Vinman

When using back button do we loose the ability to have EL locking at the same time or is there a way to turn this on and off while keeping BBF ?
I'm trying to set up mu D750 that by the way just came back from Nikon
(new sensor, clean and upgraded firmware. Great company !!

dannac Loc: 60 miles SW of New Orleans

Yes.

You could setup either the DOF or FN buttons for EL.

Being a left eye shooter, the AE/AF button is awkward for me to use for BBF.

I have mine set to use the DOF button for BBF (sits right under my middle finger)
and the FN button for EL.

The Villages Loc: The Villages, Florida

If EL button is used for BBF, you can still lock exposure with the Half Press.

JD750 Loc: SoCal

Go to the following:

MENU-> Custom Setting Menu ->
CONTROLS->AE-L/AF-L button->Press AE-AF lock->

You will see there is no option for setting BBF and Exposure lock both at the same time using only the AE-L/AF-L button.

However exposure lock can be facilitated by the 1/2 shutter press, to set that up (or disable it) go to:

MENU-> Custom Setting Menu ->
Timers/AE LOCK -> Shutter release button.

OR alternately you could set up one the of the other buttons, like the function button, to perform the AE_Lock function. To set that up go to:

MENU-> Custom Setting Menu ->
Controls -> Assign Preview Button
OR
Controls -> Assign Function Button

Hope that helps.