A lens in a small stand bending a narrow beam of warm light to one bright focal point.

Reading a prescription

Diopter

Optical glossaryReading a prescription

The unit of lens power. One diopter focuses parallel light at one meter.

Power and focal length are two sides of one number: doubling the diopters halves the distance at which the lens brings light to a focus. Drawn to scale.

Illustrative · the shape of the principle, not measured data

In more depth

A diopter is the reciprocal of a focal length measured in meters. A +2.00 lens brings parallel light to a focus 50 centimeters away; a +4.00 lens at 25 centimeters. Minus lenses spread light instead, as though it came from a point in front of them. Because the unit is a reciprocal, powers simply add: two thin lenses of +1.00 and +0.50 held together act as one of +1.50.

Prescriptions move in quarter-diopter steps, roughly the smallest change most people can reliably tell apart. The same unit measures the eye's own focusing: reading at 40 centimeters asks for 2.5 diopters of effort, which is why close work is where an aging eye first runs short.

Worked example

Power +0.50
Focus at 200 cm
Power +1.00
Focus at 100 cm
Power +2.00
Focus at 50 cm
Power +4.00
Focus at 25 cm

Related terms

Sphere (SPH)The main power of your prescription, in diopters. Minus corrects nearsightedness, plus corrects farsightedness; further from zero means stronger.Addition (ADD)The extra reading power added to the lower part of a progressive or bifocal. Typically starts near +1.00 in the early forties and usually settles near +2.50 (up to +3.00 for close working distances).PresbyopiaThe age-related stiffening of the eye's internal lens that makes close focus difficult, typically from the early forties. Not a disease — the reason reading additions and progressives exist.PrismA deliberate offset that shifts an image's position, prescribed for eye-alignment problems. Unwanted prism from bad centration causes strain.

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