            Reading your RX for your Prescription Snorkel Mask Finding the right lenses for your mask Examples of Optical Prescriptions Sample 1 RX SPH CYL AXIS DISTANCE VISION D.V. O.S. (RIGHT) -3.50 .25 O.D. (LEFT) -.25 -1.00 NEAR VISION N.V O.S. (RIGHT) O.D. (LEFT)  This Rx is a (DV) Distance reading only.  Looking at the Right (OS) the spherical reading of -3.50 and the cylinder reading of -.25 we would calculate with -3.5 plus one-half of the cylinder -.25 equals to -3.63 or rounding down it would be -3.5 diopter. Now, Looking at the left (OS) spherical reading -.025 and the cylinder reading of -1.00 we would calculate with -0.25 plus one-half -1.00 to equal to -.75 and rounding down to a plano  (zero strength) lens because half step diopter lenses start at -1.0.    Lenses needed for Sample 1  -     Right Lens -3.5 and Left Clear or Plano Lens

 Sample 2 RX SPH CYL AXIS DISTANCE VISION D.V. O.S. (RIGHT) -4.50 .25 O.D. (LEFT) -2.0 -1.00 NEAR VISION N.V O.S. (RIGHT) +2.0 O.D. (LEFT) +2.0  This Rx is  (DV) Distance reading and (NV)  Near Sighted. Looking at the Right (OS) the spherical reading of -4.50 and the cylinder reading of -.25 we would calculate with -4.5 plus one-half of the cylinder -.25 equals to -4.63 or rounding down it would be -4.5 diopter. Now, Looking at the left (OS) spherical reading -2.00 and the cylinder reading of -1.00 we would calculate with -2.00 plus one-half -1.00 to equal to -2.50 and use -2.5 diopter The Near Vision reading is a +2.0 in both eyes thus the bifocal or reading lense would be the plus 2.0  Lenses needed for Sample 2   -Right Lens -4.5 and Left -2.5 Lens for a snorkeling mask Custom ground lenses to accommodate both near and far would be needed for a scuba mask to see gauges.