Presbyopia tests: How is PRESBYOND blended vision different from monovision?
How is PRESBYOND blended vision different from monovision? I hear this all the time at ophthalmic conferences. Monovision is something that’s been used commonly for many years in contact lenses, following cataract surgery and laser eye surgery.
Monovision is asking each eye to do a different job for a patient who has presbyopia. This is the older age group of patients who have lost their capacity to accommodate their ability to shift focus from distance to near. What we do is set one eye up, usually their dominant eye, for good distance vision and then their non-dominant eye for reading vision. To have a crisp distance focus, we need a zero prescription in the dominant eye, and for really crisp focus at say 50 centimetres away, we need a prescription of minus two short-sightedness. So with monovision, the patient will have two distinct points of focus with a big gap in between. Their brain has to try to integrate these two images.
Not everyone can do this. Only about a third to forty percent of people can perform that task. Even if they can, there are some issues with maintaining good depth perception and good 3D vision.
It can be degraded by having such a big gap between the image quality of distance and near for each eye.
PRESBYOND blended vision takes this principle of asking each eye to do a different job, but links the two eyes up, by using a controlled amount of an imperfection in the optical system. Spherical aberration is an imperfection that our visual system is used to dealing with.
When we accommodate as young people, when we’re taking our focus from distance to near, we’re increasing positive spherical aberration. That’s part of how we create the near focus. So our brains know how to tune spherical aberration. If you can picture positive spherical aberration on a lens and how it’s working, it’s as if the light rays at the edge of the lens of being refracted and bend more powerfully than the light rays in the middle. It just creates this little cone of confusion or his area where the light rays aren’t all intersecting at the same point. But our brains are quite good at dealing with a little bit of this and retaining the information.
What it does, is increase the distance over which an image is in focus. So if you increase the spherical aberration of the distance eye, in a motivation type setup and increase the depth of vision on the near eye, you can create a situation where they overlap because you don’t have to pull the reading eye back so far to attain useful near version. This spherical aberration will reach back for you.
- Firstly it means that these patients have much better depth perception. They’ve got much better 3D vision because each eye is bringing something to the party, those intermediate distances.
- Secondly, it means that the acceptance of this optical set is much higher. It goes up to 97%. The vast majority of us can take this and accept it.
There is a process of adaptation and again this different for everybody. For some, it’s almost like putting on a pair of comfortable slippers, and it feels very natural. For others, it can take three, six or even nine months to feel comfortable. They’ll have moments where they’re just not quite sure which eye is doing what, but the brain will get there. It’ll blend them and become more and more seamless at shifting focus across the distances across the two eyes.
They are fundamentally different approaches. Even though they share the principle of monovision: distance eye, near-eye, PRESBYOND blended vision: distance eye, near-eye, the controlled amounts of spherical aberration allows the link up an overlap in the PRESBYOND blended vision that is different, and that makes it much more acceptable and more effective.
Now I know that’s been a long and quite technical explanation of the difference between monovision and PRESBYOND blended vision. Essentially, with monovision, each eye is doing an entirely different job, and we’re asking people to cope with it. With blended vision, we are joining the two eyes up, and they’re working together in concert to create a nice seamless view of the world.