While I continue to save for an SB-800, I've just bought a LumiQuest Soft Screen to try to calm down the harshness of the pop-up flash. The piece of paper that comes with the device claims that no exposure compensation should be required, but that's just not true. With the Soft Screen in place, pictures are just too dark. Time to tweak...
In the past I've used exposure compensation a fair bit (using the +/- button and command dial), and am used to the camera adjusting the shutter speed or aperture to alter the exposure. I believe one can adjust the power of the pop-up flash using the Flash exposure compensation (flash button and +/- and command dial) to alter the flash power. However, what I did in this case was to just use normal exposure compensation, and expected to see some evidence of the aperture or shutter speed changing, but I didn't. As anticipated, dialing in +2 EV fixed the lighting, but I'm confused just how it did it. The aperture couldn't have opened, as I was already at maximum. The shutter speed didn't seem to change - at least I couldn't hear that it was slower. So I can only guess that the flash power was increased, which would seem a logical way of increasing exposure in this situation. But if exposure compensation adjusts flash power when the pop-up flash is used, what is the flash exposure adjustment for, as it seems to do exactly the same thing!?
I could just leave it that it works, but it bugs me to not know what's going on!
So if the pop-up flash is up, changing the exposure compensation changes the flash power. I thought that's what I saw. So what on earth is the flash exposure option for, when "normal" exposure compensation does exactly the same thing????
I'm pretty sure it wasn't on Auto ISO. Also, I was able to step the exposure compensation up several times and each time achieve a better exposure. If the ISO were increasing I would have had at most 3 more exposure options.
Strange. I'll have to get the camera out again tonight and try again.
Nikon's default flash system is some form of balanced fill flash. If you are getting a pre-flash, you are useing a balanced fill flash mode. In fill flash, the camera is trying to balance both the ambient light and the flash burst. In a balanced fill flash mode, the camera is adding some -EV to the flash.
Adding flash compensation to a balanced fill flash mode will have a cumulative effect and will be unrepeatable unless you are shooting in the exact same conditions (and even then, there may be some variation).
It is best to run a controlled test to verify how the built in flash behaves in manual exposure mode (this should revert to a standard TTL mode). Depending on the lighting conditions, you can add from -.7EV to -2.0 EV to get repeatable results. As you have the Lumiquest diffuser, I recommend running a test with that, too. Note that you will lose power with the diffuser so the flash will not have the same reach.
(In a pre-flash mode, with the Luniquest diffuser, the camera will see that not enough light is reaching the subject and may try to add more light as a result.)