Standard vision screenings, like the ones given in school or at the pediatrician’s office, test a person’s visual acuities at a distance of 20 feet. Most people (depending on age and size) read at a distance of 12-16 inches, and vision screenings simply don't test your visual function at this distance. Routine annual eye exams are more comprehensive, but typically only assess eye health and the need for visual correction (glasses or contact lenses). Yes, the eye chart can help determine whether you have 20/20 acuity, but it does not test how your eyes will perform when you focus on a book page or computer screen.
During a comprehensive visual evaluation, the doctor performs a range of specialized tests to probe for problems with eye focusing, eye movement, eye teaming, and visual perceptual delays to assess all aspects of an individual’s visual performance.