Inches from eye
When people get into their 40's they often notice more problem focusing their eyes to see near objects. People who never had a problem with their vision for the first 45 years of their life notice that reading small print on documents or products they are holding in their hand has become difficult. Those people who have worn glasses or contact lenses which were good for distance and near find that with the lenses on their near vision has become worse. Even people who had LASIK surgery to correct their distance vision find that their near vision is no longer good, and start needing glasses or contact lenses in order to see close.
This blog explains how the eye focuses for close and how this ability changes through life.
The ability of the lens to accommodate, to change shape in order to focus on near objects, is dependent on the flexibility of the lens. The lens becomes harder and less flexible with age. In the 19th century Dutch physiologist Franciscus Cornelis Donders (1818-1889) postulated this in his 1864 book On the anomalies of accommodation and refraction of the eye, and he created the famous Donders Table of Accomodation (left above) to illustrate the point. From the table we can see that a teenager has the ability to accomodate about 12 diopters, and clearly see things about 3 inches in front of the eye, while a 45 year old can only accomodate about 3.50 dioptors and cannot focus on objects closer than about 11 inches. No wonder old folks like the larger screen cell phones!
This decrease of accomodation with age is a universal phenomenon. To a small extent, the decrease of near focus is offset by the fact that the pupil size tends to decrease with age and it is a fact of physical optics that aperture diameter and depth of focus are inversely proportional--that is, a smaller pupil has an increased depth of focus, and therefore a near object is clearer with the smaller pupil than with a larger pupil. This is the principle on which the camera obscura (pinhole camera) and other pinhole lens devices are based. And it is why one hears about someone in his eighties who can see at all distances with each eye without the aid of any corrective lens. If you look at the pupil size of that 80 year old, it will look more like a pinhole. The fact is that by the time one reaches the age of 45, the amount of lost accomodation generally has made the ability to focus on close objects very difficult, especially in dim light (when the pupil has naturally dilated to a larger size). This lack of accomodation due to age was called presbyopia by Donders, from from the Greek word presbys (πρέσβυς), meaning "old man" or "elder", and the ancient Greek word ops (ὤψ), meaning "eye".