Flash bracketing can be done on some Nikon bodies using the exposure bracketing function that you're already familiar with. With a flash attached, the flash power will be adjusted during the three exposures, along with ambient exposure also being bracketed by varying shutter speed or aperture.
The D90 has a custom setting E4, which gives you the following choices of exposure bracketing with flash
- Flash only - vary flash exposure only (suggest setting camera to Manual mode to fix shutter/aperture
- AE only - vary ambient only - flash will presumably remain constant
- AE & Flash - vary both, using some balanced formula?
If the D5000 does not have such a setting, then I would expect with a flash attached it would either do AE only or AE & Flash.
To test if the flash power is indeed varied, I would do the following:
- Set camera to Aperture priority (this will fix the aperture)
- Turn on exposure bracketing
- Attach a hotshoe flash in TTL/iTTL mode
- Take some shots with flash attached in a relatively dark area indoors using the exposure bracketing
Because you are in a dark area, the flash will overpower any ambient and make up most of the exposure. Because you're in aperture priority, only the shutter speed will vary. Since shutter speed doesn't affect the flash exposure, only the ambient, and the flash is overpowering the ambient, any change in ambient exposure due to the shutter speed varying will be minimal compared to the flash exposure. So in your resulting 3 frames, if you see a noticeable difference in exposure, it stands to reason it will be due to flash power being varied by the camera.
If you get three identical shots, then what has probably happened is flash power was fixed, and the ambient has varied by +/- 1EV to no effect due to being flooded by the flash. If so try the following.
Flash Exposure Compensation
If the above doesn't work for you, there is a manual way, and that is to use the flash exposure compensation. While pressing the flash button (that pops up the built in flash) rotate the rear command dial to adjust flash power. Try -1, 0 and +1 in three successive shots. This should work with the built-in flash or an attached hotshoe flash (or with CLS system if you have off-camera flash).