Multifocal eyeglass lenses contain two or more lens powers to help you see objects at all distances if you lose the ability to naturally change the focus of your eyes due to age. This condition is called Presbyopia and it is caused by the hardening of the lens of the eye. This causes the eye, when looking at close objects, to focus light behind the retina instead of on the retina.
Presbyopia occurs from age 40 and everyone is affected to some degree. Many people complain their arms have ‘become too short’ because they cannot hold reading material in a comfortable position. Other symptoms include eyestrain, blurred vision when looking at one object up close then another object further away, and difficulty reading small print.
Bifocal and multifocal lenses provide clear vision at all distances. Bifocals have two prescriptions in the same lens and multifocal have a range of powers; distance, intermediate and near vision. Your pupil alternates between the powers as your gaze moves up or down. The top part of the lens is for distance vision and the bottom part for near vision.
The invention of bifocals has long been credited to Benjamin Franklin (1705 – 1790) the American politician, statesman and diplomat. He was also an inventor so this may be true or it could be a myth. He was certainly one of the first to wear bifocals. Franklin was involved in the optical business before he left America. He imported spectacles and records of his advertisements in newspapers still exist. From the mid 1750s to the mid 1770s he spent much of his time in London and it was during this time that he was first said to have invented bifocals.
It is likely that London opticians were making ‘split lenses’ also known as ‘divided lenses’ for artists including Benjamin West (1738 – 1820) and Sir Joshua Reynolds (1723 – 1792). Both these men were in London at the same time as Benjamin Franklin and both of them have also been named as the inventor of bifocals.
There is also one recorded example of bifocal use in the animal world, Thermonectus marmoratus. This is a species of diving beetle also known as the sunburst diving beetle and the spotted diving beetle. These beetles live in ponds and lakes in Southern California, Arizona, New Mexico, Texas and Mexico. If their water source dries up they fly to a new one. Adult male beetles are about ½” or 1cm in length and females are slightly bigger. They are unique because they have, within their principal eyes, two retinas and two separate focal planes so they can switch their vision from close to distance when they are searching for food.