A tool used in optometry and ophthalmology facilitates the change of a prescription written in minus cylinder form to its equivalent in plus cylinder form. This conversion involves altering the sphere power, cylinder power, and axis of the cylindrical lens. For example, a prescription of -2.00 -1.00 x 180 can be converted to -3.00 +1.00 x 90. The initial sphere power is algebraically added to the cylinder power to find the new sphere power. The sign of the cylinder is changed, and the axis is rotated 90 degrees.
This calculation is important because different practitioners and optical labs may prefer to work with prescriptions written in one cylinder form over the other. Benefits include easier lens manufacturing in some cases, better adaptation for certain patients, and improved compatibility with specific lens designs or equipment. Historically, this conversion has been done manually, but automated tools improve accuracy and efficiency.
