The acuity— the sharpness — of vision denotes the ability to recognize small details.
The center of gaze — where we are looking — is through the fovea, which provides the greatest possible visual acuity. ￼Measuring visual acuity, which is often equated with vision, assesses the function of a small area of the retina – less than 1 degree in diameter.
Outside the center of gaze, we actually have quite low visual acuity, yet we remain unaware of this because we instinctively direct our center of gaze to where we are looking, moving our eyes, head and our body as necessary.
Measuring acuity – the letter chart
Most likely you have taken the Snellen chart acuity test many times over your lifetime.Over 150 years ago Snellen introduced his method of quantifying the resolution of the eye by using a letter chart with progressively smaller letters to measure not only how well someone can distinguish details of objects but also their sharpness of vision at a specific distance.
The Snellen chart defines its numerical value as a ratio comparing the patient’s performance to the standard observer. For people who need letters that are 2x closer or 2x larger than this standard, their visual acuity is 1/2.
Snellen described 20/20 as a size that could be easily recognized. “20/20” vision means you see at 20 feet what the normal or standard person sees, If however the last line you can read is the 40 line, this indicates that the line you correctly read at 20 feet away can be read by a standard observer from 40 feet away and your visual acuity is 1/2 (20/40 in the US, 6/12 in Britain and 0.5 in Europe).
This Snellen fraction is now the standard for measuring visual acuity.
Acuity, not vision
The quote on the right shows a common way that vision is presented. The emphasis on letter chart acuity has been so strong that we often refer to visual acuity as “vision.” It is more accurate to say that “My visual acuity is 20/40”.
Visual acuity has maintained its status as the benchmark and standard by which to measure eyesight. It is still how we determine eligibility for benefits (“legal blindness”) in the United States.
Unfortunately equating acuity with vision – for example being told your “vision” is “good” because your acuity is good – can lead one to believe that there are no vision problems when there are.
And you may be confused when you are experiencing difficulties with your vision even though your acuity is “normal.” Because your vision problems may be caused by other factors such as reduced contrast sensitivity, color sensitivity and dark adaptation.
You may find that the letters on the chart are not sharp or clear, yet given enough time you are
able to identify them. It is important to communicate this to the examiner, as the difficulty
you are experiencing can indicate a vision disability that the letter chart testing
does not pick up.