Why does the exposure vary in these Nikon flash photos?

Question

The following 4 photos were all taken with a Nikon D300 at 1/250s, f/5.3, ISO-400, in shutter priority mode, with the on-camera pop-up flash. The only difference between them is the exposure bias, which varies from -4EV to +2EV. You can clearly see that the four pictures get brighter, despite having the same exposure settings.

Can you explain how/why this is happening? I have come up with a few possibilities, but I don't know which (if any) is actually correct:

• Is the camera adjusting the output of the flash to compensate for the exposure bias? This seems like the most likely solution to me, based on how the images look, but my understanding is that exposure bias was supposed to control the amount of ambient light in the scene, while leaving the flash alone, whereas flash exposure compensation is supposed to control the power output from the flash. (Flash exposure compensation remained constant for the four images).
• Alternately, is the camera applying some sort of automatic lighting correction in post-processing after the images were shot? (These images were all taken in JPEG mode, not as Raw).
• Is there some other effect at play here that I don't understand?

Edit I've copied-and-pasted the relevant exif data, extracted by exiftool, for the photos. The data is identical in all important fields (I checked using diff), except for the Exposure Compensation and Exposure Difference fields (I don't know what the latter means):

Camera Model Name               : NIKON D300
Exposure Time                   : 1/250
F Number                        : 5.3
Exposure Program                : Shutter speed priority AE
ISO                             : 400
Flash                           : On, Return not detected
Flash Type                      : Built-in,TTL
Flash Exposure Compensation     : +2/3
Active D-Lighting               : Normal

The Exposure Compensation and Exposure Difference fields for each image, from top to bottom is as follows:

Exposure Compensation           : -4
Exposure Difference             : -0.3

Exposure Compensation           : -2
Exposure Difference             : -2.5

Exposure Compensation           : 0
Exposure Difference             : -4.4

Exposure Compensation           : +2
Exposure Difference             : -6.6

Answer 1

With Nikon, Exposure Compensation affects the overall exposure, including any automatically metered flash, while Flash Exposure Compensation (when available) affects only the flash's contribution to the overall exposure.

If the scene weren't essentially a flat plane (there isn't much depth to the scene at all), you could use Exposure Compensation to bring down the entire exposure, then boost the flash exposure to correctly expose your main subject. That would isolate the subject from the background. (The opposite can be set in theory, but it won't work unless and until somebody invents a "flash" that subtracts light from the ambient exposure.) More often, though, you'd use the Exposure compensation to compensate for the tonality of the scene (bumping it up for scenes consisting mostly of lighter tones or lowering it for predominantly dark scenes), then independently adjust the flash (usually downward) to add the desired amount of fill.

Note that with these pictures, there isn't much contribution to the exposure from ambient light at all, so if you had set the Exposure compensation to -1 EV and the Flash Exposure Compensation to +1 EV, you would have gotten pretty much the same picture as the normal exposure.

Answer 2

I checked my D7000 and set it up the same way you did - the flash is adjusted - according to the image info in playback mode when I press down.

I see +/-1 in the info against flash bias but my aperture is hasnt changed (maybe my cam dosnt think it needs to at these settings)

It is the flash that is causing the brightness change - Try dialing in the same settings but with a -5 exp. bias - You should see the aperture start to change aswell, but less pronouced than if you didn't have flash on.