I have a Nikon D90 and a D750.
I've used a SIGMA EF-610 DG Super external flash in iTTL mode on the D90 with great results.
On the D750, the flash doesn't recognize the camera as digital / iTTL capable.
The D750 manual on page 440 lists the possible flash control modes for an external flash.
Has anyone found the secret location in the manual that specifies how to select a particular mode for an external flash?
Or to see what the current mode is?