In vari-focals, the lower part of the glass contains the lens for reading and the upper part contains the lens for long distance. In vari-focals the two lenses merge across the glass without the distinct line which we see in bi-focals.
The problem arises when we are up on our feet and need to look down to the ground, the best example is when going down the stairs. As we look down, our vision looks through the lens for near vision/reading. This distorts our depth perception and the judgement of where we place our feet, increasing our risk of falls.
As we have seen from the discussions, many people use vari-focals with no problems at all, but other people have difficulty adjusting to them. We are not asking everyone to stop using their vari- or bi-focals if they like to use them. What we hope for, is that people can be aware of the risk. For people who are comfortable using their vari-focals, have an awareness that if falls do become troublesome in the future you could consider changing to single lenses. The biggest risk is for people who use them outdoors. For people who are considering using vari-focals, feel confident enough to ask your optician what all of your options are.
Vari- and bi-focal lenses are a well recognised risk for falls among experts. This is based on the results of well conducted clinical trials, small observational studies, clinical experience and expert opinion. But it remains up to each and every individual to decide for themselves whether to use them, or use separate glasses for distance and near vision.
If you would like to read more about this and make up your own mind, you may find the following links interesting:
A paper from the British Medical Journal looking at falls: BMJ 2010; 340 doi: (Published 25 May 2010, accessed 3 December 2014)