What is 20/20 etc. called?!


Question: What is 20/20 etc. called?
Like 20/20 20/40 -4.25, -6.25... etc. what is that number called? and is there a difference between +___ and -___ and 20/40 (i know the numbers mean different things but i mean like the type) oh and how bad is -4.25 :/ unfortunately thats what i have

Answers:

20/20 is a measure of visual acuity based on the Snellen chart (the standard chart with different rows of letters that you read in the eye dr's office). It is based on the row you can read from a distance of 20 feet compared to people with "perfect" vision. Ex: 20/40 means that people with normal vision can read at a distance of 40 feet what you can only read at a distance of 20.

-x.xx or +x.xx is perscription strength in diopters (another optical measurement). The plus sign is for hyperopia/farsighted prescriptions (you can see distance fine, but near objects are blurry). The negative is for myopia/nearsighted prescriptions (you can see near fine, but distant objects are blurry). Additionally, their may be a +/- and axis number after this that would indicate the level of astigmatism (irregular shaping of the cornea).

Your -4.25 means that you are moderately myopic/nearsighted. The ranges are:
* up to -3.00 = mild
* -3.00 to -6.00 is moderate
* -6.00 to -9.00 is severe
* over -9.00 is extreme

For additional reading: http://www.allaboutvision.com/eyeglasses…




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