Sony a9 Instruction Manual page 57

Face detection frame
• When the product detects a face, the gray face
detection frame appears. When the product
determines that autofocus is enabled, the face
detection frame turns white.
• In case you have registered the priority
order for each face using [Face Registration],
the product automatically selects the first
prioritized face and the face detection frame
over that face turns white. The face detection
frames of other registered faces turn reddish-
purple.
Eye detection frame
A white eye detection frame appears when an eye is detected and the
camera determines that autofocusing is possible, depending on the
settings.
[Eye AF] by custom key
The Eye AF function can also be used by assigning [Eye AF] to a custom
key. The camera can focus on human eyes as long as you are pressing the
key. This is useful when you want to temporarily apply the Eye AF function
to the entire screen regardless of the setting for [Focus Area].
For instance, if [Focus Area] is set to [Flexible Spot] and you want the
camera to focus on an eye outside of the focusing frame, you can apply
the Eye AF function by pressing the custom key to which [Eye AF] has
been assigned, without changing [Focus Area].
 MENU 
desired key, then assign the [Eye AF] function to the key.
 Point the camera at a person's face, and press the key to
which you have assigned the [Eye AF] function.
 Press the shutter button while pressing the key.
(Camera Settings 2)  [ Custom Key] 
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