DSLRs and mirrorless cameras have many buttons. If you're just starting to control the camera manually, you're probably wondering what all – seemingly unimportant – do. Let's take a look at the AE-L, AF-L, AF-ON and * buttons.
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The AE -L or * buttons
The AE-L and * buttons are the same. It's just that Nikon and Sony use AE-L, and Canon inexplicably uses the asterisk icon. AE stands for Automatic Exposure and L stands for Lock. If you press this button, the exposure settings currently selected by the camera will be locked until you take a picture or release the shutter button.  This is very useful when working in one of the semi-manual modes, such as aperture priority or shutter priority. For example, when you try to take a silhouette photo or work in a difficult situation. Do the following:
- Select the measurement mode that you believe works best.
- Press the shutter button halfway to start the meter of your camera you want. If you are taking a silhouette, measure the light background if you want to illuminate the subject well despite the backlight being bright, measure the face, etc.
- Press and hold the AE-L (Nikon) button or press the key *. (Canon) to save the exposure settings. Hold the shutter button halfway down.
- Rearrange the image as desired. The exposure settings do not change. Press the shutter button all the way to take the photo.
If you do not want to work completely manually, the AE Lock button is a very handy tool.
The AF-L Button
Some cameras also have an AF-L button or the AE-L button can also double as one. The AF stands for "autofocus". The L still stands for "Lock".
Pressing the AF Lock button sets the autofocus to the currently set range. This is useful in situations where you want to focus on a specific subject, but there is no auto-focus point where you need it.
The reason why most cameras do not have an AF lock button is that the shutter is displayed as one in single AF mode. If you press the shutter button halfway and find the focus, it remains locked. The AF Lock button is useful only in Continuous or Hybrid Auto Focus modes.
The AF ON button
The AF ON button does the opposite of the AF L button: It turns on autofocus. This is used by many professionals with a technique called "autofocus backspace".
The autofocus backspace button adjusts the camera so that the shutter button stops controlling the autofocus. Instead, autofocus will only be activated by holding down the AF-ON button on the back of the camera. This gives you much more control over how the autofocus behaves – even if your camera is a little weaker to use.
Automatic exposure lock, autofocus lock, and AF-ON buttons allow you to manually control the automatic functions of your camera. If you want to master your camera, you need to know how to use it.
How exactly these behave can usually be changed in the camera settings. For example, you can change the behavior of Nikon's AE-L / AF-L button so that both the focus and the exposure (default), only the focus, or just the exposure are saved.