Refractive errors are the most common vision problems characterized by the reduction of the visual acuity. There are different types of refractive errors:
Nearsightedness (myopia) is when the image seen far away is not clear.
Farsightedness (hyperopia) is when the image seen at near is not clear. Some young people may not notice any problems with their vision until the natural lens inside the eye starts to lose its ability to accommodate (change its shape in order to focus on the object).
Astigmatism is a when the image is not focused evenly. This is caused by an oval shape of the cornea.
Presbyopia is when the image up close becomes more difficult to see due to the age-related changes in the natural lens.