Guide:Reading glasses prescriptions
How do you read these complicated things!? Believe it or not, they're actually pretty simple to understand.
Cylinder is the additional correction in a specific direction of your eye. It is measured in diopters. Think of a straight line going across the surface of your eye at a certain degree. The degree this happens at is your axis.
Cylinder correction means your eye has some level of astigmatism.
Since cylinder correction is in addition to spherical correction, what the cylinder correction does depends on the spherical correction:
|Minus Sphere||Plus Sphere|
|Minus Cyl||Increases myopia correction||Decreases presbyopia correction|
|Plus Cyl||Decreases myopia correction||Increases presbyopia correction|
Axis is the degree at which cylinder values are relevant. The correct cylinder value but the wrong axis value would make the correction pretty worthless! When reducing your correction, you should never ever change the axis, especially if you've had that value of axis for a long time.
If you don't have any cylinder correction, you will not have an axis value either.
Prism is very rare on a prescription. It is effectively is an imbalance between how your two eyes see. It has been described as permanent double vision.