You've just climbed that mountain you've been meaning to climb. As you scan the horizon, the surrounding peaks stand out in sharp relief, clear as a picture postcard. You lean against your pack, shade your eyes, and take in the whole vista.
Just how much of that clear vista can you take in anyway? If your eyesight is good, you probably think everything you see is always in sharp focus, like a photograph. This isn't the case. To understand why, let's take a closer look at your eye. The back of your eye is covered with light sensitive cells which send messages about what you see to your brain. The cells are not evenly distributed, however. At the center is a dense cluster of ultra-sensitive vision cells. Outside this central area, the cells rapidly become more sparse. These outer cells, responsible for peripheral vision, are great at detecting motion, but not so good at forming a clear image.
This means that although your peripheral vision might be wide, only a small part of that vista is in sharp focus. How small? Hold a quarter about a foot and a half in front of your face. The size of that quarter is what you can focus clearly at any one time. If you took a snapshot of your vision, only this tiny, central area would be like a photo. The rest would blur, like a watercolor that's gotten wet around the edges.
Test this the next time you read a newspaper. Even the narrowest columns of newsprint are too wide to focus entirely. You can't read more than three or four words without moving your eyes!