Bifocals are two prescriptions combined into a single lens. Trifocals are three prescriptions combined into one lens.
Bifocals were originated by Benjamin Franklin in the 18th century when he cut the halves of two spectacle lenses and fitted them into one frame.
The basis of selection of a type of bifocal
Depending upon the occupational needs, a type of bifocal is selected.
A salesman, who has mostly outdoor work and very little reading to do, would have a small round-top bifocal fitted.
An office worker, who has to deal with close work but still has a fair deal of distance activity would use a regular medium-sized round-top bifocal segment.
An executive who deals mainly with paper work or an accountant who has masses of figures to pore over would require either a very wide round-top bifocal or preferably the straight-line executive bifocal. However, do remember that though the straight-line executive lenses are excellent for desk work they cause problems when one has to walk with them, as the complete lower field is affected. Ideally, they should be left on the desk after finishing work and a smaller D or flat-top bifocal used for activities outside.
In an effort to decrease the chromatic changes and the object jump which are problems with fused bifocals, flat-top solid bifocals have been made. They have the advantage of lowered chromatic change and of no jump or no shift in the objects when the gaze is shifted from distance to near.
Types of bifocals and their making
There are different types of bifocals. They are:
(a) Split bifocals, called
from their 18th century inventor. Actually 2 lenses cut separately and put in
one frame; extremely poor visual result; their only advantage and really the
reason for their survival being that they are extremely cheap. Franklins
(b) Cemented bifocals: a small wafer of glass is cemented or glued at the bottom. A technique used around 30 years ago with disadvantages such as poor vision at the edges and that the joint gets yellow and bubbles appear. At present, used purely as temporary device or for matching unlikely numbers; purely of historical interest; again cheap.
(c) Fused bifocals: a good commercial lens. A small circle is ground on the distance lens and another piece of glass of a higher refractive index (and therefore higher converging power) is put in the gap. It is fused in an electric furnace.
An excellent lens; however, in higher minus or plus power a fair amount of color distortion (chromatic aberration) is visible. Economical and good. Most bifocals made are the fused design available today.
(d) Solid bifocal: made from a single sheet of glass. A superb lens; no color distortion; however, it is a bit costly. The very popular straight line bifocal falls in this group. Well worth the expense.
Need trifocals or the new varifocal or omnifocal lenses
With a bifocal, distance and near are clear but the intermediate distance (between 2 and 6 feet) is blurred. Where intermediate is essential for a patient a trifocal or varifocal is required.
Take the instance of a piano player. He can see distance and near, but the music notes he has to read are too far away. Hence, he has to have an intermediate section to see them.
A lady, who plays cards, can see the cards in her hand but cannot see the cards laid on the table. Hence a trifocal.
Window shopping: some shops have deep recessed display counters and even if the nose is rubbed on the glass, without a trifocal, most of the objects on display are not clear.
A varifocal of omnifocal goes a step further and gives complete range of vision from full distance to full near. Properly fitted it is a beautiful lens (as the author can himself attest) and virtually restores full visual range. However it has to be fitted with extreme accuracy to be really effective.
Time duration to get used to bifocals
Bifocals are purely a marriage of convenience and entail the sacrifice of part of the distance field of vision. Problems encountered in the beginning, range from acute nausea and actual vomiting to virtually no discomfort.
Part of the problem for a new bifocal wearer is to learn to walk with the road slightly out of focus and to get down staircases bending the head down a little more than usual.
on the other hand requires an opposite action. For the first time in his visual
experience, he will read without tilting his head down but only turning his
eyes down. Reading
Again, no longer having a complete segment (unless he uses an executive bifocal) to read from he now has to turn his head to follow the lines. If he scans a ledger, the entire head has to be tilted up or the ledger brought down to start reading.
To be fair, a good proportion of wearers can adapt to a bifocal in about 3 weeks' time. However, some never will, and for them, separate glasses are the best.
Wearing Bifocal Spectacles for the first time
A patient who is prescribed bifocals for the first time is in a quandary. Putting them on can be a problem with the segment line persistently coming in the way of vision.
A simple way of wearing bifocals, is to put them on and focus for distance through the upper segment, then, without moving the head (this is most important—no head movement) to lower the eyes to read a book held 18" away and down at an angle of 30 degrees. One thus uses the lower segment for reading. This exercise must be repeated over and over again so that the brain and the eye adapt to a new system of reading.
In troublesome prescriptions (where the number is different in the two eyes or there is a fair degree of astigmatism) or even when the eye feels uneasy, it is simplest to use the bifocal spectacles purely for reading, the distance segment being used only when sitting down (watching T.V.). The glasses should be take off before walking for the initial period till they settle in.
Separate glasses: Still a good choice if bifocals are not working
It must be clearly understood that optically speaking, separate glasses are better. They give a wider field with sharper, better-focused optics. Bifocals or for that matter trifocals or variable focal lenses, are all, as mentioned earlier, marriages of convenience: the convenience of not having to carry extra glasses and having both, distance and near glasses, available instantly.