In short sight the eye has grown to be relatively too long so that rays of light are focused in front of the retina. Distance vision is therefore blurred while near vision remains good.
The unit of measurement is the dioptre (D) and will appear as the sphere (sph) on your prescription with a minus sign. Short sight up to -3.00D may be regarded as low, between -3.00D and -6.00D as medium and over -6.00D as high. In extreme cases short sight may be as high as -20.00D.
Illustrations © and reproduction courtesy Simon Levy.