Visual Acuity – Core Vocabulary

Visual Acuity (VA)

The sharpness of vision; a patient’s ability to recognize a minimum-sized target from a specified distance.

Snellen Principle

Design rule stating that each critical detail of a test letter must subtend 1 minute of arc and the whole letter 5 minutes of arc at the eye’s nodal point.

Snellen Fraction

Universal VA recording: numerator = test distance, denominator = distance at which the letter subtends 5′ of arc for a normal eye (e.g., 6/12).

Minimum Detectable Resolution

Ability to perceive the presence or absence of a stimulus against its background without resolving detail.

Minimum Resolvable (Recognition) Acuity

Ability to identify the smallest symbols or detail that can be correctly recognized; basis of most clinical VA tests.

Form Sense

Simple recognition task using oriented symbols such as Landolt C or Tumbling E to indicate direction.

True Minimum Legible

Recognition acuity measured with complex optotypes like letters or numbers that must be correctly named.