Positive relative accommodation (PRA) in biology, is a measure of the maximum ability to stimulate eye accommodation while maintaining clear, single binocular vision. [1] This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist during an eye examination using a phoropter. After the patient's distance correction is established, she or he is instructed to view small letters on a card 40 cm from the eyes. The examiner adds lenses in −0.25 diopter 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. [ citation needed ]
High PRA values (>= 3.50 diopters) are considered to be diagnostic of disorders involving accommodative excess. [2] Those with accommodative insufficiency typically have PRA values below −1.50 diopters. [3]