Diopter 101 – How to Choose Your Reading Glasses’ Strength

July 9, 2017

Since the Middle Ages, civilizations utilized lenses, even without fully understanding the science of optics.  In fact, people in the Middle Ages learned to use framed lenses to obtain better vision. 

Today, many people understand the connection between lenses and light refraction.  But it wasn’t until the 17th century, and the work of the scientist Willebrord Snell, that the phenomenon of light refraction became a scientific law.  Einstein and other scientists refined theories about light, and only in the latter part of the 20th century have optical systems been well understood.

Your vision and diopter strength

The human eyes bend light, allowing people to see. But when the eye’s light bending power is no longer enough to provide clear vision, the light refracting attributes of lenses are used to augment the declining optical power of the human eye. But how much light refraction is needed to compensate for decreasing vision?  

The light bending ability of the lens is based on its focal length.  If the focal length is shorter, then the lens has a greater ability to bend light. And if the focal length is longer, then less light is refracted by the lens.  For people buying reading glasses, the focal length of the lens is important.  For a lens that has a focal length of 1 meter (about 40 inches), its optical power is reported as 1 diopter. The term “diopter” is the unit for optical power, just as “meter” is the unit for distance.

The average human eye has an optical power of 40 diopters (about 1.574 inches or 131 feet).  The human eye is a sophisticated optical system that can instinctively adjust its focus by 20 diopters nearer and farther. This ability of the human eye to adjust its focus is known as “accommodation.”

Unfortunately, despite the sophistication of the human eye, its “accommodation” abilities may deteriorate over time. A severe deterioration of the eye’s accommodation is a condition known as presbyopia. A person who is 25 years old and older can generally accommodate only by 10 diopters. And a person who is 50 years old and older can usually accommodate only by 1 diopter. To offset the deteriorating optical power and still have clear vision, people use reading glasses.

How to calculate the right diopter strength for your reading glasses

Reading glasses refract light in a manner that enables a user to see clearly.

Remember, the optical power of reading glasses are also expressed as diopters. If a specific pair of reading glasses has a diopter of 2.00, it means that the lenses can refract light to focus on objects that are ½ meter (about 20 inches) away. If the diopter strength is 3.00, the lenses can refract light and focus on objects that are 1/3 meter (about 1 foot) away. The higher the value of the diopter strength, the closer the focus of the lens. 

The eye’s ability to accommodate typically deteriorates gradually. That means the eye will require reading glasses with lenses of different strengths over time. To meet the needs of the human eye, the optical power of reading glasses’ lenses vary in small increments. The diopter strengths can be 1.00, 1.22, 1.50, 1.75, and so on, up to 3.00.  People in their 40s generally need reading glasses with a diopter strength of 1.25. And people in their 50s use a diopter strength of 2.25 and higher for their reading glasses.

To determine which diopter strength is best for you, you can test your eyes by using the chart found by clicking here:  READING GLASSES EYE TEST. Print the chart and read each line. When you have reached the line where you can no longer see clearly, you have found the diopter strength you need.

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