Sony ILCE-6500 Manual page 157

Press the button to which [FEL Lock toggle] is registered, and set the flash amount.
3
A pre-flash flashes.
The (FEL lock) lights up.
Adjust the composition and shoot the image.
4
When you want to release the FEL lock, press the button to which the [FEL Lock
toggle] is registered again.
Flashes that support FEL lock
HVL-F60M, HVL-F43M and HVL-F32M can be used as clip-ons, off-camera connected by a
cable, and for wireless flash shooting (controller/remote).
HVL-F20M can be used as a clip-on, off-camera connected with a cable, and for wireless
flash shooting (controller).
Note
The FEL lock cannot be set when a flash is not attached.
If a flash that does not support the FEL lock is attached, an error message is displayed.
When both AE and FEL lock are fixed,
Hint
By setting [FEL Lock hold], you can hold the setting while the button is pressed down. Also, by
lights up.