How to understand it
Keep the sign: minus and plus powers change light in different ways. Read SPH with CYL and AXIS when these are present.
An example
SPH −2.00 D and +2.00 D have the same numerical magnitude but are not interchangeable.
A common mix-up
Sphere in a prescription does not mean the manufactured lens must have a spherical surface design.
See the idea in a full guide →
Sources & verification
Page updated: