But that was before someone noticed that Apple had made a small but somewhat crucial design change: it moved the ambient-light sensor a few millimeters to the left. That created a problem for many rugged cases that covered more of the front of the iPhone: instead of peeking out through a hole cut into the case, the sensor was now hidden behind the case.Case companies are adjusting their cases to suit the changes. Case manufacturer Otterbox told us that the main differences between the old and new cases "will be a larger cutout around the camera and proximity sensor, as well as a few minor adjustments to ensure the cases fit all versions of the iPhone 4 and iPhone 4S."
With no light penetrating, the sensor thinks the phone is in a darkened room and automatically dims the screen to look less blinding and save battery life. In the settings menu, you can manually set the screen brightness, but when in "auto-brightness" mode, the ambient-light sensor adjusts the brightness on the fly according to the ambient lighting conditions.
The CDMA iPhone 4 moved the mute switch slightly lower to make room for the upper left antenna. Some cases from the GSM iPhone 4 don't allow access to the moved mute switch. The iPhone 4S has the mute switch in the same location as the CDMA iPhone 4 and thus some early iPhone 4 cases prevent easy access to the mute switch as well.
Users having difficulties with screen brightness or dark pictures from the rear camera are encouraged to contact their case manufacturers for remedies.
Image courtesy iFixit