For Patients: Presbyopia II: how your accommodation changes during your life
Presbyopia is the loss of the ability to refocus from distance to near (accomodation) that occurs as we age. In this article, I will take a slightly more technical look at what happens to accommodation as we age. For an introduction to presbyopia, click here.
Accomodative Amplitude describes the amount that we can focus from distance to near. It is measured using the unit Diopter. The following table shows how near vision changes as accommodative amplitude is lost:
Accommodative Amplitude Inner limit of good near vision
1 Diopter 1 meter (100 cm)
2 Diopters 1/2 meter (50 cm)
3 Diopters 1/3 meter (33 cm)
4 Diopters 1/4 meter (25 cm)
10 Diopters 1/10 meter (10 cm)
We usually hold things about 33 cm away to read. When our vision is corrected to distance, we begin to struggle to see close up when accommodation drops to about 3-4 Diopters.
The graph below demonstrates the loss of accommodation that occurs with age, measured in Diopters. Note the rapid reduction of accommodative amplitude from 40-50 years of age.
This chart shows us some other interesting things. Note that a 10 year old has an accommodative amplitude of 14 Diopters. This allows the 10 year old to focus as close as 1/14 meter or 7 cm. A 20 year old can accommodate 10 Diopters- images become fuzzy at 1/10 meter or 10 cm. The slow march of presbyopia between ages 10 and 20 is not noticeable because the nearest focus only goes from 7 cm to 10 cm- but it is happening. It may seem amazing, but a 20 year old cannot accommodate as well as a 10 year old!
Now, note how quickly accommodation drops off beyond 40 years of age. A 45 year old may still have about 4 Diopters of accommodation, allowing them to see clearly at 25 cm, but by age 50 accommodation has dropped to 2 Diopters- images get blurry at 50 cm! Nobody has arms that long!
This explains why most of us notice presbyopia in our 40’s. According to the above graph, in just a few years, accommodation drops from 6 Diopters (near vision is clear at 17 cm) to only a half a diopter (near vision is clear at 50 cm). What began in childhood only becomes noticeable when we get to the end of our arm length. If our arms were shorter, we would get bifocals earlier!