Learn · Your prescription, number by number

What is prism on your prescription?

Most of a prescription corrects how sharply you see. Prism corrects something different — where your two eyes are pointing. It appears on a minority of prescriptions, and when it does it always comes with a direction called the base.

What it does

A prism does not change focus; it shifts the whole image sideways or vertically. If your two eyes do not aim at exactly the same point, your brain has to work to fuse the two slightly different images — and when it cannot do so comfortably, the result is eye strain, headaches, or double vision. A prism moves the image to where the eye is already looking, so the two line up without effort.

The two numbers

Amount — prism dioptres (Δ)

How much the image is displaced. Written as a number followed by a triangle, e.g. 2Δ. Corrections of 0.5 to 4Δ are common; larger amounts get thick and heavy, so bigger misalignments are usually managed differently.

Base — the direction

Which way the thick edge of the prism faces, and therefore which way the image moves. Written as BI (base in, toward the nose), BO (base out, toward the ear), BU (base up) or BD (base down).

Prism is prescribed per eye, and can be split. A 4Δ correction is often written as 2Δ in each eye rather than 4Δ in one, which keeps the lenses thinner and better balanced. Two prescriptions can therefore look different while doing the same job — another reason to keep the original rather than a remembered version.

Why it appears

Common reasons include a latent tendency for the eyes to drift (a phoria) that becomes symptomatic with age or fatigue, convergence difficulty at reading distance, and alignment changes after eye-muscle surgery or a neurological event. Prism is also sometimes used temporarily, to be reduced once the underlying issue settles.

Worth tracking closely

Prism is the part of a prescription where change carries the most clinical meaning. A stable small prism over years is unremarkable; a prism that appears suddenly, or grows steadily, is exactly the pattern an ophthalmologist wants to see documented with dates. If it is on your prescription, it belongs in your record — with the base direction, which is easy to lose and impossible to reconstruct later.

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