Vision is a worthy testament to the beauty and complexity of the human mind, but as with any other brain process it is explainable as an interaction between brain activity and the environment.
Electromagnetic waves enter the eye and are focused through the pupil onto the eyeball's back wall, called the retina. Millions of cells lining the retina respond to different wave frequencies from different regions of the visual field. When activated, they form a multi-dimensional map of the visual world. This map gets simplified and organized by the brain's visual cortices. But if the focus point for the light beams is either short of or behind the retinal wall, the map will lose representational clarity, resulting in a fuzzy interpretation by the brain.