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It can be challenging to make sense of the difference between visual acuity and visual strength/glasses strength, when your child has had a visual assessment and been told they will need to wear glasses. But the two terms are vastly different.

Visual strength

It is possible to correct your eyesight using glasses. Visual strength is a general term used to describe how thick or thin the lenses of the glasses need to be – i.e., the strength of the glasses. The main purpose of the glasses is to give the user the best possible vision.

Visual acuity

Visual acuity means how well one sees AFTER eyesight has been corrected with glasses. You cannot necessarily see 100% just because you are wearing glasses. If you have a lazy eye, for example, you may have reduced vision down to 60% in the lazy eye, after you have corrected your eyesight with glasses.

Visual acuity is often referred to in percentages or decimals.
It is visual acuity that is measured when testing using letters and symbols on an eye chart. If your visual acuity is 6/6, then this means that you are able to see the smallest letters on the eye chart from six meters away. When this is converted to a percentage, this equates to 100% sight (6 / 6 * 100 = 100%). A person who is said to see normally has a visual acuity of 6/6.

Similarly, a visual acuity of 6/18 means that a person has to stand six meters away from the eye chart to be able to see the same as a person with normal sight can see from 18 meters away. When converted to a percentage, this gives a score of 33% sight. (6 / 18 * 100 = 33%).

Visual standards for a diving license

In most countries the requirements for acquiring a driving license are a minimum of 50% eyesight, which equates to a visual acuity of 6/12 in at least one eye. This means that you can have just 10% eyesight in one eye, as long as the other eye is at least 50%.

Understand the glasses prescription reeipt

It can also be challenging to make sense of your glasses prescription receipt. Read our post: ‘Understanding your child’s glasses prescription receipt’.