When I use a macro bellows with flash on my Sony A55, I get half-exposed frames. Why?
If you are getting half exposed frames then you have the shutter set outside the flash synch range. Basically, the shutter has two curtains, the front and back curtain. The front curtain opens up the camera, and the rear curtain closes the shutter again. For really fast shutter speeds (1/1000, for example, where the shutter is open for only 1/1000 of a second) the curtains are pretty much both moving at the same time, with only a sliver of the shutter open at any moment. Now the flash is a burst of light, and light moves fast, faster even than a 1/1000th shutter. And the duration, the length of the burst, depends on a number of factors, but the longer the burst, the more power it drains from the flash batteries. For this reason camera systems carefully set up the flash to synchronize at certain speeds, where the flash will go off at the moment the whole shutter is open. For most cameras the "flash synch" speed is 1/60. Some of the newer cameras can synch at 1/125, and a few of the really high-end pro cameras synch at 1/250. To get rid of the half-exposed frames try shooting at 1/60. If the picture is still too bright even at the smallest aperture, then the solution is to get a neutral density filter for the lens. Neutral density filters darken the lens without changing the color balance. They are mostly used when you want to use wide apertures in bright light, but they would work on a bellows setup too. They come in different strengths - ND1 reduces by one f/stop, while ND4 cuts the light down to 1/4. And they can be stacked.