I have been all around the manual and cannot get my D750 to set options on Page 185 Flash Control Mode i-TTL balanced fill-flash for digital SLR using built in popup flash.
It is confusing, since these options are not directly set in the camera. My notion is that they should be.
Balanced fill flash is called TTL BL mode, which is
also not directly set in the camera. Some bigger Nikon flashes have a TTL vs TTL BL menu on them. Only current model is the SB-910 now. And older SB-800, SB-600, but all the others are TTL BL by default. No menu for TTL vs TTL BL.
Nikon is a TTL BL system, and TTL BL is simply the default for the camera internal flash too, so you do already have it (fill flash).
"Fill" can be disabled (to be the other "standard iTTL) by selecting Spot Metering mode (see last line there, page 185).
However Spot has lots of other issues for ambient. Ambient users need to understand Spot metering. Without a clue about Spot, all you will get is radically variable exposures in ambient situations.
But all Spot means to the flash (speaking indoors, where ambient does not matter to flash) is that it comes ahead strong, using the actual flash metered intensity, instead of backing off to be reduced for fill flash.
I'm using matrix metering mode, 'A' priority and do not see the Flash Mode option on page 183: 'Fill Flash: The flash fires with every shot.
The manual is confusing about this. The top icon on page 183, called Fill flash, does NOT SAY Fill flash in the LCD. It shows just the same blank icon (no words or icons in it), same as the manual shows. It actually just means regular Front Curtain Sync, and specifically means NOT one of the other choices there. The camera internal flash is TTL BL, and so is already Fill Flash, by default. You already have it (unless you select Spot metering).
I took some shots behind a nice enclosed dark brick archway. The foreground was obviously very dark, whilst through the arch was good daylight metered and exposed correctly.
Using a lowish ISO of 150 -200 I was able to pull up the brick arch 'shadows' in post, but what I needed was fill flash technique: Expose for distance, adjust or meter flash power for foreground. I've tried a number of test options since and I appear to be fighting the camera metering. As I read my manual (several times), the functionality of foreground flash metering should be built in and fill shots even with the flip up flash should be easy.
Fill flash normally has little meaning or effect indoors, unless very high ISO to pull the ambient up. Ambient is usually way down indoors (reason we are using flash), and ignored, and "fill" has little meaning then.
But in your case, the ambient was strong daylight through the arch. The fill effect in bright daylight will be to reduce the flash level by about two stops, to be lower fill level. Otherwise, the added TTL flash will overexpose the regular ambient that is already metered correctly there. But TTL BL will try to reduce the flash level if there is meterable ambient, and that is one meaning of fill flash. Filling shadows is the regular meaning.
However, daylight is very bright, and I suspect your foreground was very dark, and any fill effect would just make it darker.
Fill flash means reduced relative to the ambient, which was NOT your goal there. Your test had an extreme dramatic range, not a regular "fill flash" situation.
Solutions to your question:
You could increase the flash with Flash Compensation, however it only allows +1 EV, which is not going to make much difference (in your dark wall case).
In this extreme case, you could aim the camera and the center focus point left or right, directly at the dark wall. Then after setting up camera FV Lock, and reading how to use it, you could push the FV Lock button aimed at the dark wall. That will do a little preflash and will meter the flash for that situation (assuming flash power capability is sufficient for the distance range).
Then recenter camera and take picture, and it will use the previous FV Lock flash value (much stronger flash then, metered for the dark area it was previously aimed at). Do read about FV Lock.
Can anybody else confirm with latest Nikon firmware they are seeing a Fill flash control option? If not how are you all achieving fill flash?
Thanks
To learn about fill flash, start with simpler, more usual and regular flash tests. A photo of a persons face outdoors. Or it could be a flower pot, etc, standing with back or side to the sun, so there are harsh shadows on the face or front. Then (if distance is close enough for little internal flash), then regular fill flash will make a dramatic improvement.
The tiny internal flash might have range for reduced fill flash level up to about 6 or 7 feet in bright daylight (at aperture settings for sun). Bigger speedlights can do a lot more.
See
Flash pictures are Double Exposures- Outdoors for an example. Point & shoot fill flash with default TTL BL.