RAF Near Point Ruler: Complete Guide to Convergence & Accommodation Testing

What Is the RAF Near Point Ruler?

The Royal Air Force (RAF) Near Point Ruler — commonly called the RAF rule — is a standardised optometric instrument used to measure two critical aspects of binocular vision: near point of convergence (NPC) and amplitude of accommodation. It is one of the most widely used tools in routine eye examinations, orthoptic assessments, and sports vision testing.

Originally developed for Royal Air Force aircrew vision screening, the RAF rule has become a universal clinical tool in optometry and orthoptics due to its simplicity, repeatability, and diagnostic value. SciMed supplies the RAF Near Point Ruler to optometrists, ophthalmologists, orthoptists, and educational institutions across the US and internationally.

What Does the RAF Rule Measure?

1. Near Point of Convergence (NPC)

NPC is the closest point at which both eyes can maintain simultaneous binocular fixation on a target without one eye breaking away (deviating outward). It is the standard clinical test for detecting convergence insufficiency (CI) — the most common binocular vision disorder, affecting an estimated 2–8% of the population and frequently causing symptoms during near work such as reading and screen use.

Normal NPC: Break point ≤5 cm; recovery point ≤7 cm. A break point beyond 10 cm suggests convergence insufficiency.

2. Amplitude of Accommodation

Accommodation is the eye’s ability to change focus from distance to near. The amplitude of accommodation is the full range of this ability, from the far point to the near point of clear single vision. It decreases predictably with age (presbyopia), making it a key measurement in spectacle and contact lens prescribing.

Expected amplitude by age (Hofstetter’s formula):

  • Age 10: ~14 D
  • Age 20: ~10 D
  • Age 30: ~7 D
  • Age 40: ~4.5 D
  • Age 50: ~2.5 D
  • Age 60: ~1 D

Components of the RAF Near Point Ruler

The RAF rule consists of:

  • A calibrated rod marked in centimetres (measuring working distance) and dioptres (measuring accommodation amplitude)
  • A chin rest or nasal bridge rest that anchors the rule against the patient’s face, ensuring a consistent and reproducible starting position
  • A sliding target holder carrying four test targets (rotating drum or card): letters for monocular accommodation, and a fixation target for binocular NPC testing

How to Measure Near Point of Convergence (NPC)

  1. Seat the patient comfortably; ensure adequate room illumination
  2. Place the end of the RAF rule against the patient’s nasal bridge or lower orbital rim
  3. Select the small fixation target (a pen point or RAF target symbol)
  4. Position the target at 40–50 cm to start
  5. Slowly move the target toward the patient at approximately 1–2 cm per second, instructing them to keep the target single
  6. Record the break point (cm from the bridge of the nose) — the point at which the patient reports double vision or one eye is observed to deviate outward
  7. Continue moving the target briefly, then move it back out and record the recovery point — where binocular fixation is re-established
  8. Repeat 3 times and average the results

How to Measure Amplitude of Accommodation

  1. Occlude one eye (test monocularly)
  2. Place the target at arm’s length (approximately 40 cm) with the letter target facing the patient
  3. Instruct the patient to report when the letters become blurred and cannot be cleared
  4. Slowly move the target toward the patient; note the point at which letters first blur persistently
  5. Record the distance in cm; convert to dioptres: Amplitude (D) = 100 / distance in cm
  6. Repeat for the fellow eye
  7. Compare findings to age-expected norms

Clinical Significance

Convergence Insufficiency

A receded NPC (≥10 cm break point) is the hallmark of convergence insufficiency. Symptoms include eyestrain, headaches, blurred or double vision with reading, difficulty concentrating, and loss of place when reading. The RAF rule provides the fastest, most cost-effective screening tool for CI in both clinical and school-based settings.

Post-Concussion Assessment

Convergence and accommodation are frequently disrupted following mild traumatic brain injury (concussion). NPC measurement with the RAF rule is now included in standard concussion assessment protocols (including the Sport Concussion Assessment Tool, SCAT5) as an objective vision-based marker of recovery.

Presbyopia Management

Reduced accommodation amplitude measured with the RAF rule confirms presbyopia and guides the addition power required for reading spectacles or multifocal contact lenses.

Shop RAF Near Point Ruler at SciMed

SciMed supplies the RAF Near Point Ruler — the gold standard tool for NPC and accommodation testing — to optometry practices, orthoptic departments, universities, and sports medicine clinics. Durable, lightweight, and calibrated to clinical standards.

Shop RAF Near Point Ruler →

Back to blog