What defines atorics in terms of meridian flattening?

• A. Different rate of flattening in each meridian that's independent of the other meridian's flattening
• B. Identical flattening in all meridians
• C. Flattening only horizontal
• D. Flattening only vertical

Answer: (A)

Atorics are defined by independent, different rates of flattening in the two principal meridians. In other words, the amount one meridian is flattened can differ from the amount the perpendicular meridian is flattened, and these changes aren’t tied to each other. This creates a surface whose power varies between meridians. If flattening were identical in all directions, you’d have a spherical situation with no astigmatism; if flattening were limited to only one orientation (horizontal or vertical), that would not describe true atoric behavior. The key idea is the independent, unequal flattening of each principal meridian, which characterizes atorics.