A thin glass wedge in front of a bar of light, the bar seen through it stepped sideways but still sharp.

Reading a prescription

Prism

Optical glossaryReading a prescription

A deliberate offset that shifts an image's position, prescribed for eye-alignment problems. Unwanted prism from bad centration causes strain.

A prism bends light toward its base, so what you see through it appears shifted toward the apex. Any lens looked through off-center does the same.

Illustrative · the shape of the principle, not measured data

In more depth

Prism moves where an image appears without changing its focus. It is measured in prism diopters — one moves the image one centimeter at a distance of one meter — and written with a base direction: in, out, up or down. Prescribed prism helps eyes that do not naturally point at the same place, easing double vision or strain.

Prism also arrives by accident. Looking through any lens away from its optical center bends light like a prism, by an amount given by Prentice's rule: the distance off-center in centimeters times the lens power. That is why the centers are placed from measurement, not by eye.

Worked example

Lens power
+4.00
Looking off-center by
5 mm
Prism you receive
2.00 Δ

Related terms

Optical centerThe point of a lens where light passes without deviation. Should sit directly in front of your pupil.Pupillary distance (PD)The distance between your pupil centers in millimeters. It positions each lens's optical center in front of each eye; errors cause strain even with a correct prescription.DiopterThe unit of lens power. One diopter focuses parallel light at one meter.

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