Refractive errors of the eye include myopia, astigmatism, hypermetropia and presbyopia.
The characteristic of refractive errors is that the rays of light do not all end up together in the center of vision at the back of the eye as is normal, due to structural abnormalities in the optical media. Depending on the error, the rays may end up in front of the focal point that should be (myopia), behind the focal point that should be (hyperopia) or in 2 lines perpendicular to each other in front or behind the focal point that should be (astigmatism).
In presbyopia, the crystalline lens of the eye has difficulty zooming in when the person looks closely, i.e. it curves, due to hardening of its material after the age of 40, so the person cannot focus clearly on near objects.