Refractive Errors

Emmetropia:  In an emmetropic eye, parallel rays of light focus at a point on the retina (the center of macula, called Fovea).

Near sightedness Or Myopia:  In this condition, parallel rays of light focus in front of the retina. Close objects are seen clearly, but objects farther away are blurred. Most of the times there is an environmental factor like close working distance or sustained near vision tasks could potentially induce Myopia. Apart from environmental factor there is hereditary factor involved, especially high myopia. 

Farsightedness or Hyperopia:  In this condition where the parallel rays of light would focus behind the retina. In which distant objects are seen clearly most of the times, but close objects are blurred. 

Hyperopia is often related to short axial length (From the front part of the eye to back of the eye). Hyperopia is also caused by less refractive dioptric power of the eye (Flat cornea) or under developed eye.

Astigmatism:  In this condition the parallel rays of light are not focused in one point instead they are dragged in a line or stretched could be in front of the retina (Myopic astigmatism) or behind the retina (Hyperopic Astigmatism) or dragged between in front of the retina and behind the retina (Mixed Astigmatism). The word Astigmatism literally means "lack of focus on a specific point".

 
Presbyopia:  Is a refractive condition where the eye muscles are unable to focus on a near object due to lack or inability to accommodate. This condition occurs secondary to hardening of lens capsule which in turn makes it hard to focus on a near object.