Volker, The fundamental difficulty you face is that the best quality cameras are digital SLR. These are the cameras with the best lenses (including macro), but SLR, by definition, doesn't provide a constant digital feed, even to its own screen. They are designed such that you look through the viewfinder -- the image is only transferred to the image capture device when you press the shutter. (Incidentally, this leads to a huge saving in battery life.)
Something else you might not realize is that any camera with more than about 5 mega-pixels is capturing so much data that it is not easy to transfer it in real-time down a FireWire or USB 2.0 cable. The dedicated microscope cameras I have seen are pushing the envelope -- the image updates in a jerky manner. A good SLR will have even higher-resolution. Having said that, SLR cameras are very good. If your concern is that the image you capture will be not be as good as you expect, or framed correctly, I wouldn't worry about it. A good SLR will do an excellent job of exposure, and the framing will be pretty much exactly what you see though the viewfinder. The bottom line is: you are better off trusting a good camera with a good lens than trying some jury-rigged contraption just to get a real-time feed. You CAN use SLRs in tethered mode, so that you see the image instantly AFTER taking it. Hope this helps, Gareth
