Diabetes is multi organ disease which mainly affects the small blood vessels supplying nutrition to the organ and eventually damaging it.
In the eye diabetes mainly affects the back part of the eye -the Retina. This in the initial stages leads to blurring of vision & if neglected leads to permanent loss of vision. So if you are a Diabetic for some years any blurring of distant or near vision,black spots in front of the eye & flashes of light should not be neglected.