#1. "RE: Flash & shutter speed (D800 /910 flash)" In response to Reply # 0
I don't have a D800. But don't worry...I won't let that stop me from answering!
The quick & dirty answer is to set M mode or S mode and set the shutter speed directly, either by setting a specific speed or by selecting the next speed after the slowest speed. In this case, an "X" will appear, indicating that the shutter speed has been set according to Custom Setting e1:Flash Sync Speed.
Expanded Answer Most likely you're in A mode or P mode. When you are in A or P mode, and enable your flash, the slowest shutter speed that the camera will set for itself is defined by the Custom Setting e2:Flash Shutter Speed...which defaults to 1/60s. The e2 setting is a not explained well by Nikon. Basically, it defines the point where fill-flash turns into the key light for A and P modes.
When ambient light conditions (along with your aperture choice in A mode) require a shutter speed of 1/60s or faster, then ambient light will be exposed normally and the flash will act as a fill-flash. But when ambient light conditions require a shutter speed slower than 1/60s, then shutter remains at 1/60s, underexposing the ambient light. In this case, the flash acts as a key flash.
As you are in A or P mode, you cannot control shutter speed, and so your only alternative to force a faster shutter speed than what the ambient conditions call for, is to use M or S modes.
#3. "RE: Flash & shutter speed (D800 /910 flash)" In response to Reply # 0
San Diego, US
Excellent advice from Gray_star!
One more piece of advice: if you have a smartphone or tablet, it's always a great idea to have a PDF copy of your camera and flash user manuals saved to the devices so they'll always be with you in case you ever need them. I have PDF versions of all my cameras and flashes, as well as those of friends and family, stored on my phone and tablet. There have been a few times when I've had a brain fart and couldn't remember how to change a setting or perform a specific task, so I just whipped out my phone and was able to look it up immediately! It can be a real live-saver!
_________________________________ The camera doesn't make a bit of difference. All of them can record what you are seeing. But, you have to SEE. Ernst Haas, 1985
#5. "RE: Flash & shutter speed (D800 /910 flash)" In response to Reply # 4
Yes, listen to Chris. I have the .pdf on both my smartphone and netbook. I just recently upgraded from a D90, and while the user interface isn't that drastically different, it's different enough that I've already found myself sitting there staring at the back of the camera and thinking, 'Wait. Now, how do you...?' If you shoot at night, the smartphone option (at least for my phone) works best because the screen is backlit. Best of luck!