20/40 visual acuity is a common pass-fail mark for vision screenings. Visual acuity testing measures the absolute smallest letter size that a person can see at a specific distance. This testing of eyesight is different than sustainable acuity. Sustainable acuity is the eyesight level at which a person can comfortably and easily read for an extended period of time. A common rule-of-thumb is that a student’s visual acuity should be three times better than the size of the words that are being read in order to read comfortably for an extended period of time. This means in order to read a typical 8th grade book comfortably for an extended period of time, a student should have 20/20 eyesight. In order to read a typical 4th grade book comfortably, a child should have approximately 20/25 acuity. Someone who only has 20/40 eyesight would only comfortably be able to sustain reading print that is the size of newspaper sub-headlines or a typical 1st – 3rd grade book. When a child becomes uncomfortable, he/she will often be more easily distracted while reading.
There are other visual skills that can affect sustainability of reading. Some children, even if they can see 20/20, have a difficult time reading for lengthy periods of time or reading at a proper rate because of poorly developed visual skills. Visual acuity, though, is a contributor to decreased reading rates. If a child is only seeing 20/40, please schedule an eye exam for him/her.