Most vision tests examine a person's sharpness of VISION (visual acuity) and often of the field of vision (see VISION, FIELD OF). Refraction tests assess whether a person has an error that can be corrected with glasses such as ASTIGMATISM, HYPERMETROPIA or MYOPIA. Visual acuity is tested using a Snellen chart, when the patient tries to read letters of differing standard sizes from 6 metres away. The optician will prescribe lenses to correct any defects detected by such vision tests.