AE-L/AE-F button Vs Half Shutter Press
By default most camera’s will only lock focus with a half shutter press. This means when recomposing a shot with a half shutter press the camera could adjust exposure. Pressing AE-L/AE-F button will lock both exposure and focus even when recomposing. However mainy DSLR camera’s such as Nikon’s allow this functionality to be changed so a half press will lock exposure as well.