We’ve all heard the term ’20/20 vision’ and most of us know that this is considered ‘normal’ vision. But, what does that really mean? We hear this question a lot at Mandel Vision. The simple answer is that 20/20 vision is a standard for the measurement of visual acuity and represents ‘normal’ or average vision.
Here’s a little more detail on 20/20 vision:
A Snellen eye chart, or projector, quantifies visual acuity which is a measurement of the sharpness of vision. The letters on this chart are constructed based on the Hemholtz theory. This theory states that in order for you to recognize an object, a minimal area of your retina needs to be stimulated. It also specifies that the minimal amount of your visual field involved would create at least 1′ of arc.[i]
For this reason, the letters on the Snellen Chart are not only a specified size for each line, but have very precise construction and spacing between letters. The ‘Big E’ is situated at the top of the chart. This is the 20/400 line. The letters get smaller as your eye travels down the chart. Your recorded vision is the smallest line you can see.
The numerator of the 20/20 fraction denotes 20 feet, because 20 feet is the standard for distance vision measurement.
The denominator tells you at what distance the letter on the chart was designed for. This is also the distance at which a normal-sighted patient can see the same size object. (By “normal-sighted”, we mean someone who has 20/20 vision without wearing contact lenses or glasses.)
What 20/20 vision really means:
If you have 20/20 vision, you can see objects 20 feet away from you that a normal-sighted person can see at the same distance. If your vision is 20/40, to see an object clearly, you need to be 20 feet away from it, whereas a normal-sighted person can see the same size object clearly from 40 feet away.
It’s also possible to have better than 20/20 vision. For this reason, eye charts include letters that are even smaller than the 20/20 line. For example, the line below 20/20 is 20/15. The letters on this line are smaller than those on the 20/20 line. If you have 20/15 vision, this means that you can see an object clearly at 20 feet that a normal-sighted person can only see clearly from a distance of 15 feet.
Here’s some perspective:
To pass a vision test to obtain a driver’s license in New York, your uncorrected vision must be at least 20/40 with both eyes together.
Of course, some patients can’t achieve 20/20 vision. There are many reasons for this. In some cases, the eye just doesn’t develop normally and will never be capable of 20/20 vision. A few reasons for this are ambylopia and congenital conditions, such as cataracts. Other eye conditions responsible for blurry vision are myopia, hyperopia and astigmatism. These conditions can usually be treated with glasses or contact lenses.
If you are nearsighted or farsighted and don’t want to wear glasses or contact lenses, LASIK surgery or PRK surgery may be an option. If you’re interested in LASIK or PRK laser vision correction surgery, click here to schedule an appointment with Dr. Mandel for a FREE evaluation.
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