Your eye is a lens. I am making these numbers up but the analogy holds.
If you turn all the lights off, you can't anything see until your eyes adjust. They do this by enlarging the pupil (the black hole in your eye) and the purpose is to let more light in so you can see in the dark. The size of the pupil is called Aperture and is measured as a fraction. Lets say this would be like f/2. The "f" stands for the focal length, or mm of the lens.
Now turn the metal halides on in your tank and stare at them (pretend they come on at full power instantly). You will be blinded because there is way too much light and your pupils are too big. Your pupil must get smaller to let less light into your eye. This would be like f/22.
If you stare at something when your pupils are big (like f/2) everything before and after what you're staring at will be blurry.
If you stare at something when your pupils are tiny (f/22) much more of what you can see will be in clear focus.
Everything you see is based on perception. If you are close to someone looking at a distant mountain, they will look large and the mountain will look small. A 50mm lens will show the size comparison just as your eyes see it.
If you step back and take a picture at 100mm, the person would look a little smaller and the mountain would look a little bigger. If you stepped forward and took a picture at 25mm, the person would look even bigger than he would in real life, and the mountain would look smaller.