When selecting the spherical equivalent, which description best describes the choice?

• A. most minus or least plus
• B. most plus or most minus
• C. least minus or most plus
• D. least plus or least minus

Answer: (C)

The main idea is that the spherical equivalent is a single power that best represents a mixed sphero-cylindrical prescription by averaging the powers in the two principal meridians. It always lies between those two principal powers.

Because it is the average, it tends toward the less extreme end of the pair: toward the least minus when one meridian is more minus, or toward the most plus when the other meridian is more plus. In other words, the spherical equivalent sits toward the end of the range that is less negative or more positive, respectively. This is why the description fits: least minus or most plus.

Example: with a sphere of -1.00 and a cylinder of -2.00, the two principal powers are -1.00 and -3.00, so the spherical equivalent is -2.00, which lies toward the less minus end of the range. Example: with a sphere of +1.00 and a cylinder of +2.00, the two principal powers are +1.00 and +3.00, so the spherical equivalent is +2.00, which lies toward the less plus end.