Whether you are using an entry-level DSLR like Nikon D3100 or a top of the line DSLR like Nikon D3x, there is a special button on the back of your camera labeled “AE-L / AF-L” that can be quite useful in many situations. After I wrote the Autofocus Modes article, I received several requests from our readers, asking me to explain what the AE-L / AF-L button does, when it should be used and how it can be combined with different autofocus modes. In this article, I will try to go through this button in depth and explain how I personally use it on my cameras.
1) AutoExposure-Lock / AutoFocus-Lock
The AE-L / AF-L button stands for “AutoExposure-Lock and AutoFocus-Lock” and its primary function is to lock camera exposure and/or focus. What does this exactly mean? If you are using any of the camera modes like Program, Aperture Priority, Shutter Priority or other scene modes, the button could be used to force the camera to use a certain value for shutter speed, aperture, ISO and white balance. Since in any of the automatic modes the camera uses its metering sensor to determine the optimal exposure, instead of having your camera re-evaluate the light every time you recompose, you could lock the exposure to a value you are comfortable with – hence the term “AutoExposure-Lock”. There are many cases where using this feature is very helpful. One example is when you photograph panoramas. It is extremely important to use exactly the same exposure from frame to frame in panoramic photography. If one exposure differs from another, it is practically impossible for panoramic software to stitch images together in a consistent, continuous form. Another good example is if you are photographing a subject with a constantly changing background and you want to expose the subject exactly the same way from shot to shot. Basically, any time consistency of exposure is required and you do not want to switch to a full manual mode, the AE-L button can be very useful.