Articles
Reading your prescription
What sphere, cylinder, axis and add actually mean, why a minus number is not worse than a plus one, and why the number that changed most is not always the one that matters.
A spectacle prescription is a short list of numbers and abbreviations that means nothing until somebody explains it once, after which it never needs explaining again. Four things appear on most of them: sphere, cylinder, axis and add. This piece uses the word “prescription” a great many times, because that is what the document actually is — and explaining what is written on it is a different thing entirely from offering to write you one.
Sphere (SPH)
The main power of the lens, in dioptres. A minus number bends light out and corrects short sight (blur at distance); a plus number bends light in and corrects long sight (blur up close). The sign tells you which type of correction it is, not how strong it is — a -1.00 is a mild correction and a -8.00 is a strong one, and the same is true on the plus side.
Cylinder (CYL)
The extra power needed where the eye is not perfectly round — astigmatism, in the ordinary sense of the word. Most eyes have a small amount of it. It is written as a second number because it corrects a different kind of blur from the sphere and is added on top of it, not instead of it.
Axis
Where the cylinder power sits, given in degrees from 0 to 180. It only means anything alongside a cylinder value — a prescription with no cylinder has no axis to speak of. On a small cylinder the axis can look like it has moved a long way between two visits without the correction itself changing much at all, which is why the axis is the number most likely to look dramatic and mean the least.
Add
The extra power for close work, added to the distance sphere for reading, an occupational lens, or the near portion of a bifocal or varifocal. It is always a plus number, because near focusing always needs more converging power, whatever the distance prescription already is.
Why a minus number is not worse than a plus one
The sign in front of the sphere describes which way the eye focuses wrongly, not how badly. A -6.00 and a +6.00 are the same size of correction pointed in opposite directions, and neither is a harder prescription to fill than the other — the number that actually says how strong a correction is is the one after the sign, and a bigger one usually means a thicker lens rather than a worse eye.
The number that changed most is not always the one that matters
Comparing two prescriptions by looking for the biggest single change is a natural thing to do and a misleading one. A cylinder axis can swing a long way for a very small real change when the cylinder itself is weak; the sphere is usually the steadier number to watch. And a prescription on its own does not say anything about the health of the eye — it is the output of one part of the examination, not the whole of it.
You get a written copy at the end of every examination here, unasked, whether or not you buy anything, along with your pupillary distance — the one measurement a prescription does not include and a lens cannot be made without.
Get a set of numbers to read
A written prescription at the end of every examination, whether or not you buy anything here.