| Age |
Diopters |
Inches |
| 10 |
14.00 |
2.81 |
| 15 |
12.00 |
3.28 |
| 20 |
10.00 |
4.94 |
| 25 |
8.50 |
4.63 |
| 30 |
7.00 |
5.63 |
| 35 |
5.50 |
7.16 |
| 40 |
4.50 |
8.75 |
| 45 |
3.50 |
11.25 |
| 50 |
2.50 |
15.75 |
| 55 |
1.50 |
26.25 |
| 60 |
.75 |
52.49 |
| 65 |
.25 |
157.48 |
| 70 |
0 |
0 |
The diagram from Peter Kaiser's Joy of Visual Percetion shows elements of the eye needed for focusing on near objects. The structure of the normal healthy young eye allows the cystalline lens inside the eye to focus on near objects when a nerve impulse from the brain via the third cranial nerve causes the ciliary muscle, behind the iris, to contract, this causes the fibers (called zonules) attached to the lens capsule surrounding the crystalline lens to loosen, causing the lens to become more convex (bulging more in the center), which shortens the focal length of the lens. Dr Wallace's LA Sight website has a wonderful Flash animation showing how the crystalline lens becomes convex when focusing on a near object. The change of focus of the lens to allow clear focus of near objects is called accomodation and can be measured in diopters (Dioptor = 1/focal length of lens in meters).
The ability of the lens to accomodate, 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 Donders (1818-1889) knew this, and created the famous Donders Table of Accomodation (see Table to left) to illustrate the point. From the table we can see that a teenager has the ability to accomodate about 12 diopters, while a 40 year old can only accomodate about 4.50 dioptors. 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 is called presbyopia (See Diseases and Conditons).
