If you have 20/20 eye sight, it does not guarantee you have a vision that is perfect. This is because reading a chart containing letters and numerals in different light conditions is just a specific task, whereas our eyes have to perform many different and challenging tasks in our daily lives. There is a term called binocular vision that refers to the fact of our eyes working as a team to make sure we see clearly in all conditions.
Bad eyesight, or blurred vision, is most commonly caused by a refractive error like nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. Refractive errors develop when the eye is unable to focus light directly on the retina. This inability to focus stems from imperfections in the eye’s length, corneal curvature and/or internal lens curvature. The refractive error you experience depends on which anatomical flaw your eyes have.
For example, eye length is normally associated with farsightedness, corneal curvature is linked to astigmatism and lens curvature can determine nearsightedness or farsightedness.
Amblyopia (lazy eye) — caused by a lack of communication between the eyes and the brain — is another condition that can cause poor eyesight. The brain is supposed to receive visual signals from both eyes, but in young patients with amblyopia, the brain ignores signals from one of the eyes. If caught early, amblyopia can be treated very successfully, but if left untreated, it can cause permanent vision loss. Presbyopia: Experiencing blurred vision after age 40 is often an indication of presbyopia, or age-related farsightedness. As your eyes age, the lenses that used to focus on up-close images become stiff, making it difficult to adapt and causing your near vision to blur.