Testing eye convergence is straightforward: you slowly move a small target toward the bridge of someone’s nose while watching whether both eyes track inward together. The key measurement is called the near point of convergence (NPC), and in healthy eyes, both eyes should be able to follow the target to within about 5 centimeters of the nose before breaking apart. This simple test can reveal convergence insufficiency, one of the most common binocular vision problems, which causes eye strain, double vision, and difficulty reading.
The Near Point of Convergence Test
The NPC test is the primary way convergence is evaluated, both in clinical settings and basic screenings. You need good lighting, a small visual target, and a ruler or measuring tape. The person being tested sits directly in front of you, wearing their glasses if they use them full-time.
Hold the target about 18 inches (roughly 45 centimeters) from the person’s face, centered between their eyes. Then move it slowly and steadily toward the bridge of their nose at about 1 centimeter per second. As you move it closer, watch both eyes carefully. They should turn inward smoothly and symmetrically. Look for any shaking, uneven tracking, or one eye drifting outward while the other continues to follow.
Two things can signal the endpoint. The person may report that the target “splits” into two images. Or you may observe one eye physically losing its inward position and drifting out. The distance from the nose where either of these happens is called the break point. Once you’ve found it, slowly pull the target back outward until both eyes lock on again and the person sees a single image. That distance is the recovery point.
Repeat the test three times and note the measurements each time. A normal break point is 5 centimeters or less from the nose, and a normal recovery point is 7 centimeters or less. If the break point consistently falls beyond these thresholds, it suggests convergence insufficiency.
What to Use as a Target
The choice of target matters more than you might expect. There are two main options: an “accommodative target” like a small letter printed on a card, or a penlight. An accommodative target forces the eyes to focus on fine detail, which naturally helps drive convergence. A penlight, being a simple point of light, doesn’t engage the focusing system the same way.
Research comparing these targets shows they produce statistically different results. People tend to converge closer with an accommodative target because their focusing reflex gives convergence an extra push. A small letter (around the size you’d see on an eye chart’s 20/30 line) is the standard first choice. If results are borderline, clinicians often retest with a penlight combined with red and green glasses, which eliminates the focusing boost and reveals the eyes’ raw convergence ability. For a basic screening, a small letter or the tip of a pencil with a tiny detail on it works well.
Testing Children
Children don’t always give reliable verbal feedback about when a target looks double. With younger kids, you’ll rely more on objective observation than on what they tell you. Watch their eyes directly as you move the target inward. The moment one eye breaks outward or stops tracking, that’s your break point, regardless of what the child reports.
A cover test is another useful tool for children because it’s entirely objective. The examiner covers and uncovers each eye while the child looks at a near target, watching how the eyes move to pick up fixation. Expected results are a slight outward resting posture of up to 6 prism diopters at near. Any inward resting posture at near is considered abnormal. For very young or inattentive children, a simpler alignment check involves holding a small light at about 40 centimeters and observing where the light reflects on each cornea. If the reflections aren’t symmetrical, one eye may be misaligned.
Symptoms That Suggest a Problem
Convergence insufficiency doesn’t always announce itself with obvious double vision. More commonly, people notice eye strain, headaches centered around the forehead, blurred vision after reading, or a sense that words are moving on the page. Some people just feel unusually sleepy or unable to concentrate after short periods of near work. In children, this can look like an attention problem rather than a vision problem.
A standardized questionnaire called the Convergence Insufficiency Symptom Survey (CISS) scores these complaints on a numerical scale. A score above 15 is the threshold that flags a likely problem, picking up about 72% of true cases. While you won’t formally administer this survey at home, recognizing the pattern of symptoms (strain, headache, and reading difficulty that worsens with time on task) is a useful screening step before formal testing.
What the NPC Test Can and Cannot Tell You
The NPC test is a good first screen, but it measures only one dimension of how your eyes work together. A full convergence evaluation also includes measuring fusional vergence ranges, which test how well your eyes can sustain convergence under increasing demand using prisms. During this test, progressively stronger prisms are placed in front of the eyes until the person reports blur (the blur point), then double vision (the break point), and finally the prism strength is reduced until single vision returns (the recovery point).
Convergence problems fall into distinct categories that require different measurements to distinguish. Convergence insufficiency means the eyes struggle to turn inward enough for near tasks, producing more outward drift at near than at distance. Convergence excess is the opposite pattern: the eyes over-converge at near, causing more inward turning than necessary. These two conditions can look similar on a basic screening but behave very differently when tested with lenses and prisms. Convergence insufficiency often involves weak fusional convergence reserves or a low ratio between focusing effort and convergence response. Convergence excess typically involves a high ratio, where small changes in focus trigger exaggerated convergence.
This is why an NPC test that falls outside normal range is best followed up with a comprehensive binocular vision exam from an optometrist. The NPC tells you something is off. The additional tests tell you exactly what, and which treatment will help.
How to Do a Basic Screen at Home
You can perform a reasonable NPC screening at home with a pencil and a ruler. Tape a single small letter (cut from a magazine or printed small, around 3 to 4 millimeters tall) to the eraser end of a pencil. Have the person sit in a well-lit room, looking straight ahead. Hold the pencil at arm’s length, centered on their nose, and move it slowly inward.
Watch their eyes, not just the pencil. You’re looking for the moment one eye stops converging and drifts outward. Ask them to tell you if the letter doubles. Use the ruler to measure the distance from the bridge of the nose to the pencil at the break point. Do this three times. If the break consistently lands beyond 5 centimeters, or if one eye visibly loses tracking well before the other, that’s worth bringing up with an eye care provider.
The main limitation of home testing is measurement precision. Without a standardized tool like an RAF rule (a calibrated ruler with a sliding target carriage), it’s hard to move the target at a consistent speed and read the exact distance simultaneously. You’re also both the tester and the observer, which makes it difficult to watch both eyes closely while managing the target. Home results are useful as a rough screen, not a diagnosis. Consistent readings well beyond the normal range are meaningful, but borderline results need professional confirmation.