Long and Short Sight.
When your eyes are longsighted the object you are looking at in the distance may be clear but as it come closer to you it begins get to more of a stain to see and can be more blurred.
Lenses that correct this type of vision are called convex lenses. They have a shape that is thinner at the edge but thicker towards the centre of the lens.
The numbers written in the prescription will have a plus sign to denote this e.g. +2.00
When you eye are short sighted. The object that you are being in the distance blurred but as it becomes closer to you it become clearer and less of a stain to see. Lenses that correct short-sightedness are concave in shape and so thicker at the edges and thinner towards the centre.
The numbers written in the prescription will have a minus sign to denote this e.g. -2.00