Most advanced cameras allow you to separate exposure and/or focus lock from a half press of the shutter button to allow each photographer to choose how and when focus and exposure are locked for a given composition. Even what happens by default in the camera's "factory" settings will often vary based on what shooting modes in terms of exposure and focus are selected.

Why is it advantageous to separate exposure lock and focus lock from the shutter button? If both focus and exposure are locked by a shutter half press, then pressing an AEL or AF-L/AF-On button allows you to update either exposure or focus without changing the other value. They also allow you to maintain the same exposure or focus settings for more than one frame.

• Suppose you want to take a series of shots, such as a panorama, with different composition but the same exposure settings. Press the button on your camera that locks exposure and focus and hold it down while you take the series. I saves you the trouble of metering and then switching to M exposure mode to preserve the exposure settings for more than one shot.

• If you are using a form of continuous focus and find the need to lock focus. With most advanced cameras there are several ways to do this. You can usually set the camera to either allow a half press to start focus (in AF-C/AI Servo AF the camera will focus continuously without locking as long as the shutter is half pressed) and a press and hold of the AF-L or AF-On will lock the focus OR you can set the camera so that a half press does not start focus and pressing the AF-L/AF-On button is required to focus. In the second case you would always focus by pressing the AF-L/AF-On button. If you are using AF-C/AI Servo AF, as long as the button is pressed the camera would continuously focus. If you wanted to lock the focus at a particular distance you would simply release the AF-L/AF-On button while the camera is focused to that distance.

• You can hold down the AF-L/AF-On button to take multiple shots at the same focus setting. Just as with exposure, there are times you may want to maintain the same focus distance for a series of shots. Holding down the AF-L/AF-On button allows you to do this if you are using an AF-S/One Shot focus mode. Otherwise your camera would refocus between shots. Another advantage of this method is that it may increase your burst rate because the camera isn't refocusing or confirming focus between each frame.

• If you are using a form of "One Shot" or AF-S focus. In this scenario if the shutter half press tells the camera to find focus and then lock it as well as begin metering, you can set the AF-L/AF-On button to refocus after you have recomposed. By continuing to hold down the shutter at half press the exposure settings will remain locked while you refocus.

• If you are shooting in a mode that locks focus and exposure at the shutter half press, pressing the AE-L button will allow you to update the metering without altering the focus. This is useful for focus and recompose or when the lighting conditions of a scene may be changing rapidly. The AE lock button allows you to update the metering without changing the focus.