Your Prescription explained

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The prescription seen here is for illustration purposes and may look different to the one you are given following your eye examination.

• SPH - this stands for "sphere" and is the most basic aspect of your prescription. If you need glasses there will always be a value for the sphere. if it is negative then you are shortsighted whereas if it is positive you are longsighted. If it says "Plano" then no lens is needed.
• CYL - this stands for "cylinder" and is the astigmatism reading. It may not always be present as not everybody has astigmatism. You may just have a spherical reading in which case you may see a "DS" following the "SPH" figure, this means Diopters Sphere. If there is no CYL reading then there will be no AXIS reading either.
• AXIS - this is the orientation measurement of the cylinder power. Imagine a protractor from school marked up from 0-180 degrees the axis is the position that the astigmatism power has to be orientated at. Here the -1.50 lens must act at 145 degrees and oly at 145 degrees (that's about 10 o'clock on a clock face). If the CYL orientation is shifted but the power kept the same your vision will still blur significantly, that's why contact lens fitting can be more complicated in astigmatism patients.
• PRISM - this is a lens only used if you have difficulty using both eyes together, it can help with eyestrain and double vision. It is not often required.
• BASE - this is the orientation of the prism lens needed; a little like the axis above. There are only four possible positions, however, UP, DOWN, IN or OUT. Often the prism is spread between the two eyes.

Positive & Negative Cyl Prescriptions

The prescription can be written in two entirley different ways but still mean exactly the same. In the example above the prescription for each eye is actually exactly the same. This is because the "CYL" is demarcated by a positive sign in the right eye and a negative sign in the left eye. Different Optometrists will write the prescription down in different forms so they can look vastly different between eye examinations when in fact the prescription is fairly similar.

The rule is:
                          sph + cyl = new sph
                          swap sign of cyl
                          add / subtract 90 degrees from axis
                          remember axis is never higher than 180
                          if axis is above 90 degs then subtract 90

Example as above:

     Right Eye (-1.00) + (-1.50) = -2.50
                      swap cyl from -1.50 to +1.50
                      subtract 90 fro 145 axis = 55

      New prescription = -2.50 / +1.50 x 55

Remember negative adding negative = more negative and negative adding positive = more positive!

The prescription can be written in either positive or negative cyl forms it just depends on how your optometrist has decided to perform your eye examination. It should, however, have the same cylinder for each eye and not different for each eye as seen above.

Additions

An addition is the power needed for your reading glasses, if needed. Sometimes they will be written as above and sometimes they will be written as a prescription, depending on how the optometrist prefers. An addition is just that, you add the lens power to the "sph" on the "base prescription" which is that written above, the prescription required for distance. 

In this example the reading prescription would be:

                          RE: +1.50 / -1.50 x 145          LE: Plano / +1.50 x 55 (as seen above)

In certain circumstances you may also have an "intermediate" prescription, this is because you have told your optometrist that you have specific close tasks which require a slightly different addition lens because they are done at a slightly different distance than reading., this will include computer use and craft work. If you require a shorter working distance then the addition will be stronger than that for reading and if you need to see slightly further away, for a computer screen perhaps, then the addition will be weaker that that for reading. Again the addition could be written as a simple top-up lens as above or as a prescription. The abbreviation "BE" stands for "Both Eyes".