the idea is that they put lenses into the machine near your eyes until you can see correctly.....therefore if they duplicate those lenses in your spectacles it should correct your vision..simples...
I knew someone who went to an opticians a year after being prescribed glasses. Her glasses were OK but they insisted she had another pair on the second visit.
Don't you get the point Chrissa? Of course your eyesight deteriorates but they are in the business of making money and thats by selling more specs sometimes, but not always, unnecessarily.
One is under no obligation to buy any glasses. The sight test gives a definitive answer to the level of correction required and the change since the previous sight test. Just say 'no' to the question 'which pair of frames would you like'. At long last there is a faint smattering of competition in this market; about time too.
I have bought 4 pairs over the last few years. I each case the prescription has differed. However an otician in Turkey had a look and told me that all were close enough not to bother changing!