Vision screenings are not comprehensive eye exams. Screenings usually take only a few minutes and are often performed by volunteers who are not eye care professionals.
In many cases, vision screenings are nothing more than a visual acuity test where you're asked to identify the smallest letters you can on a vision chart across the room.
Vision screenings typically are designed to only detect subnormal visual acuity and major vision problems — as quickly and cost-effectively as possible. They generally are ineffective for detecting more subtle vision problems and potentially sight-robbing eye diseases.
People who fail a vision screening (usually because their visual acuity is worse than 20/40) are made aware of this and are encouraged to visit an eye doctor so they can have their vision problem professionally diagnosed and treated with eyeglasses, contact lenses, medicine or surgery.