In short
A spectacle prescription is four or five short numbers, each describing a different thing. The eye test also records measurements that have nothing to do with glasses, and those are the ones worth reading carefully.
Emergency if: Sudden loss of vision, a curtain across the field, a shower of new floaters with flashes, or a painful red eye with haloes and vomiting — go to an emergency department now.
On your report
HbA1c, Fasting Glucose, Total Cholesterol, LDL Cholesterol +1 more
On this page
01
Sphere: the main number
SPH, the sphere, is the main lens power, measured in dioptres. A minus value corrects short sight, where distant objects blur; a plus value corrects long sight. The further the number is from zero in either direction, the stronger the lens — so minus 4.00 is a stronger correction than minus 1.00.
The value is given separately for each eye, labelled right and left or with the Latin abbreviations OD and OS. A difference between the two eyes is common and normal; a large and new difference, particularly in a child, is worth asking about because it can lead to one eye being favoured.
02
Cylinder and axis: astigmatism
CYL, the cylinder, describes astigmatism — the eye's surface being shaped slightly more like a rugby ball than a football, so light focuses unevenly. The axis, a number from 1 to 180, states the orientation of that correction in degrees. It is an angle, not a strength, so a high axis number does not mean a worse eye.
The cylinder and axis only mean anything together; a cylinder value with no axis is incomplete. Small amounts of astigmatism are very common and often need no correction at all.
03
ADD, and why it appears after forty
ADD is extra magnifying power for the lower part of the lens, used for reading. It appears as the eye's lens becomes less flexible with age, typically from the early forties, and it usually increases gradually over the following years before settling. This is presbyopia, and it is an expected change rather than a disease.
Prism and base, if present, correct alignment between the two eyes rather than focus. A prism appearing for the first time, or new double vision, is a finding to discuss with a doctor rather than simply to have made up into glasses.
04
Visual acuity, and why it is written as a fraction
Acuity is recorded as a fraction such as 6/6 or 20/20 — the same measurement in metres or feet. The top number is the testing distance and the bottom number is the distance at which a typical eye could read that line. So 6/12 means you read at six metres what a typical eye reads at twelve.
Acuity is usually recorded both without and with correction. Vision that corrects fully with glasses is a focusing problem. Vision that does not correct fully is the important finding, because it points to something other than lens power, and that is the line on the report most worth noticing.
05
The parts of the test that are not about glasses
An eye examination also checks eye pressure, the health of the optic nerve, and the retina, and it may include a visual field test. These screen for glaucoma, which damages sight painlessly and irreversibly from the periphery inward, and often produces no symptoms until a great deal has been lost. Raised pressure is treatable, and finding it early is the whole point.
The retinal examination also detects diabetic changes, macular degeneration and high blood pressure. This is why a routine eye test is a genuine general health check, and why people with diabetes are recalled for retinal screening even when their vision is perfect.
When to act
Routine — see a doctor
A small change in prescription with vision correcting fully: update your glasses and keep to the recall interval you were given.
Same-day — call promptly
Vision that does not correct fully with glasses, a rapidly changing prescription, or new double vision — these need examining rather than dispensing.
Emergency — act now
Sudden loss of vision, a curtain across the field, a shower of new floaters with flashes, or a painful red eye with haloes and vomiting — go to an emergency department now.
Values mentioned in this story
Each one opens a visual guide with its normal range and what moves it — upload a report and we place your own numbers on the same scale.
Sources
- Eye Care — MedlinePlusmedlineplus.gov
- Glaucoma — NHSnhs.uk
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