@nickjb I genuinely don’t know but this off the interweb
“Instead, many buy several cheap versions to stash around the house so that they can always find a pair.A researcher at consumer champion Which? checked 14 pairs from seven high street chains.
He found problems with half of them, with those carrying a higher prescription – +3.5 to +4 – considered to cause the most concern. ‘Off-the-peg glasses could cause eye strain, blurred vision, headaches or double vision,’ the Which? researcher said.
‘For people with higher prescriptions, they’re not suitable for walking or other mobile activities.’ They could even ‘cause a nasty accident’, he warned.
The biggest problem is that the centre point of the two lenses might not be aligned.
This means the sight in one eye might be clear while the other is blurred. This was the case in a pair from Poundland – which also had a prescription strength that differed from the +3.5 on the pack – and a £16 pair from fashion eyewear store Sight Station.”