Positive relative accommodation
Positive relative accommodation (PRA) is a measure of the maximum ability to stimulate accommodation
Accommodation (eye)
Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image on an object as its distance changes....

 while maintaining clear, single binocular vision
Binocular vision
Binocular vision is vision in which both eyes are used together. The word binocular comes from two Latin roots, bini for double, and oculus for eye. Having two eyes confers at least four advantages over having one. First, it gives a creature a spare eye in case one is damaged. Second, it gives a...

. This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist during an eye examination
Eye examination
An eye examination is a battery of tests performed by an ophthalmologist, optometrist, or orthoptist assessing vision and ability to focus on and discern objects, as well as other tests and examinations pertaining to the eyes....

 using a phoropter
A phoropter is an instrument commonly used by eye care professionals during an eye examination, containing different lenses used for refraction of the eye during sight testing, to measure an individual's refractive error and determine his or her eyeglass prescription.Typically, the patient sits...

. After the patient's distance correction is established, he is instructed to view small letters on a card 40 cm from the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...

s. 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
Accommodative insufficiency
Accommodative insufficiency involves the inability of the eye to focus properly on an object. Approximately 80 percent of children diagnosed with CI also demonstrate AI....

 typically have PRA values below -1.50 diopters http://www.aoa.org/documents/CPG-18.pdf.

See also

  • Amplitude of accommodation
    Amplitude of accommodation
    Amplitude of accommodation is a measurement of the eye’s ability to focus clearly on objects at near distances . This eye focusing range for a child is usually about 5–7.5 cm . For a young adult, it is 10–15 cm . The focus range for a 45-year-old adult is about 50 cm...

  • Convergence insufficiency
    Convergence insufficiency
    Convergence insufficiency or Convergence Disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by an inability of the eyes to turn towards each other, or sustain convergence.-Symptoms:...

  • Negative relative accommodation
    Negative relative accommodation
    Negative relative accommodation was proposed by Prof. Joseph Kearney of Oxford University in 1967, is a measure of the maximum ability to relax accommodation while maintaining clear, single binocular vision. This measurement is typically obtained by an orthoptist, ophthalmologist or optometrist...

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