When a cataract is removed, the eye loses the focusing power of its natural lens, which provides around a third to two-fifths of the eye’s total. Before lens implants, people wore very thick glasses after surgery to make up for it. Today, an intraocular lens (IOL) is placed in the eye during the same operation.
What an IOL is
An IOL has two parts: the optic, the clear centre that focuses light, and the haptics, two small arms that hold it in position. It sits inside the capsule, the thin membrane that held the natural lens. Over time the capsule closes snugly around the implant and keeps it steady. The lens is meant to stay for life.
Early implants were rigid plastic. Today’s are made of soft acrylic that can be folded, passed through a tiny opening and left to unfold inside the eye. Many also filter ultraviolet light.
Choosing the strength
Every eye has its own measurements. Before surgery, the curvature of your cornea and the length of your eye are measured precisely (see biometry). The surgeon uses these in established formulas to choose the lens power.
This means cataract surgery can also correct short or long sight you had before. Measurements and healing are never perfectly predictable, so a small leftover prescription is possible, and glasses may still sharpen your vision. Sometimes a small prescription is left on purpose, for example to make reading easier.
The main types
Monofocal
A monofocal lens gives one sharp point of focus, usually for distance. Many people see well far away without glasses but need reading glasses for close work. Some designs have a refined shape that gives sharper, brighter images, especially in dim light.
Toric
Astigmatism comes from a cornea that is curved more in one direction than the other. It blurs vision at all distances. A toric lens has a correction built in, and is lined up carefully in the eye to cancel the astigmatism. Like a monofocal lens, it usually focuses at one distance, so reading glasses may still be needed.
Multifocal and extended-range
These lenses split or stretch the light into more than one focus, so you can see at more than one distance. The brain learns to pick the clearest image. Many people need glasses less often, but some notice halos or glare around lights at night, and the lenses do not suit every eye. Toric versions are available for eyes with astigmatism.
Which lens is right for you?
That depends on the health of your eye, your astigmatism, your work and hobbies, how you feel about glasses and halos, and your budget. Your surgeon will go through the options at your consultation. The choice is yours to make with them, not one a website can make for you.
This guide explains; it does not diagnose. Your doctor’s advice about your own eyes comes first.