We determine the line that the user can merely recognize on a Snellen chart. If that line is twice as wide as the reference norm (20/20), we say that individual has an MAgnification Requirement (MAR) of 2x. The visual acuity is 1/2 (20/40) if the magnification required is 2x. If the line the person sees is equal in width to the reference norm, the vision is said to be at its best quality (HIGH VA). If the line seen by the person is thinner than the reference norm, the vision is described as poor (Low VA). There are different types of charts available for testing visual acuity. The Snellen eye chart is the most common type of chart used by optometrists and ophthalmologists. It consists of letters that range in size from 3mm to 7mm high. The test person reads the letters out loud. They then identify the smallest letter that they can read correctly. This tells us how far their vision is down to the level of light perception (LP). Vision worse than LP indicates a need for medical attention. Snellen charts work on the principle of logarithms. You take the reciprocal of the highest number that can be read correctly and multiply it by 20 to get the MAR. So if someone could read only items with a height of 4mm or less on the chart, their MAR would be 40 times.