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

What about astigmatism?

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
    ADD: 2.00

Presbyopiais a condition that causes your crystalline lens to lose some of its elasticity. This makes near work much more difficult and requires the use of additional power to allow the wearer to read and view objects up close. Often a patient will look at their prescription and think that 2.00 is the reading prescription. To their surprise when they purchase over the counter readers they just don't work. That is because the 2.00 must be added to the distance prescription. The add power is always a plus power and when added to the prescription above, which is anegative number, we get -1.50 -1.00 x 180 for the right eye, and -2.00 -1.25 x 170 for the left. This is the patient's actual reading prescription. It's easy to see why the +2.00 readers would not work.

At times a person's eyes may not work well together. In such an instance, something called prism may be prescribed. Prism actually displaces the image towards its apex. Prism is prescribed in what is termed in base up, base down, base in and base out or a combination like base in and base up. If you imagine a lens made from two prisms one stacked base to base (plus lens) and one apex to apex (a minus lens) you can get a better picture of what is meant by moving the base of the lens in or out. Often a prescription with prism looks like the example below:

    O.D. -3.50 -1.00 x 180 1Δ B.U.
    O.S. -4.00 -1.25 x 170 2Δ B.U. 1Δ B.I.

The triangle placed after the number is the Greek letter Delta which stands for diopter a unit of measurement that is also used to measure the amount of prism needed for the lens. The initials stand for Base Up (BU), Base In (BI), Base Down (BD), Base Out (BO). In the case of the left eye we have a combined prism.

Understanding your vision

The eye is a very complicated system. So are the prescriptions needed for visual correction. The above examples are simplified to help the average patient understand what their prescriptions might mean. To understand in detail what is going on with your eyes you should discuss this with your doctor.