Why the eye is measured
Every eye is a slightly different size and shape, and even your two eyes differ. How long the eye is, and how steeply the cornea is curved, decide most of its focus. Long eyes tend to be nearsighted; short eyes tend to be farsighted.
In cataract surgery, the natural lens is removed and replaced with an artificial lens. Its strength has to match your eye. Biometry takes the measurements used to calculate that strength.
What is measured
- Keratometry: the curve of the cornea. It is measured in two directions. If they differ, the cornea causes astigmatism, which a toric lens can correct.
- Axial length: the length of the eye. This is measured from the front of the cornea to the centre of the retina, usually with a fine beam of light. If a cataract is so dense that light cannot pass through, ultrasound is used instead.
- Other measurements, such as the depth of the front of the eye and the thickness of the lens, which refine the calculation.
What the test is like
You sit at the machine with your chin on a rest and look at a light for a few seconds per eye. Nothing touches your eye for optical biometry. If ultrasound is needed, a numbing drop is put in first and a small probe touches the eye briefly.
If you wear contact lenses, you may be asked to leave them out for a while beforehand, because they change the shape of the cornea.
From measurements to a lens
The surgeon puts the measurements into established formulas to choose the lens power. Unless something different is planned with you, the aim is to leave as little prescription as possible for distance.
Measurements and healing are never perfectly predictable, so a small leftover prescription is possible, and glasses may still fine-tune your vision. The choice of lens, your eye’s health and how it heals all affect the result.
This guide explains; it does not diagnose. Your doctor’s advice about your own eyes comes first.