How does a dog see the world?
Eyesight is quite different between dog and man. While dogs have a better visual field of view to see beside and behind them, they see with less visual detail than we do but function far better in dim light. Dogs also see colors differently than we do and function much like a color blind individual with difficulty distinguishing between close shades. The dog eye is not designed for fine detail work like completing crossword puzzles but rather tailored to things like chasing things in dim light.
For more information on understanding your dogs world, please read Understanding Your Dog Better.