If you want to maintain or hold a specific point of focus and exposure while using the native camera app for iOS devices (iPad, iPod Touch and iPhone) simply press and hold on the area of the frame that you wish to focus on. After about 2 seconds you should see the message “AE/AF Lock” indicating that the focus and exposure are now locked. You can still manually adjust exposure by sliding your finger up or down the screen and any new adjustments to exposure will now remain locked.
This feature works when taking either photos or videos. This is a great feature for any videographer that wants to maintain focus on a subject during a shot and prevent the camera from continually refocusing on different elements in the frame or from automatically adjusting to changes in lighting. As an example, this would be important when shooting subjects in front of a greenscreen. Exposure and focus lock would also be important if you were taking a series of photos for stop motion animation. To disengage the AE/AF lock, simply tap anywhere on the screen again.
NOTE: You will need the latest version of iOS 8 in order to manually adjust the exposure while still in the locked mode.
– K. Takahashi