When taking photos with the camera on Android 4 cell phones, it can be useful to lock the exposure in backlit situations, and the focus in some scenes.
Backlighting is where your subject has bright daylight behind them, making them look like a silhouette, instead of a person. Locking the exposure and the focus forces the camera to override its automatic settings. Cameras get fooled, because they try to average out the lighting.
First, here is a backlit photo, where the light from the window and the computer monitor fool the camera into thinking the scene is brighter than it really is:
Android cameras can bias the exposure by up to 2 f-stops, plus or minus, but I find that they have little effect on the image.
Quick by accident, I found that the Android 4 camera has exposure/focus lock. It works like this:
1. Move the camera to a part of the scene that has good lighting.
2. Hold your finger on the shutter button
3. After a second or two, you hear a double-toot sound. Android locks the exposure. Keep your finger on the button!
4. Move the camera to frame the scene you want.
5. Lift your finger from the shutter button; the photo is taken.
Here is the same scene, but here I used exposure lock by first moving the camera down to my printer, locking the exposure, and then moving the camera back up to the window.
(I found this function by accident after my aunt had difficulty taking a photo of my wife and me. As she tried taking the photo with my cell phone, I could hear a double-toot sound I hadn't heard before, and my aunt complained the photo wasn't taken. After 3-4 tries, the photo was taken. Then she realized she had held the shutter button too long. When I got back the cell phone from her, I experimented to find why she had problems with the camera and the strange noise. I finally found that holding the shutter button locked the exposure and focus.)
Naturally, you can use this hidden function to reverse the process to deliberately overexpose photos, underexpose them, or to make them unfocused. Here I used exposure- and focus-lock to overexpose and blur the scene... (LoMo, here we come!)... (the blue tone is made by setting white balance mode to Tungsten)...
... and here in this last photo I used the lock function to unfocus the image. (I moved the camera close to the printer, engaged focus lock, and then move the camera back out again.)
Bonus tip: if you don't want a photo taken when you hold down the shutter button, slide your finger off the shutter button.