1. How the Eye Focuses on Close Objects Using the Cornea and Crystalline Lens
The eye has two lenses that bend light to form an image on the retina at the back: the cornea in front and the crystalline lens behind it. Around the crystalline lens is a muscle called the ciliary body, which changes the thickness of the lens to adjust focus. The Japanese Ophthalmological Society calls this ability "accommodation" [3].
The way the muscles work is interesting. When looking at something far away, the ciliary muscle relaxes. This pulls on the lens, making it flat. When looking at something close, the muscle contracts. This releases the pulling force, and the lens becomes thicker and rounder due to its own elasticity [1]. So, when the muscle contracts, the pulling force weakens.
Think of a rubber bag filled with water. If you pull on the edges, it flattens; if you let go, it bulges out. This is similar. However, the real lens is not rubber but transparent tissue inside the eye. The basic explanation for this mechanism was proposed by Helmholtz in 1855. However, researchers are still studying the details. A report from the International Myopia Institute (IMI) states that the exact mechanism is not yet fully settled [1].
2. Why Distant Objects Look Blurry After Long Screen or Book Use
After looking at a smartphone or book for a long time, distant objects may look blurry. This temporary shift in focus is called "near work-induced transient myopia" and is being studied. In an Indian study, participants looked at text 20 cm (about 8 inches) from their faces for 5 minutes. Then, researchers measured their focus on distant objects with a machine.
For the 24 people who were already nearsighted, the average shift was 0.21 diopters (D) toward myopia. For the 24 people who were not nearsighted, there was almost no shift; instead, it shifted slightly in the opposite direction by 0.07 D. The value showing how fast the shift disappeared was about 6 seconds. Calculations suggest it takes tens of seconds for the shift to nearly vanish. Even after looking closely for 60 minutes, the shift for nearsighted people was only 0.31 D, with a recovery speed of about 8 seconds [4].
The cause is not certain. Some explanations suggest it takes time for the muscle to relax, but researchers debate this [5]. It is unclear if the everyday "blur" people feel is exactly the same as the shift measured in these experiments.
Safety Note: If blurriness lasts a long time, or comes with headaches or strong tiredness, you should see an eye doctor for a check-up.
3. How the Eye's Ability to Change Focus (Accommodation) Starts Declining in Your Teens
The ability of the eye to change focus for near vision is called "accommodation." Accommodation power is expressed in diopters (D). Calculating "1 ÷ D" gives a rough estimate of the closest distance where focus works (though actual measurements vary by method).
In a study of 154 people, average accommodation was 14.67 D in teens, 11.13 D in their 20s, and 5.53 D in their 40s [6]. Using the "1 ÷ D" calculation, the closest focusing distance is about 7 cm (about 3 inches) for teens, about 9 cm (about 3.5 inches) for people in their 20s, and about 18 cm (about 7 inches) for people in their 40s.
This power decreases slowly from the teenage years. The speed and accuracy of adjustment do not change much until around age 40. However, around age 40, the accommodation force begins to fall below the range needed for close-up work [7]. The Japanese Ophthalmological Society notes that people often start feeling fatigue when doing near work in their 40s [3].
Presbyopia does not start suddenly at age 40. It is a change you notice when the power, which has been decreasing over many years, finally fails to reach the distance you need.
4. What Changes Inside the Eye in Presbyopia
What changes inside the eye during presbyopia? Researchers Glasser and Campbell tested 27 crystalline lenses removed from human eyes aged 10 to 87. They applied force through the ciliary body and zonules (suspensory ligaments) to see if the focal distance changed.
Young lenses changed their focal distance significantly when pulled. Lenses from people over 60 did not change, even when pulled [9]. This means the lens itself has become harder to reshape. A review by Charman also states that changes in the shape and elasticity of the lens play a major role in reduced accommodation. However, other factors may be involved, and the full picture is not yet fully understood [7].
Whether presbyopia is called a disease or a natural change varies by source. The Japanese Society of Presbyopia calls it a disease requiring some form of intervention [15]. Wolffsohn and others propose redefining presbyopia as the point where age-related reduction in accommodation range, which is physiologically normal, no longer meets an individual's needs for near vision [10]. Both agree it is a change that can happen to anyone and can be corrected with glasses. One estimate suggests that in developed countries, most people spend about half their lives in a state of presbyopia [11].
5. How Nearsightedness, Farsightedness, Astigmatism, and Presbyopia Differ
Nearsightedness (myopia), farsightedness (hyperopia), and astigmatism describe the state of the eye before it focuses. According to the Japanese Ophthalmological Society, an "emmetropic" eye is one where light from far away focuses exactly on the retina without using accommodation. If the focus falls in front of or behind the retina, it is called myopia or hyperopia [12]. Presbyopia is about the ability to change focus, which is a different issue.
Here is how they differ:
- Nearsightedness (Myopia): Light focuses in front of the retina. Distant things are blurry; near things are visible.
- Farsightedness (Hyperopia): Light focuses behind the retina. Both distant and near things can be hard to see. Children can often compensate with their accommodation power.
- Astigmatism: The cornea or lens is distorted, so light does not focus at a single point. Vision is blurry; things may look double or triple.
- Presbyopia: The ability to change focus for near objects decreases. Near things are hard to see.
6. Why Astigmatism Causes Blurry or Double Vision
The main cause of astigmatism is distortion in the cornea or lens. If an eye had a perfectly ideal lens, theoretically, astigmatism would not occur. In reality, most eyes have some degree of distortion, so light does not focus at one spot, causing blurry images. Even when looking with just one eye, objects may appear double or triple; this is called monocular diplopia [12].
Imagine the side of a rugby ball. It curves differently in the vertical direction than in the horizontal direction. Thinking of an astigmatic cornea as curving differently depending on the direction makes it easier to understand. However, most astigmatism is not as large a distortion as a rugby ball.
A little corneal astigmatism exists in almost everyone's eyes. One review states that the surface of the cornea is often flatter horizontally than vertically [13]. A small amount of distortion is normal.
The US National Eye Institute (NEI) writes that doctors do not know the cause, and there is no way to prevent it. Some people are born with it, some develop it in childhood or youth, and others get it after injury or surgery. Mild astigmatism may go unnoticed. Children especially may not realize their vision is unusual, so eye exams are important [14].
Safety Note: Astigmatism can be present from birth, appear in childhood or youth, or result from injury or surgery.
7. How Glasses Lenses Differ for Nearsightedness, Farsightedness, and Presbyopia
Glasses lenses come in two main types: concave lenses (thinner in the middle) and convex lenses (bulging in the middle). Myopia is corrected with concave lenses, and hyperopia with convex lenses. Astigmatism uses cylindrical lenses that have different strengths in different directions [12].
Reading glasses are also convex lenses. Although they use the same type of lens as those for hyperopia, their role is different. Hyperopia glasses compensate for insufficient focusing power. Reading glasses compensate for a lack of focusing power when looking at near objects.
According to the Japanese Society of Presbyopia, glasses are the "safest and most reliable first choice" for correcting presbyopia. From early presbyopia to old age, options like bifocals (for distance and near) or progressive lenses (where power changes gradually) are chosen based on each person's eye condition [15]. Contact lenses and surgery are also options. Since the correct prescription varies by person, you should get checked at an eye clinic or optical shop.
In 1554, Maurolico is said to have first distinguished that myopia can be corrected with concave lenses and hyperopia with convex lenses [2].
Safety Note: The Japanese Society of Presbyopia considers glasses the "safest and most reliable first-choice method" for correcting presbyopia.
8. A Study of Tea Pickers in India Who Needed Reading Glasses
In 2017, a study took place in tea plantations in Assam, India. It involved 751 tea pickers aged 40 and older who struggled with near vision but did not have glasses. One group received free glasses immediately, while another group received them later [16].
The group that got glasses right away picked an average of 25.0 kg (about 55 lbs) of tea leaves per day before, and 34.8 kg (about 77 lbs) after. The group that got them later increased from 26.0 kg to 30.6 kg. Comparing the difference in increase, the immediate group picked an average of 5.25 kg more tea leaves per day, which is about a 21.7% increase. Workers' income was linked to the amount picked. The cost of glasses, including delivery, was $10.20 USD per person [16].
A 2015 estimate suggests there are about 1.8 billion people with presbyopia worldwide. Of these, about 826 million have impaired near vision because they lack glasses or have incorrect ones. This was especially common in rural areas of low-income countries [17].
Presbyopia is a change that can happen to anyone. However, access to glasses varies greatly depending on where you live.
Safety Note: The study focused on people with good distance vision but poor near vision who could be corrected with reading glasses and had no other eye diseases.
9. How to Check the Closest Distance Your Eyes Can Focus On
You can try measuring the closest distance where your eyes can focus on something near. Do not push yourself too hard. Use a ruler and a piece of paper with small text (like a newspaper). Cover one eye with your hand and slowly bring the paper closer from far away. Stop when the text starts to blur, and measure the distance from your face with the ruler.
If you get it too close, your eyes will tire. Stop immediately if they feel tired. In the study by Park et al., the accommodation power of teenagers was an average of 13.35 D in the morning (9-10 AM) and 16.15 D in the afternoon (3-4 PM), meaning it is lower in the morning [6]. If you measure in the morning and afternoon, you might find a difference. However, calculating with "1 ÷ D" shows this difference is only about 1 cm (about 0.4 inches) in distance. A ruler may not clearly show this. This is not a medical test. If difficulty seeing continues, please see an eye doctor.
You can also compare reading materials from the Japanese Ophthalmological Society pages on "Presbyopia" and "Myopia/Hyperopia/Astigmatism" [3][12], and the Japanese Society of Presbyopia's page for the general public [15]. What does each organization emphasize about presbyopia?