will be fuzzy. If you need or desire clear vision at that distance, the in-between segment of the trifocal will supply it. A typical user would be a computer opera‑

Tor who would use the mid-range portion to read the characters on the video screen.

In normal bifocals or trifocals there is an abrupt change in the prescription from one section to the next. One class of lens designs gaining in popularity has a continuously changing prescription from the distance to the reading power. These are called progressive addition lenses. Theoretically, all seeing is clear if you look through the appropriate part of the lens. In practice, it takes quite a bit of adaptation because the size of the intermediate and reading zones are rather small. Also, there are distortions at the edges of these zones. Most people do adapt within about a week and are quite pleased with the results. These multifocals do away with the normally visible dividing lines and are called “invisible”. They are, therefore, more acceptable cosmetically.

When you get your first bifocals, you’ll have to develop new seeing habits combined with different patterns of head and eye movement relationships. For one thing, when walking down stairs, don’t just lower your eyes. If you do, you’ll be looking through the reading segment and the steps will be blurry. Just lower your chin to look through the upper section and the stairs will be clear. Another common problem for beginning bifocal wearers is trying to read a notice on a bulletin board. Simply raise your chin until the reading section is in position.

Most people learn to use bifocals fairly rapidly, and within a week or ten days have everything mastered. A few people just never seem to be able to get the hang of it and must revert to using two pairs of glasses with its inherent nuisance. If you are one of these people, have the optometrist re-asses your vision needs. It’s possible that you were trying to use the wrong type of bifocal for your particular needs.

It was not until I started to improve my eyesight that I started to analyse the subtleties of the gestures of seeing. In my research I read about a connection between posture and eyesight that suggests that poor eyesight might cause or affect poor posture or vice versa. I feel that eyesight and posture develop together as a result of our activities and how we are using our muscles. They are both an expression of the gesture of seeing. Perhaps this way of explaining it can help us see how to reverse the condition by creating the opposite action.

The two basic ways to see include distance focus and close focus. In walking and athletic activity, the head is usually upright, the shoulders and back are straight, the chest is expanded to breathe deeply, our eyes are pointed straight ahead and the ciliary muscle is relaxed for distance focus. We have good peripheral vision. For close work, the head is usually positioned forward and down, the shoulders are rounded slightly to extend the arms, the muscles of the eyes contract to focus close and point the eyes inward. We are narrowing our peripheral vision.

Moving from one posture to another can naturally trigger the eye to change focus; however, a person with poor eyesight may have lost touch with his ability to change focus effectively and might try to incorporate the opposite gesture. To see into the distance, a short-sighted person may push his head forward and squint, often in a futile attempt to bring things close enough to focus on clearly. A more effective gesture is to pull the head upright and open the eyes wide. This should help to trigger the ciliary muscle to relax and the lateral recti muscles to pull the eyes outward to improve peripheral vision. People who are a little far-sighted or have presbyopia, faced with reading a menu, for example, often pull their head back and lengthen their arm in order to focus better. They may be attempting to create a distance focus. A gesture of bringing the head forward and down and focusing the eyes towards the end of the nose might be more effective in triggering the oblique and ciliary muscles.

Start practising going from one posture to another. At first, you may want to exaggerate them so that you can experience how different they feel. As you learn how to increase the flexibility of the eye patterns, you may also notice that you can reverse the intensity of frown lines, crowsfeet and sagging eyelids.