Jim, I used to have the EF-500 DG ST model, which is the slightly earlier model. It is identical except for it having slightly lower power Guide Number 50 instead of the 530's Guide Number 53.
The problem with the ST models is the lack of manual power level adjustments. As Don has said in post #2, the only manual power adjustments are full power or 1/16th power. I used to find full power often too much, and 1/16th power too little. You can work around the issue in a variety of different ways, especially if you use the flash off-camera. But, and it's a big BUT, knowing the kind of social documentary work you do, I don't think this is the gun for you; it simply hasn't got the manual adjustability you'll need.
You can use it on-camera and use the flash compensation feature in your camera's settings, this will give the flexibility you need, but I think you'll soon get frustrated with this flash. It's cheap and cheerful, with limited settings. A good gun for a beginner to get them going into the world of flash, but in reality, you'll soon grow out of it. The build quality is nothing to write home about either - adequate, but doesn't inspire confidence.
Options to look at would be the Sigma EF-530 DG Super, a bit more cash at £200 instead of the ST at £120, but has a lot more extra features. My recommendation would be the Metz 48AF-1 at £175. I have the 58AF-1 and can highly, highly recommend that, but accept that it's probably beyond your budget. The 48 is the lower power version, has all the adjustability of the 58 (full power to 1/256th power in lots of very small increments) and doesn't have the secondary fill-in lamp of the 58, otherwise has all the features you need.
Get the Metz 48, the extra £55 over the cheap Sigma is money well spent. You'd get frustrated by the cheap Sigma very quickly.
Duncan
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