I got my D610 back in November 2015 & on one of my shoots in late December decided to try out the AutoFP (High speed sync) flash mode.
I was using my onboard popup flash to wirelessly trigger a slave SB700 (attached to a softbox) and a 85mm 1.8 lens.
I was able to get shutter speeds of up to 1/1000 or even 1/2000 of a second with '1/250 AutoFP' flash mode enabled with the setup described above. Thus allowing me to shoot nice wide apertures of f1.8 in the daytime.
However recently (last week) I tried the same thing with the exact same setup but I wasn't able to go below 1/250 of a second for my shutter speed!
I have tried to resolve this issue with Nikon but they haven't been very helpful at all.
I've attached a couple of photos from the first shoot I mentioned where I was able to get shuttter speeds of 1/2000th of a second.
Can anyone help me out here please?
All the best!