The eye and refractive errors

The eye is a magnificent organ. Clear and colorful images are created in the retina of the eye with cooperation of the cornea, lens and pupil. The light that reaches the eye meets first cornea as it breaks and focuses towards the central part of the eye. Then it passes through the pupil where the size can change to adjust the amount of light that goes on into the eye. Finally, the light passes through the lens, there it is further focused and further refined before it lands on the retina. In the retina there are 3 color sensors along with high contrast black / white sensors activated by light, this image is then transmitted to the brain via the optic nerve.

Normal vision (Emmetropia)

If you see everything clearly: Normal vision (emmetropia)
Eye shape is normal and media is clear. The light is focused directly at one point in the central part of the retina of the ocular fundus. You see things well, both far and near.

FarVision

Nearsightedness

If you only see things that are near clearly (Myopia)
Either the eye is too long (large) or the cornea is too convex. Light is thus the focus at a point in front of the central part of the retina. So you can see clearly only when things are very near. Images in the distance are not in focus.
Treatment
The cornea is made flatter by laser treatment reduces the nearsightedness by moving the focus point to the central part, this mimics "shortening the eye".

Farsightedness

If you see poorly up close (Hyperopia)
Here is the eye either too short (small) or the cornea too flat with the result being light is focused at a point behind the retina. So things that are close to you are not in focus.
Treatment
With laser treatment of cornea is made convex and its steepness increased. It directs the image to the centerpoint, so that things near to you will be in focus.

Astagmatism

Here light rays are not focused at one point, but in two points. This will create a distortion in the image called visual error.
Treatment
The treatment for astigmatism is usually with laser treatment, the laser takes into account the deformation of the cornea and corrects so that its appropriately convex and will be the same in all 360 degrees of the cornea. It focuses light at a single point in the central part of the eye.