Vision

Vision begins when light rays enter the eye through the cornea, the first transparent tissue.

Then, the rays pass through the pupil whose size varies according to the quantity of light that enters the eye.

The light rays then pass through the crystalline lens, which, by changing its shape, focuses the light rays on the retina.

The information then travels to the brain via the optic nerve, for interpretation.

Vision
Myopia (short-sightedness)
Short-sighted people can see close up but objects at a distance are blurred.
This is because the image perceived by the eye is no longer projected perfectly on the retina but in front of it.
Hypermetropia (long-sightedness)
Long-sighted people can see objects at a distance but those close up are blurred.
This is because the image perceived by the eye is no longer projected perfectly on the retina but behind it.
Astigmatism
People with astigmatism have distorted vision at all distances. 
This condition is due to an abnormal curve of the cornea and/or crystalline lens. 
Presbyopia (age-related long-sightedness)
People with presbyopia have blurred near vision (for reading in particular) as, with time, the crystalline lens 
loses its elasticity and its ability to focus on images perceived.