Spherical error, or the overall lens power, is the basic part of your prescription. You’ll notice (+) and (-) signs on your prescription. When there’s a (+), it means you have trouble seeing near, and when there’s a (-), it means you have trouble seeing far away.

A cylinder value indicates astigmatisms, or perpetually blurry vision. The axis — measured between 1° and 180°— tells us the orientation of the astigmatism.

The ADD number is sometimes written NV for near vision, and is used for multifocal lenses and readers.

Prism correction is used to help correct diplopia, or double vision. The prism bends light to help align the vision so only a single image is seen.

Vertical prism correction is used to help correct vertical eye misalignments, and is prescribed in opposite directions (upward in one eye, and downward in the other).

Horizontal prism correction is used to help correct lateral eye misalignments, and is always prescribed in the same direction for both eyes.