I'm sure I MUST be doing something wrong. I've set the AF assist beam on on both my D600 and my D750 but it only seems to work on both cameras when they are set to Auto or Scene (even in the exact same light conditions). To set it to On I went to "Custom settings menu", then "a Autofocus", then "a7 Built-in AF-assist illuminator" and then ON. What am I missing??