How to read an eyeglass prescription

You leave the appointment holding a small slip full of abbreviations. The numbers on it answer one question: what lens has to sit in front of each eye for the image to land exactly on the retina?

What follows is here so you can read the slip, not so you can make decisions from it. Interpreting a prescription and changing it when needed is your doctor's work.

OD marks the right eye and OS the left; some prescriptions simply write R and L. Mixing up the two rows is a common error and worth checking when you collect the glasses.

If you see OU, that is a value applying to both eyes together.

The SPH column gives the sphere, the eye's overall refractive error, in dioptres. A plus value indicates hyperopia, a minus value myopia.

The size of the number is the degree of the error, and the gap between +1.00 and +5.00 is not a small one. Still, no single figure is good or bad in isolation; what it means depends on the child's age and on how the two eyes compare.

Mild hyperopia is an expected finding in children and often needs no glasses at all. It commonly decreases as the eye grows.

The CYL column gives the amount of astigmatism. In astigmatism the cornea is not equally curved in every direction, so the image focuses along a line rather than at a single point.

AXIS is the angle between 0 and 180 stating which direction that runs in. It is often misread: 180 does not mean a worse astigmatism than 10, only a different orientation.

If CYL is blank or zero, no astigmatism was recorded, and the axis carries no meaning in that case.

PD is the distance between the pupils. It is needed so the optical centres of the lenses sit in the right place; a wrong PD can make even a correct prescription uncomfortable.

ADD is extra power for near work. It is common in adults for age-related near vision and appears in children only in particular situations, such as certain patterns of inward turn.

Prism and base direction are written when a correction towards eye alignment is needed. They do not appear on every prescription.

Most parents look for the biggest figure. For lazy eye, though, the decisive information is the difference between the two eyes. A child who is +4.00 in both eyes gives the brain a similar quality of image from each side; a child who is +1.00 in one and +4.00 in the other has one side permanently behind.

This is why a difference that looks small is still taken seriously. Anisometropia is one of the most common causes of amblyopia, and it is the one finding you can spot on the slip itself.

In children the values come from a cycloplegic refraction. The drops relax the focusing muscle and reveal the hyperopia a child otherwise hides by focusing through it. A measurement taken without drops may not show the true figure in a child.

Doctors sometimes prescribe deliberately less than the measured value. That is not an oversight; it is a judgement based on how much correction the child can adapt to.

For the same reason, comparing prescriptions between siblings or acquaintances is misleading. The same number means different things at different ages and in different clinical pictures.