AE-L Auto Exposure Lock
* This button is used to lock exposure in one area of the frame then recompose the photograph.
Set your exposure mode to “center weighted” or “spot metering” then position this point on the area you wish to have correctly exposed. Push in the AE-AL Lock button to set and lock the exposure, then (without releasing this button) recompose the frame then push the shutter release button to capture the image. Use this feature when a composition contains areas with different intensities of lighting.
* This button is similarly used to lock focus in one area of the frame then recompose the photograph.
Set the Focus Mode Selector to S (Single). Position the viewfinder’s focus point on the area you want to be in sharp focus, then push the AE-AL Lock button to lock the focus. Without releasing this button, recompose the image then capture the image by pushing the shutter release button. Use this button when you want sharp focus on an area of the frame which will not be in a focus point in the final composition.
 button locks your cameras exposure for as long as the button is pressed.){kind=link}