Positive relative accommodation

Positive relative accommodation (PRA) is a measure of the maximum ability to stimulate accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by an ophthalmologist or optometrist during an eye examination using a phoropter. After the patient's distance correction is established, he or she is instructed to view small letters on a card 40 cm from the eyes. The examiner adds lenses in -0.25 increments until the patient first reports that they become blurry. The total value of the lenses added to reach this point is the PRA value.

High PRA values (> or = 3.50 diopters) are considered to be diagnostic of disorders involving accommodative excess. Those with accommodative insufficiency typically have PRA values below -1.50 diopters.