Eyeglass Prescriptions Explained

So you just had your eye exam and before you leave you are handed a little card with a bunch of numbers. At some point you may have asked yourself 'I wonder what they mean?'. Here we will examine a sample prescription and discuss how refractive errors are measured.

Before we go any further we need to explain what a diopter is. In 1872 a french Ophthalmologist, Felix Monoyer, developed a new method of measuring the focal length of ophthalmic lenses called the diopter. One diopter equals a focal length of one meter. So a lens that is -4.00diopters has a focal length of 1/4 meter. The focal length of a lens is determined by this equation, 1/diopter = focal length. So now that weknow what form of measurement is used lets take a look at a sample prescription.