AE-Lock & AF-Lock
Digital cameras are set to default to lock focus and exposure together when the shutter release button is pressed halfway. Some digital cameras have a feature to lock focus and exposure independently. AF-Lock: controlling focus
Gain more control over where the camera focuses when you depress the shutter-release button half-way. Say you want to focus on the subject that is off to one side in a scene. Move your digital camera so the focus area indicator is on the subject. Then press the shutter-release button down half way to lock focus. While holding the button in this position, recompose and then fully depress the button to take your shot. This helps ensure that the main subject is in sharp focus rather than a random object selected by the camera.
AE-Lock: controlling exposure
Obtaining better exposed photos in difficult lighting situations involves moving the camera to the main area you want properly exposed. It should of equal distance as the subject since the shutter-release button controls focus too. For example, if you want to capture detail in an area that would otherwise come out dark, lock exposure on the area, recompose and press the shutter-release button all the way down.
Sample of Auto Exposure Lock:-
