There are two ways of expressing a prescription so don't panic.

There's a quick way to find out whether or not you've been given the same prescription in two forms - it's known as transposition.

To convert one from the other you should do the following:

• SPH + CYL = SPH of the other prescription
• Both CYLs should be of the same value but one is negative while the other is positive
• For AXIS, add or subtract 90 degrees to the original axis value to obtain the new value which must be between 0 and 180 – for example if the original is 30 you add 90 to get 120; if the original is 140 you subtract 90 to get 50.

If the results of your conversion don't match, your optician will be able to tell you which of the two prescriptions to use.