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.

O.D. -3.50
O.S. -4.00

The first thing that we notice are the two initials in front of thenumbers. These designated which eye the prescription is for. O.D. stands for Ocular Dexter or right eye and O.S. stands for Ocular Sinister meaning left eye. More commonly you may see just the simple R for right and L for Left.

After the right/left eye designation you see a symbol that looks like eithera minus or plus symbol. A plus lens magnifies and a minus lens minify objects. A plus lens is used in the correction of hyperopia, a condition where light comes to a focus behind the retina, the plus power of a lens brings it to focus on the retina. A minus lens is used for the correction of myopia, a condition where light comes to a focus in front of the retina, the minus power of the lens brings light to a focus on the retina.

O.D. -3.50
O.S. -4.00

The numbers highlighted above are called sphere powers. This person has ano astigmatism, meaning that the shape of their cornea is the same in all meridians.

O.D. -3.50 -1.00 x 180
O.S. -4.00 -1.25 x 170

This next person has astigmatism. It’s correction is given in the highlighted numbers above. This means that the person has a cornea shaped more like a foot ball as opposed to a sphere. Therefore each meridian needs its correction. A football has two radii of curvature. One from point to point and the other from side to side. From point to point is a much larger radius of curvature resulting in a flatter curve. From side to side is a much smaller radius of curvature resulting in a steeper curve. An astigmatic eye will resemble these curves. For a lens to correct this it must have two meridians as well. This is were a toric or sphero-cylinder lens will be used.

The second highlighted numbers are called cylinder power. This cylinder power is the additional power needed for the correction in the second meridian. So in the above example the right eye will have -3.50 in the spherical meridian and additional -1.00 in the cylinder resulting a power of -4.50 for astigmatism correction.

To determine which meridian the cylinder power is in a 180 degree system is used, often referred to as the axis, the axis is distinguished from the other numbers by an x then the number. The typical circle (180?) after the number showing that the number is a degree is not used. In a poorly written RX that symbol could look like a zero resulting in an axis error. On occasion you may see the cylinder power with a plus symbol in front of it. This is called writing the prescription in ‘plus cyl’. It’s just another way of writing the same prescription. To change the prescription from plus cyl to minus cyl or visa versa you would need to do the following. In our example we will change the prescription to plus cyl.

1. Add the sphere and cyl power.
2. Change the symbol from – to + for the cyl power.
3. If the axis is more than 90 subtract 90. If the axis is less than 90 add 90.

Now our sample prescription has become:

O.D. -4.50 +1.00 x 90
O.S. -5.25 +1.25 x 80

To change it back you do the reverse of the above steps.  The prescription has not changed, only the way it was written.

Changes over time

As some of you are aware when you get older you need a little help reading. At the bottom of your prescription you may see:

O.D. -3.50 -1.00x 180
O.S. -4.00 -1.25x 170