Your prescription: what is on it, and what to do if you have lost it

"I think I have lost it" is one of the things I hear most often, said in the voice of somebody who assumes the appointment is over. It is not. But before getting to that, it is worth knowing what is actually on the paper.

What the numbers mean

• OD and OS: your right eye and your left eye, from the Latin. Some prescriptions simply write R and L.
• SPH, the sphere: the main correction. A negative number means nearsighted, a positive one means farsighted.
• CYL and AXIS: the astigmatism correction and the angle it sits at. Plenty of people have some. It is not bad news.
• ADD: the reading addition. This is what tends to show up after forty, and it is what makes a progressive lens necessary.
• PD, the pupillary distance: often missing from the prescription, because it is not a measurement of your sight but of your face. I take it myself at the fitting.

How long a prescription stays good

Generally up to two years, but the person who wrote it decides and puts the date on it. Sight that is changing, diabetes, a child still growing — any of those can make it shorter. The date on the paper overrides any general rule, including this one.

If yours has run out I will tell you before anything is ordered. Having lenses cut to an expired prescription means paying for a correction that may no longer be yours.

If you no longer have it

Ask the clinic where you had your exam for a copy. It is an ordinary request, made every day, and there is nothing awkward about making it.

If the clinic has closed or your optometrist has retired there is usually nothing left to request, and that one needs a new exam — I can point you to someone.

A prescription is not a contract

It describes your eyes. It does not tie you to any particular shop. An exam and a pair of glasses are two separate purchases, and there is nothing unusual about doing one in one place and the other somewhere else. Most of my customers do exactly that.