Two lenses in beams of warm light: one gathers its beam to a bright point, the other spreads its beam outward.

Reading a prescription

Sphere (SPH)

Optical glossaryReading a prescription

The main power of your prescription, in diopters. Minus corrects nearsightedness, plus corrects farsightedness; further from zero means stronger.

An example prescription, one line per eye. The sphere is the first number: the main power, minus for nearsightedness, and plus for farsightedness.

Illustrative · the shape of the principle, not measured data

In more depth

The sphere is the lens's main power, the same in every direction across it. A minus sign corrects nearsightedness: the lens spreads light so the eye focuses it further back, onto the retina. A plus sign corrects farsightedness: the lens adds focusing power the eye lacks. The further the number is from zero, the stronger the correction.

It is written SPH, or simply as the first number in each eye's line. "Plano", "PL" or 0.00 means no spherical correction, and "DS" after a number means there is no cylinder to follow. The sphere also does most to decide how a lens looks: minus lenses thicken toward the edge; plus lenses, in the center.

Related terms

DiopterThe unit of lens power. One diopter focuses parallel light at one meter.Cylinder (CYL)The correction for astigmatism, where the eye's optics focus light along two different lines instead of one point. Always paired with an axis.MyopiaNearsightedness: distance is blurred because light focuses in front of the retina. Corrected with minus lenses.HyperopiaFarsightedness: light would focus behind the retina, making near work effortful. Corrected with plus lenses.

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