Many falls could be prevented if older people do not wear varifocal glasses while out and about, a study in the British Medical Journal suggests.
Falls are a leading cause of death and disability in the elderly, but the research says 40% could be prevented if single-lens glasses were worn outside.
Varifocal lenses are widely prescribed as they correct problems seeing both near and far in one pair of glasses.
But the Australian study says they may create problems impairing balance.
This risk of falling over while wearing varifocals, also called multifocals or progressives, has been studied before, particularly when going up and downstairs or walking outside.
But this risk has to be offset against their benefits when it comes to carrying out tasks that require changes in focal lengths, like driving, shopping and cooking.
Researchers from Sydney, Australia recruited more than 600 people aged either over 80 or over 65 with a history of falls. All of them used varifocals and not single lens glasses, and wore them outside at least three times a week.
Half were given single lens glasses for going out, the other half were not. In all, the first group had 8% fewer falls than the second. For those who regularly went outside, the difference was significant - with 40% fewer incidents.
Based on the findings, the authors recommend that older people who are active outside should be advised to switch between single and varifocal glasses - despite the inconvenience.
However for those who spend most of their time indoors varifocals remain the best option.
The UK's College of Optometrists welcomed the study, but stressed some people did find it frustrating to have two pairs of glasses.
"However, everyone is different and we are pleased that the report highlights another factor that can be taken into account when deciding which type of lens is best for the individual person's needs," said the college's advisor, Dr Susan Blakeney.
"It is not just a 'one size fits all' approach to spectacle lens selection."