Edit: Not how but Why?
Why do 2D games have the main character locked in the middle of the screen when the character starts to move to the middle of the game screen?
Mario NES, Kirby GBA, Megaman NES, Ghost and Goblins NES all do this.
There are exceptions where the main character to "break out of the lock". This happens when Kirby needs to go to a door or when megaman needs to climb a ladder
Viewport (Camera) is locked on the main character's coordinates, as the character moves, so does the camera.
The centre of the screen is the best framing to give the player enough visual information of what's coming ahead as well as what's behind him, if he needs to head back.
It has to do with the ability to react the Mario brother games for example once you get moving and pass some items you could potentially have stuff come at you from behind. Remember not everything on those screens moves in just one direction. If you were stuck against the left side of the screen or close to it you first may not see it before it could effect you and you just may not have the time to react.
The centre of the screen is the best framing to give the player enough visual information of what's coming ahead as well as what's behind him, if he needs to head back.
It has to do with the ability to react the Mario brother games for example once you get moving and pass some items you could potentially have stuff come at you from behind. Remember not everything on those screens moves in just one direction. If you were stuck against the left side of the screen or close to it you first may not see it before it could effect you and you just may not have the time to react.
It is all about game fairness. If your player loses, it should be his own fault. How many times have you died in a game(not stealth type either) simply because you couldn't react to an enemy or obstacle quick enough? Did it feel fair to you?
For example, you will notice in faster moving games, if they did it right, the camera could actually show more ahead than behind, simply so you have more time to see what is coming as you move. Since you are moving faster in that direction, most likely what is behind isn't near as important since you are likely outpacing it, while what is ahead is much more important. If you slow down or stop, than the camera should recenter you considering that what is behind has more chance to catch up, and because you no longer as far of a view distance ahead since you are slower or stopped at this point. Note that the above applies as well in top-view as it does in side-view.
For example, you will notice in faster moving games, if they did it right, the camera could actually show more ahead than behind, simply so you have more time to see what is coming as you move. Since you are moving faster in that direction, most likely what is behind isn't near as important since you are likely outpacing it, while what is ahead is much more important. If you slow down or stop, than the camera should recenter you considering that what is behind has more chance to catch up, and because you no longer as far of a view distance ahead since you are slower or stopped at this point.
I've noticed this too, in StarFox 64 and some other games.
I was testing this behaviour and made a simple demo for Windows illustrating it if anyone's curious:
For example, you will notice in faster moving games, if they did it right, the camera could actually show more ahead than behind, simply so you have more time to see what is coming as you move. Since you are moving faster in that direction, most likely what is behind isn't near as important since you are likely outpacing it, while what is ahead is much more important. If you slow down or stop, than the camera should recenter you considering that what is behind has more chance to catch up, and because you no longer as far of a view distance ahead since you are slower or stopped at this point.
I've noticed this too, in StarFox 64 and some other games.
I was testing this behaviour and made a simple demo for Windows illustrating it if anyone's curious:
http://www.mediafire.com/?jg3akhm678zvm3c (612 kB)
Neat application. There is something weird about two buttons. Why does the game have up key makes ship go down and down key makes go up?
