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PD errors are the fastest way to remake glasses. This guide keeps your measurements accurate and exam-ready.
A perfect lens prescription can still feel wrong if the PD is off. That’s why the ABO exam loves PD questions. Expect 25+ questions testing monocular vs binocular PD, near vs distance, and when digital pupilometers outperform a simple ruler.
PD measurement looks basic, but the exam often hides tricky details: frame PD vs patient PD, measuring high prescriptions, and common parallax mistakes. If you’re off by even 1 mm, you can create unwanted prism and patient discomfort. That’s the “why” behind this topic.
This guide walks you through manual and digital techniques, shows you how to handle near PD, and gives you quick formulas that make exam questions easy to spot.
A pupilometer is a device that measures the distance between a patient’s pupils. It can measure binocular PD (distance between pupils) or monocular PD (from midline to each pupil). Digital pupilometers reduce parallax error and give fast, consistent results.
Manual PD measurement uses a ruler and your eyes. It’s simple but easy to mess up if you’re not aligned or if the patient moves. The ABO expects you to know both methods and when to use each.
Average adult distance PD is 54–74 mm, with 62–64 mm being common. Near PD is typically distance PD minus 3 mm. That’s a classic exam shortcut.
A digital pupilometer includes viewing windows, a nose rest, and ameasurement display. The operator aligns the patient’s corneal reflexes with the target, and the device calculates PD automatically.
Manual tools are simpler: a millimeter ruler and a fixation target. The key component is you—your eye alignment determines accuracy.
[Image: Labeled diagram of digital pupilometer]
[Image: Technician measuring PD with ruler]
Record PD as binocular or monocular. Example: 32/31 means OD 32 mm and OS 31 mm. Binocular PD would be 63 mm.
Near PD is smaller because eyes converge. If a patient’s distance PD is 64 mm, near PD is about 61 mm. That’s a quick check for accuracy.
Frame PD matters for decentration. If frame PD is wider than patient PD, you’ll decenter inward; if narrower, you’ll decenter outward.
Accurate PD ensures optical centers align with patient gaze, reducing unwanted prism and visual discomfort. It’s essential for high prescriptions, multifocals, and progressive lenses.
Monocular PDs are especially important when facial asymmetry is present. Most experienced techs measure monocular PDs by default for premium lenses.
Expect PD questions on near vs distance, monocular vs binocular, and typical adult ranges.
Memory aid: “Near PD = Distance PD - 3 mm.” It saves time on the exam.
Try these before you check the answers.
A patient’s distance PD is 64 mm. Approximate near PD?
Answer: B. 61 mm
Near PD is typically distance PD minus 3 mm.
Practice with Opterio and build confidence in every instrument-focused ABO question.