The power of contacts can be calculated from that of glasses.Here is a formula :Fo=Fs/(1-dFs).The Fo means your power of your contacts and Fs means the power of glasses.dFs means the distance between rear vertex of glasses and your cornea.Usually the dFs should be measured by meter it is 0.012m for easterners.For example,the RX of glasses is -4.00,the dFs is 0.012m,we can know that your RX for contacts should be -3.82,but this result is not a regular RX,so we choose the proximal and a lower number , it should be -3.75 .
