Talking about my eyeglass prescription?if the sphere value is -6.50 and the cylinder value is -0.50, then my prescription strength is -7.00 or -6.50?

Glasses half full vs: Are the glasses half full vs half empty... as the old saying sort of goes. It depends on how one wants to look at it ($6 plus 50 cents is the same as $7 minus 50 cents). Likewise, -7.00 diopters plus 0.50 diopters in one axis (direction) would be the same as -6.50 diopters minus 0.50 diopters in the perpendicular axis (opposite direction).