Dioptre

The dioptre or diopter is a unit for measuring lens power, A lens with a higher dioptre will make parallel light focus closer to the lens. Lenses can bulge in the middle (they are convex) or can be thinner in the middle (they are concave). Convex lenses have dioptre numbers bigger than zero - a positive dioptre. Concave lenses have dioptre numbers less than zero - a negative dioptre.

Optical power in dioptres and focal length in metres.

Dioptre of the lens is measured as one divided by the focal length of the lens, in Metres. So if the focal length was half a metre, 50cm, the dioptre would be 2. If the focal length was 20 cm, the dioptre would be 5. If the focal length was 2 meres, the dioptre would be 0.5.

Dioptre is often used to describe corrective lens power. Thicker glasses will typically have a bigger dioptre number. If the glasses are to help with seeing close things, then the dioptre number will be bigger than zero. If the glasses are to help with seeing things far away, then the dioptre number will be negative.