There’s a long-standing bug in the D500 firmware, where you can’t get the camera to focus in Live View by pressing the AF-ON button. This problem occurs when you use “custom controls” and assign different AF modes to different buttons. When you do this, the AF-ON button is dead, and the only way to auto-focus in Live View is to touch the Live View LCD.
You may be a big fan of using the touch-screen to focus, and this issue is therefore a “don’t care” for you. My fingers are so hard-wired to use back-button auto-focus that it messes me up virtually every time that I switch to Live View.
As of firmware version 1.15, this bug still exists.
Custom controls ruin the Live View focus via AF-ON
My D500 is configured so that my depth-of-field preview button is assigned AF-ON with an AF-area mode of “Group Area AF” (nearest subject focus). My joystick button is assigned AF-ON with “Single-point AF” (precision focus). My AF-ON button is assigned with AF-ON and “Dynamic-Area AF-25” (far subject focus).
The custom auto-focus button assignments are one of my absolute favorite things about the D500. They solve most of the missed-shot focus problems.
One solution to get the AF-ON button to work for Live View focus is to simply un-assign the custom controls for different AF-ON modes. I find this solution unacceptable, because I rely heavily on these different instant-on AF modes.
A better solution, which I use, is to assign my “Custom Settings Bank D” for Live View mode, and I un-assign all custom controls related to AF-ON for just this single bank. Finally, my AF-ON button works normally for auto-focus in Live View mode.
This solution is a bit of a hack, but it works. Fortunately for me, I was only using the other 3 settings banks (A=sports, B=portrait, C=manual). Now, I have a use for the “D” bank: Live View.
My settings banks
If you tend to forget what each settings bank is used for (like I sometimes do), make sure that you give each bank an identifying name.
Good luck using the LCD touch-focus with an LCD viewfinder
If you have ever tried using Live View in the sunshine, you have a pretty good idea of how well you can see and focus; Live View is nearly useless. That's what LCD viewfinders are for. LCD viewfinders don’t cooperate too well with touch-focus features. It’s mandatory to have a way to force the focus via buttons instead.
I had assumed that Nikon would get around to fixing this D500 bug, but I’ve long since given up hope. I guess that in their own minds, this is merely a feature and not a bug.