The eye, operates very much like a regular camera. Light is focused by the cornea and lens, at the front of the eye and the image falls on the retina at the back of the eye, which acts like a film in the camera.
Deviations are referred to as ' Refractive Errors', and the vast majority of these can be corrected with spectacles, contact lenses or surgery.
Your vision is normal if you can see objects at distance and near, clearly. Light entering your eye is focusing on the retina, as it should. This is also referred to as uncorrected 20:20 or 6:6 vision, during an eye test.
Short Sighted (myopia)
This is the name given when the image does not reach the retina and actually falls short of it ( or in front of it). This happens when the eye is too long for the refractive power of the cornea and the lens.
A person who is short sighted would be able to see objects at close range, but distant and even mid range objects would be blurred.
Long Sighted ( Hyperopia or Hypermetropia)
If the eye is too short for the refractive power of the cornea and lens, the image produced will be focused beyond the retina. The result of this means that the person will be able to see distances clearly, but close up images will be blurred.
Astigmatism is caused by irregularities in the curvature of the parts of the eye responsible for focusing light rays ( the cornea and the lens). An even curve ( a round eye) results in even distribution of the light rays making up an image which will be clear. If the curvature of the eye is uneven ( rugby ball shaped), light rays will be unevenly distributed, causing blurred images at distance and at near.
From aged 40+ the muscles responsible for the movement of the lens within the eye, (that allow it to change focus), weaken. This results in the need for reading glasses for most people. Long-sighted people may be affected more quickly than short-sighted people who may benefit for a short period by being able to read without spectacles.