PMFJI. I don't agree fully with the reasons that Anupam gave you. I see several problems with using auto-TTL flash closeup.
Originally Posted by glbeas
It seems that some camera/flash combinations are badly, um, flustered by specular highlights.
The camera can't be counted on to recognize the main subject and illuminate for it. This is exacerbated by falloff of illumination when the background is far behind the main subject.
Intelligent use of a lens-specific flash rig that is calibrated for magnification avoids these problems. I have a flash rig for my wife's 55/3.5 MicroNikkor, another for my 105/2.8 MicroNikkor AIS. And I have two for use with my Graphics, one that attaches to the lens (conceptually very similar to the rigs I use on 35 mm) and is calibrated for a 105/5.6 El Nikkor and another that attaches to the camera body and is calibrated for a 100/6.3 Neupolar.
Using a pre-calibrated flash rig is equivalent to metering incident. For unusually reflective subjects, stopping down a bit below the aperture indicated in the calibration table is helpful. Similarly, for unusually dark subjects opening up a little is helpful. If it isn't obvious, the problems with auto-TTL flash that I gave above are due to metering reflected light.
I also sometimes use a single hand held variable-power flash (Vivitar 283 with VP-1) with my Graphics and do the GN arithmetic, including adjustment for magnification, in my head. No big deal.
I don't understand why people make such a big fuss over using flash closeup. I made my first closeup flash rig in 1971, used part of a roll of film to calibrate it, and since then have had very few poorly exposed closeup shots. Ain't much to it.
Anupam made the point, but not strongly enough, that on-camera flash makes underexposed backgrounds possible. Ponder my patented (yeah, sure) idea of "depth of illumination" and you'll see why. The only way to guarantee that the background will be illuminated as desired (as many, including no, stops down from the main subject) is to position a flash off camera to get the desired result. A pain, but doable.