How do multifocal contacts work?
When I first started wearing contact lenses, I never gave a thought to what might happen when I got older. Even when my mum started wearing bifocal eye glasses to be able to see clearly over both short and long distances, I didn't even think that one day, I might also need a lens with a dual prescription.
Last year, however, when I started to struggle to read fine print in bad light, I suddenly thought I should find out a little more about multifocal contact lenses, how they work and whether they would be suitable for me in later life.
And my first question was "How can a contact lens allow a person to see both short and long distances clearly if they require a different prescription for clear vision?"
How multifocal lenses work is a fascinating topic. Here's what I found out when I did some research.
First, there are different types of bifocal or multifocal lenses, both soft and gas permeable options. A bifocal lens is where there are two different prescriptions within the lens and a multifocal lens means there are either two or more prescriptions.
The way they work seems almost unbelievable, but some force your eyes to alternate between two different powers (these are called "alternating vision" contact lenses) and others require you to look through different powers at the same time. I must admit that this type of lens, called a "simultaneous vision" contact lens, sounds completely unworkable...
It seems that once the eye gets used to looking through a lens with two different powers, it "learns" which power to actually use depending on whether you are looking at a short or far distance.
In order to understand how they work a little better, let's take a look at the types of multifocal contact lenses in more detail:
Alternating vision contacts are bifocal lenses with two powers where one is at the top of the lens and the other at the bottom, with a distinct line in the middle of the lens. It's a bit like looking through bifocal glasses where you look downwards to read small print.
For this to work, the lens needs to stay perfectly in place and, as soft contacts tend to rotate in the eye, most alternating vision bifocal contacts on the market are rigid gas permeable lenses.
Aspheric multifocal contact lenses are the most popular soft bifocal lenses among wearers right now. They have the near power in the middle of the lens and the distance power towards the outside and they work as the eye learns to "look" through the right power for the appropriate target.
Some people can't get used to this type of lens so an alternative is to prescribe the near power in one eye and the distance power in the other eye.
It still all sounds pretty complicated to me and what's clear is that what works for some, won't work for all. So, if and when I get round to needing contact lenses with different powers in old-age, it's certain that it will probably take a little trial and error to find the right bifocal contact lens for my needs.