Eye Strain
Performing near tasks (such as staring at a computer monitor or a cell phone, or even reading a book) too close, or for too long causes eye strain.
Assuming you have perfect vision (or you are wearing glasses or contact lenses with a very recent prescription), the Ciliary Muscle has to change the shape of the Lens to make you focus near.
At 20 feet, the muscle is doing zero work. At 3 feet, it is doing approximately 1 unit of work (known as "diopter").
After that, every time you half the distance, you double the amount of work the muscle has to do to focus.
Therefore, if you hold something at 2 inches from your eyes, the muscle has to work 20 diopters versus only 3 diopters if you hold it at 13 inches.
Assuming you have perfect vision (or you are wearing glasses or contact lenses with a very recent prescription), the Ciliary Muscle has to change the shape of the Lens to make you focus near.
At 20 feet, the muscle is doing zero work. At 3 feet, it is doing approximately 1 unit of work (known as "diopter").
After that, every time you half the distance, you double the amount of work the muscle has to do to focus.
Therefore, if you hold something at 2 inches from your eyes, the muscle has to work 20 diopters versus only 3 diopters if you hold it at 13 inches.
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Overworking the muscle will lead to pain starting behind the color of the eye in the eyebrow area. Later spreading to the forehead and possibly the rest of your head.
Harmon's Rule
Position your computer monitor between 20 to 26 inches (50 to 65 cm) from your eyes.
You can also use the distance between your shoulder and your knuckles as an approximation of this length.
For all other near activities (such as cell phone, reading, playing with hand-held video games, etc.), 14 to 20 inches (35 to 50 cm) is considered appropriate.
You can also use the distance between your elbow and your knuckles as an approximation of this length. This is known as "Harmon's Rule".
20/20 Rule
Even if your not looking at something too close, you can still overwork the muscle by making it work for too long. For every 20 minutes spent looking at a near object, stare at 20 feet for 20 seconds.
Note: Even if you follow these recommendations, your vision may still worsen, due to other factors involved, such as heredity for example.
Harmon's Rule
Position your computer monitor between 20 to 26 inches (50 to 65 cm) from your eyes.
You can also use the distance between your shoulder and your knuckles as an approximation of this length.
For all other near activities (such as cell phone, reading, playing with hand-held video games, etc.), 14 to 20 inches (35 to 50 cm) is considered appropriate.
You can also use the distance between your elbow and your knuckles as an approximation of this length. This is known as "Harmon's Rule".
20/20 Rule
Even if your not looking at something too close, you can still overwork the muscle by making it work for too long. For every 20 minutes spent looking at a near object, stare at 20 feet for 20 seconds.
Note: Even if you follow these recommendations, your vision may still worsen, due to other factors involved, such as heredity for example.
