To have a sharp sight rays reaching the eye should be refracted and focused on the retina. Refraction is accomplished in corneal and lenticular layers of they eye. Eyes in which rays are focused on the retina are called normal/emmetropic eyes.
In cases where rays are focused not on the retina causes complaints like blurred/deformed sight, which has 2 causes:
1. too long/short anteroposterior length of the eye
2. refrective indexes of the corneal/lenticular layers out of the normal range