How Can Multifocal Lenses Show Both Near and Far?
You are reading a message on your phone, then you look up at a sign across the street. With progressive glasses you would read through the lower part of the lens and see the sign through the upper part, but a contact lens sits on your eye and moves with your gaze. So how can one lens, always in the same position, show you both far and near?
Most multifocal lenses send distance light and near light to the retina at the same time, so you see a focused image with a blurred image laid over it. The lens does not change itself to suit the distance; it offers both answers at once.
This article covers optical principles only. The adaptation process and prescriptions for first-time wearers are set out in our article on first multifocal lenses in your 40s.
Why it is hard to divide a lens into top and bottom
Multifocal lenses work in two broad ways. According to the summary in a 2020 academic review, one is the alternating approach, in which the lens moves when you look down so that you look through a zone of different power. The other is simultaneous vision, which sends images from several distances into the eye at once.
The alternating approach needs the lower eyelid to hold the lens up while the eye alone turns downwards. It is used mainly in rigid lenses. The British Contact Lens Association's 2024 presbyopia report, BCLA CLEAR, explains that among current designs this is the only approach that gives a complete single focus for each of distance and near.
Soft lenses fit closely to the eye, which makes this movement hard to achieve, and the eyelids also lose tension with age. That is why, the report sums up, soft alternating lenses never came into wide use.
Looking at two photographs laid over each other
The path soft multifocals took is simultaneous vision. In its simplest form, two zones of different power sit together in front of the pupil.
Think about looking at a distant sign. Light passing through the distance zone comes to a sharp focus on the retina, while light passing through the near zone misses its focus and spreads. When you look at your phone, the roles reverse. It is like a sharp photograph with a blurry one laid semi-transparently over it.
The report describes this distribution of light as both the reason multifocals work and their greatest weakness, because blurred light laid over a sharp image lowers contrast. The reduced vision in dim places, ghosting and halos that wearers describe also come from this.
Which distance gets the centre?
Designs divide according to how the zones are arranged. With the near power in the centre of the lens, it is a centre-near design; with the distance power there, a centre-distance design. A 2017 study in an ophthalmology journal also looked at these two separately.
According to the report, most contact lenses prescribed for presbyopia today are centre-near designs. Many include a section that joins the two powers gradually rather than breaking between them, so in theory intermediate distances can also be seen to some extent. That is why the name “multifocal” is used instead of “bifocal”.
In centre-distance designs, the near power grows stronger towards the outside, and they are prescribed less often. When light spread by the outer zone forms a ring, it can appear around oncoming headlights at night, like the halo around a moon.
The pupil changes the lens's proportions
The pupil is the window that lets light in. When the window changes size, so does the question of which zone of the lens carries the most light. In the report's example of a centre-near bifocal, distance and near light are split roughly 50 to 50 when the pupil is large, but this shifts to about 25 to 75 when the pupil becomes small.
When the pupil becomes smaller than the near zone, the lens effectively works like a lens with a single power. Pupil size varies not only with brightness but also with viewing distance, age and emotional state.
The 2017 study mentioned above also wrote that a lens's power distribution varies with pupil size and from person to person. This is one reason two people wearing the same lens can have different experiences. How night vision differs from daytime vision continues in our article on night-time blur.
Other attempts to lessen the pupil's influence
Multizone designs, with several concentric rings, try to reduce the effect of changes in pupil size by increasing the number of zones.
There were also diffractive lenses, which use the wave nature of light to create two focal points. In theory they are less affected by pupil size, but they can lose up to 20% of incoming light, and no presbyopia product was on the market when the report was written.
Extended depth of focus designs aim to give usable vision across a continuous range of distances rather than at two distances.
Does the brain really erase the blurred image?
It is often said that the brain picks out the image it needs. The 2017 study also wrote that the brain must choose the sharp image, and that failing to suppress the blurred one lowers contrast. But this area is less certain than you might think.
The BCLA CLEAR report judges that the evidence for adapting to simultaneous vision is not strong. Some results show people becoming more aware of ghosting or halos in the first week of wear, and it says it is uncertain whether that trend later reverses. The same report's chapter on rigid lenses says neural adaptation is required, so the weight of the explanation differs even within one document.
There are clues from the laboratory. A 2014 study showed 4 participants footage imitating the blur of simultaneous vision for 60 seconds and found that what looked in focus to them shifted. This was, however, a brief test with a device rather than real lenses. That the brain responds is one thing; that it erases the blurred image in lenses worn every day is another, and the two should be read separately.
The compromise in a single lens
A multifocal lens is a design that shares light between far and near. How much goes to each depends on the lens design, the pupil and the characteristics of the eye. That is why the report wrote that several designs may need to be tried and that vision chart numbers alone make it hard to tell whether a lens will succeed.
If your vision changes suddenly or discomfort continues, it is best to check with an ophthalmologist or optometrist.
Next time you move from a bright window to a dim corridor and have a chance to look in a mirror, take a look at the size of your dark pupil. The window that decides how much light comes through each zone of the lens is moving like that all the time.