PRESBYOPIA
"Doc, I have to hold my books further back..."
The ability to focus our eyes from the distance to a close object (such as reading a book) involves the lens inside the eye changing shape. There are hundreds of muscles inside the eye that physically reshape the lens everytime we change our focus. It is this ability that allows us to see objects clearly at many distances. Almost all of us will notice a decrease in our near vision after the age of 40 because the lens loses flexibility. This is a very normal occurance and should be of no surprise and it is called Presbyopia. Oddly enough for myopic (nearsighted) people they may note they have to remove their glasses now for reading.
As a young child the lens is very elastic and changes shape relatively easy. Over time the lens loses this flexability and a careful observer will note that year after year their closest possible focus distance moves further and further away from their eyes. The average person reads at about 16 inches from their eyes and mathmatically if you calculate how long it takes the average person to lose focus ability at 16 inches it is approximately 40 years. Again, this is very normal, expected and happens to all of us.
If you have myopia (nearsighted) the need for reading lenses/ bifocals may be delayed or avoided by simply reading without glasses. It depends on the degree of myopia and the convience of removing your glasses for reading. Conversly, if you have hyperopia (farsighted) the need for reading lenses/ bifocals can occur more rapidly if your hyperopia is undercorrected.
The corrections for presbyopia include reading glasses, multifocal spectacles and contact lenses (multifocal or monovision) and surgical procedures. The typical considerations in correcting Presbyopia is to establish a correct balance between your distance prescription and your reading prescription (which are different). If you look through reading glasses the distance will be blurred. If you look through distance glasses the reading will be blurred. It is for this reason the majority of people use multifocal design glasses or contact lenses which incorporate both the distance and the reading prescription . In spectacle lenses the most common lenses used are computer glasses (business lenses by Zeiss) and Progressives. The Business Lenses have no lines visible and are designed so the top of the lens is for intermediate distances (computer) and the bottom is for reading. Progressive lenses have actually three powers basically (again without visible lines) and are designed so that the top portion is distance, the middle intermediate (computer) and the bottom is for reading.
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Dr. Daniel L. Mason, Optometrist
(619)267-9900