Question: What’s the difference between plus and minus cylinder?

Answer: Glasses are made to correct a certain amount of spherical error. However, people also have a certain amount of astigmatism that also needs to be corrected. Astigmatism is when the surface of the cornea is not perfectly round like a basketball, but more round like a football. The extra steepness along one axis needs to be corrected with a cylinder.

The cylinder correction can either be made with a positive convex lens or a negative convex lens, the end result is the same.