What Is 20/20 or 6/6 Normal Vision?
In this short video Mr Glenn Carp explains 20/20 and 6/6:
Mr Glenn Carp – “The main concern of having 6/6 vision or 20/20 normal vision really comes from whether you are based in the United Kingdom or you are based in America. It is really just to do with feet and metres. 6/6 means you see at 6 metres what an average person can see at six metres, and 20/20 is the equivalent in feet, twenty feet versus twenty feet. So the average normal vision in the general population would mean that what someone sees at twenty feet an average person can see. If you have better vision than 20/20 for example 20/16 it means that you can see at twenty feet what an average person can only see at sixteen feet and of course the opposite if you are 20/40 for example it means you have poorer vision than an average person; you see at twenty feet what they see at forty feet from much further away.”
How you can achieve 20/20 or 6/6 normal vision or better
In the most common visual acuity test, an optometrist places an eye chart at a standard distance, twenty feet, or six metres, depending on the customary unit of measure. At this distance, the symbols on the line representing “normal” acuity on the eye chart, designated 20/20, is the smallest line that a person with normal acuity can read at a distance of twenty feet.
It is possible to see better than the norm, which optometrists express as visual acuity of 20/16 or 20/12.5, or better. For example, 97% of our short-sighted patients see 20/20 or better after surgery (which includes patients who see 20/16 or better, and patients who see 20/12.5 or better).
Three lines above, the letters have twice the dimensions of those on the 20/20 line. The chart is at a distance of twenty feet, but a person with normal acuity can read these letters at a distance of forty feet. This line is the ratio 20/40 (or 6/12). If this is the smallest line a person can read, the person’s acuity is 20/40, meaning, in a very rough kind of way, that this person needs to approach to a distance of twenty feet to read letters that a person with normal acuity could read at forty feet.
For example, 100% of London Vision Clinic patients (-1.00D to -9.00D) see 20/32 or better, and 100% of patients (+1.00D to +6.00D) see 20/40 or better after Laser Eye Surgery.
The biggest letter on an eye chart often represents acuity of “20/200″, the value that is “legally blind.” Many people with refractive errors have the misconception that they have “bad vision” because they “cannot even read the E at the top of the chart without glasses.” However, in most situations where acuity ratios are mentioned, they refer to best corrected acuity.
Many people with moderate myopia “cannot read the E” without glasses, but have no problem reading the 20/20 line or 20/15 line with glasses. A legally blind person is one who cannot read the E even with the best possible glasses.
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In Laser Eye Surgery, the surgeon’s goal is to get your vision without glasses after surgery to the same level as your “best corrected visual acuity” (with glasses or contact lenses) before surgery, or better. When optometrists or surgeons say “gaining or losing a line of vision”, they mean the change in ability to read a line on the eye chart without glasses after surgery, from the ability to read the line with glasses, before surgery.