The autorefractor is a machine which measures the ability of your eyes to focus and gives an approximation of your prescription.

You will be asked to stare into a machine through two lenses at a picture, for example of a hot air balloon at the end of a long straight road, and focus on the picture. The machine makes the balloon appear to be brought closer and further away.

As it does this the machine calculates an estimation of your prescription needs with the results of how well your eyes focus on the image.