Check out the photo below. What problem can you see?
It is quite obviously over-exposed. The photo was taken using an iPhone 4S in a local aquarium. The big lizard was in a cage with uneven lighting. The automatic exposure decision made by the iPhone4S was clearly poor. But the phone is only partially responsible. It really doesn’t know how I wanted the exposure when facing a difficult scene. Unlike most point-and-shot camera, the iPhone 4S doesn’t have anyway of adjusting exposure (exposure compensation).
Fortunately, iPhone has a nice touch-to-focus feature, which also sets the exposure level based on where you touch on the preview image. In this case, I simply focused on the back of the lizard where it was the brightest to take the next photo. The resulting photo (shown below) was much better.
This trick can work in many situations. Potential focus problem may arise if main point of interest falls out of the depth of field when you focus on somewhere else in order to get proper exposure.
Are we out of luck?
No! By accident, I discovered another feature of iPhone 4S that can help greatly in tricky exposure situations. The iPhone 4S has an AE/AF lock function built-in! Unfortunately it may not be very intuitive because there is no button to push. But it is simple to use. Just touch and hold on the spot where you want to have exposure and focus locked until “AE/AF Lock” is displayed on the screen. This works great for focus and recompose. For our particular need, we need to set and lock exposure but not focus. There seems to be no way to do this separately on the iPhone but we have a work around: first set the AE/AF Lock then move the camera forward or backward until the point of interest is in focus again. Now both exposure and focus will be correct.