Sight, that is, the ability to distinguish visible elements in the world around us is measured a number of different ways, depending on the age and reading ability of the person being tested. One thing, however, is commonly misunderstood: Glasses prescriptions do not result from reading an eye chart. Eye charts (and there are many different ones), are similar to tape measures in that they give you a standard measurement from person to person. At the same time, and eye chart can no more give a glasses prescription than a measuring tape can build a house. It is simply a tool or guide in the process.
Most people are familiar with some sort of eye/acuity chart. The most common is the ‘Snellen’ acuity chart – typically represented with a large ‘E’ at the top. The letters get progressively smaller and the relative spacing between the letters increases as you go down the chart. Testing is done with each eye alone, then both eyes together; frequently people will not realize they have a significant impairment in one eye until it is demonstrated to them in clinic under controlled conditions. The process is generally repeated at near distances because what we see up close is often different from what we see off in the distance.
If a person has trouble with letters, such as for young children or those unfamiliar with the English alphabet different charts and techniques are employed. For example, children might use picture charts. It is quite possible to even check visual acuities in infants. In preschool-aged children, the ‘tumbling E’ chart is quite effective – this is a chart with capital ‘E’s rotated 0, 90, 180, or 270 degrees, starting with the same large ‘E’ as in the Snellen chart. In fact, in most respects it is very similar to the Snellen chart. However, rather than asking what the letters are (they are of course all the same, just rotated), we ask people (children) to show us ‘which way the fingers are pointing’, explaining that the three bars of the ‘E’ ‘are like fingers on your hand’, pointing left, right, up, down. A child does not have to know her left from right, even what ‘up or down’ means. They simply point their fingers in the same direction as the fingers on the selected ‘E’ on the chart.
If a child can see a letter on the chart, it is only a small part of the equation. My main concern is: How much effort is required to see that letter? If too much effort is expended simply seeing letters clearly enough to read, this can become an obstacle to reading and thus learning. For example, a nearsighted child will squint to see in the distance and will not likely see the smaller letters on a chart. Copying from the whiteboard to a sheet of paper can be a chore. A farsighted child will avoid looking at things up close (which often appears like a problem with attention), but can still see them; this same child will see things in the distance as well. Glasses, even for a mild prescription, can provide visual ‘grease’ making it easier for the child to scan over text. Once the child’s vision is balanced so that they expend minimal energy to resolve images, only then can you address the real impediments to reading: Motor control and visual perceptual skills.
Glasses are the single most cost-effective means of improving reading potential. In the end, the chart itself is helpful, like a yardstick, but it does not reveal much about the actual glasses prescription a child might need or other concerns that may be present. To determine this, optometrists use principles of optics and human physiology combined with an understanding of how children use vision to gather information and direct action – what is seen is never so important as how it is seen.