What the numbers on your prescription mean
A prescription card lists three numbers for each eye: sphere, cylinder and axis. Each one changes the world you see in its own way.
OD is your right eye, and OS is your left.
Most cards use Latin shorthand. OD stands for oculus dexter, the right eye, and OS for oculus sinister, the left. Some cards write R and L instead.
Sphere tells you how far your eye sees sharp.
Sphere, or SPH, measures the eye's power in dioptres. A minus sign means myopia, or nearsightedness: the eye focuses too strongly, so far things blur. Divide one by the number, and you get the farthest distance a minus eye sees sharp, in metres.
- A −1 eye sees sharp up to a metre.
- A −2.25 eye sees sharp up to about 44 centimetres, about where you hold a book.
- A −6 eye loses focus past 17 centimetres.
A plus sign means hyperopia, or farsightedness: the eye focuses too weakly. A young plus eye squeezes extra power out of its own lens and can hide the problem for years, but the effort tires it, and close work suffers first. With age, that spare power fades and the blur shows.
Cylinder and axis describe astigmatism.
Cylinder, or CYL, adds power in one direction only. The cornea curves more one way than the other, like the back of a spoon, so lines running one way look sharp while lines across them blur. The axis, a number from 1 to 180 degrees, says which way the cylinder lies. A card with no cylinder leaves the axis blank.
A lens cancels the error.
A minus lens spreads light out just enough for a minus eye to bring it to a point on the retina. A plus lens gathers light in for a plus eye. That's why the lens in Refrac has the same number as the eye it fixes.
Try your own numbers.
Open Refrac and turn the dial to your sphere. Then use the Astigmatism panel to add your cylinder and pick the axis closest to yours. The scene shows what your eye sees without glasses; drag the lens over it to see what your glasses do.