This test, called refraction, involves placing different lenses before your eyes to check your eyesight from your own viewpoint.

There might be a slight difference between the prescription that is estimated using the instruments and the prescription that you find most comfortable to wear. You will be asked to decide between a choice of two lenses or to say whether a lens makes what you see better or worse.

Many people feel that they have to make a choice, even if they can't see a difference, or that they'll get the answers wrong, but in fact there is no right or wrong. However it is important that you tell your optician if you are unable to tell the difference as this means something to them as well.