AEL/AFL
To use thumb-operated autofocus
You can choose the operation used to initiate autofocus and set the exposure.
You can choose to initiate autofocus and set the exposure only using the button to which the [AEL/AFL] function is assigned, or control in combination with shutter button operation. This setting can be made for each [AF Mode] option.
When using continuous AF [C-AF] with [mode3] or [mode4] selected for [AEL/AFL] > [C-AF], you can perform so-called “thumb-operated AF” where the camera tracks the subject while the AEL/AFL button is held down and shoots when the shutter button is pressed.

Take sequential photos while tracking the target subject