There are two ways of expressing a prescription, so don’t panic! There is a quick way to find out whether or not you have been given the same prescription in its two different forms, this process is known as transposition.There are several mathematical rules to transposition, however, in general in order to convert one from the other you should do the following;SPH + CYL = SPH of the other prescriptionBoth CYLs should be of the same value but one is negative while the other is positiveFor 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 original is 30 you add 90 to get 120; if original is 140 you subtract 90 to get 50.If the results of your conversion are not the same please return to your optician to establish which of the two prescriptions is the one you should use.