iPhone 5 extreme brightness problem

Sometimes brightness problem (if you excluded the screen factor) may be caused by a faulty filter on the logic board. When you pump the brightness, the Backlight driver IC and Coil act to pump the voltage on the line, thus making the screen brighter. If your screen has the same "mid brightness" even when at maximum manual brightness on the system, the filter maybe close to burning, which causes the voltage to not flow properly, making a "bottlecap" affect, preventing the higher voltages to power the screen. But before fiddling with the logic board, try changing the screen again, because backlight problems tend to be a "0 or 1" problem. It works normally, or dont work at all (on the iphone 5 at least.)