Sharness of sight, also known as ACUITY, is the ability to see clearly and sharply. ACUITY is measured by how well a person can distinguish between two points of light or two lines that are close together. It is an important part of vision and is often tested during eye exams. ACUITY is usually measured in terms of visual ACUITY, which is the ability to distinguish letters or symbols at a certain distance. Good ACUITY is essential for activities such as reading, driving, and recognizing faces.