In myopia, rays of light from a distant object are focused in front of the retina. This is either because the eye is too long (from ‘front to back’) or the cornea (the front window of the eye) is too steeply curved and hence has too much focusing power (or a combination of these 2 factors).
A negative or minus spectacle or contact lens is required to correct this, i.e. to cancel out this excess of positive focusing power. When not wearing spectacles a short-sighted person will have blurred distance vision. The greater the amount of myopia the more blurred is their view of the world. Even a relatively modest amount of myopia will cause a considerable blurring of distant objects.
For example, a person with minus 3 dioptres of myopia would have great difficulty recognising even the largest letter on the standard optometrists’ chart. For myopic individuals there is a point however at which objects are seen clearly. This will be at a relatively short distance in front of the eye (i.e. ‘short-sight’) and the exact distance depends on the amount of myopia present. For example, in the case of someone with minus 3 dioptres of myopia the point of sharpest focus is 1/3 of a metre (12 inches) in front of the eye. In the case of a highly myopic individual of minus 10 dioptres this point would be only 10cm away. This can sometimes be an advantage since without spectacles even small print will be visible at this 10cm point. Distance vision however would then be extremely blurred.