Two eyes, aimed and focused as one.
Both eyes have to point at the same spot and focus together. When the brain can’t hold them in line, close work gets hard, and in a child, an eye that turns can stop sight developing normally.
Read the in-depth series →- Aiming
- Convergence: both eyes turn in
- Focusing
- Accommodation: the lens thickens
- Reading at
- About 40 cm
- Common signs
- Eyestrain, headaches, words that blur, double or move
Who it helps
Children who avoid reading
Losing their place, skipping lines, or rubbing their eyes during homework.
Adults with screen strain
Tired eyes or headaches after close work, even with up-to-date glasses.
An eye that wanders
One eye drifting in or out, especially when tired. In children, this needs prompt assessment.
Sudden double vision in an adult needs same-day care, not an online booking. What to do
How the two controls work
Top view, five stepsLook far.
Eyes and distances not to scale. Values for a 62 mm pupil distance.
What the assessment involves
Sequence / 04History
Your symptoms, when they start, and how long you can read comfortably.
Measurement
How close the eyes can turn in, how far each can focus, and how they hold alignment.
Diagnosis
Which control is falling short: aiming, focusing, or the link between them.
Plan
Glasses, vision therapy in the clinic and at home, or prism, reviewed until the symptoms settle.
Want the detail? Read the series
In depth · 06 partsSix short, illustrated parts for parents, patients and referring optometrists. Each part has a patient section first, then notes for optometrists.
Hidden drift or visible turn?
Phorias, tropias, and why a turned eye in a child matters.
Read →Part 02Focusing and aiming, working as a team
Accommodation, convergence, AC/A and how alignment breaks down.
Read →Part 03Signs the eyes are working too hard
Symptoms, and the reading habits parents spot first.
Read →Part 04How we test, and how it’s treated
The assessment, and a treatment ladder that starts simple.
Read →Part 05Duane syndrome
Present from birth, stable, and easy to mistake for a palsy.
Read →Part 06Sudden double vision
Why it’s never normal, and when to go to hospital now.
Read →