I have been blessed with visual acuity which means I have pretty clear vision. 
Congrats ,that's good to know.
Many people who have have low visual acuity is because there is a refractive error in the refraction of light in their eyeballs.
How does refraction of light work in the eyeballs?
the shape of the eyeball or the cornea, and reduced flexibility of the lens. Too high or too low refractive error (in relation to the length of the eyeball) is the cause of nearsightedness (myopia) or farsightedness (hyperopia) (normal refractive status is referred to as emmetropia). Other optical causes are astigmatism or more complex corneal irregularities. These anomalies can mostly be corrected by optical means (such as eyeglasses, contact lenses, laser surgery, etc.).
Neural factors that limit acuity are located
in metres.
This is often measured with a Snellen chart or with a logMAR base Velorum Visual Acuity System.
Basic examination Refraction.
In physics, "refraction" is the mechanism that bends the path of light through the eye. In an eye exam, the term refraction is the determination of the ideal correction of refractive error. Refractive error is an optical abnormality in which the shape of the eye fails to bring light into sharp focus on the retina, resulting in blurred or distorted vision. Examples of refractive error