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Refractive Power



Myopia

What is it: Eye’s ’power’ is too high causing light to focus in front of the retina. Moving closer to the object being viewed will increase clarity.

Symptoms: Difficulty in viewing distant objects.

How it’s corrected: Eyeglasses with ‘negative power’ lenses are used, for instance -3.00.


Hyperopia

What is it: Eye’s ‘power’ is too low causing light to focus behind the retina. The eye may be able to compensate and increase the ‘power’ for tasks that require clarity.

Symptoms: When mild, there may be no symptoms; when moderate, near work can be difficult and cause eye strain; when severe, even distant objects can look blurry; it also depends on age and time spent on visually demanding tasks.

How it’s corrected: Eyeglasses with ‘positive power’ lenses are used, for instance +3.00. The amount of correction (or none) depends on various factors such as the age, symptoms, visual tasks, and amount of hyperopia.


Presbyopia

What is it: Eye’s ability to focus is reduced. This happens naturally with age and typically affects people above 40 years old.

Symptoms: Difficulty in viewing near objects when wearing a pair of eyeglasses corrected for distant vision.

How it’s corrected: Eyeglasses with a ‘power’ that is more positive than the distant eyeglasses, for instance a -3.00 for distance eyeglasses and -1.00 for near eyeglasses. Progressives, which combine both a far and near prescription within the lenses, may be considered.

Read more about the solutions for presbyopia.



Astigmatism What is it: Eye’s ‘power’ is different along each ‘axis’, i.e. not spherical Symptoms: Elongation of images along one direction, sometimes causing the effect of a double-vision where there there are two edges to a word or object right next to each other. How it’s corrected: Eyeglasses with a ‘cylindrical power’ component to the lenses are used, with neutralises the ‘astigmatic power’. You often see an ‘axis’ tagged to the corrective astigmatic power, such as -1.00X180 which means an astigmatic correction of -1.00 at 180 axis. See our buying guide and prescription reading guide. FAQs: Isn’t hyperopia the same as presbyopia (老花)? Someone recommended progressives for my child? Is the TV causing astigmatism? What is considered high myopia? Are there risks to having high myopia? If I lower my child’s myopia prescription, will it be beneficial? [1] (Chung, Mohidin, O’Leary, 2002) [2] (Li, Li, Zhou, et. al., 2015) [3] (Tyler and Wagner, 2019)

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