The brightness of your iPhone is actually one of the basic aesthetic settings that you have. Go to the iPhone Settings icon on the desktop. Once in Settings go down to Brightness, which is in the second down block of options, directly below Sounds and above Wallpaper. Here is when you are going to get two main options for your iPhone touch screen brightness.
On top is a bar that you can use to choose exactly how bright you want the touch screen to be. The more to the right you go the brighter it will be, and the more you shift it to the left the more you will darken the over all image. Below that is the Auto-Brightness function for the iPhone, which you can choose to either leave On or turn Off.
The iPhone's Auto-Brightness feature responds to the amount of light that is around the iPhone. If it is very bright outside the Auto-Brightness function will respond by making the image easier to see. If it is dark, it will lower the intensity of the image because you do not need it to be so bright. It lowers the brightness in these situations mainly to save your iPhone's battery life.