Your optician will tell you that eyes work similarly to a camera. Light passes through the Cornea into the Pupil, which can change its optical aperture to regulate the amount of light entering the eye. 

Light then passes through the optical lens which focuses the light onto the Retina at the back of the eye. The Retina transforms light into electrical impulses which are interpreted by the brain into the optical images you see. 

Short- sightedness or Myopia

If light admitted to the eye focuses short of the Retina, then objects can only be clearly seen at close range. 

To correct the optical condition called Myopia a minus spectacle lens is used. This is thicker at the edge that in the centre and alters the path of light to focus it correctly within the eye. 

Long - sightedness or Hyperopia 

Hyperopic eyes try to focus the light behind the Retina, which results in nearer images appearing blurred. 

Hyperopia can be corrected with plus spectacle lenses which are thicker in the centre than at the edge and alter the path of light to focus it correctly on the Retina. 

Presbyopia

In middle age the small ocular muscles in the eye, which help the lens to focus on near objects, begin to lose efficiency. 

This is called Presbyopia and is similar in effect to Hyperopia. The optician can correct this by adding plus power to any distance correction. 

Astigmatism

Most eyes are spherical in shape (like a table-tennis ball), but in some cases they are slightly flattened (similar to a rugby ball). In these cases light does not focus evenly onto the Retina. This optical condition is known as Astigmatism. 

Optical astigmatism can be present by itself or in combination with Myopia or Hyperopia and is corrected by the inclusion of a cylindrical surface in the spectacle lens.