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?
I know that most errors with reractive results are due to aberrations in the shape of the eyeball. The shape of the cornea can also come into play as well as the reduced lens flexibility.