How does the eye work?
In a normal healthy eye, the image we see of an object (far away or up close) is focused clearly onto the retina at the back of the eye. The retina is multi-layered light sensitive tissue that captures light rays and converts them to electrical impulses. These impulses then travel along the optic nerve to the brain where they are turned into images.
When these images are not focused on the retina, the vision is blurred. This can be due to many types of visual defects, such as cataract, macular degeneration, long sightedness, shortsightedness, and many others.