Keith,
With DSLRFocus, you continue in your prime-focus setup but the camera is connected to the laptop via an interface cable (usually parallel or serial if you want exposures greater than 30 secs) and the software will continually download images as you are playing with the focus knob.
In one senses you are emulating a real-live feed on some camera LCD's such as the older Nikon Coolpix 995 and the newer Canon EOS 20d.
Further to my reference about the 30 sec limit, this applies only if you also plan to use DSLRFocus for programming your exposure sessions (ex. 10 exp @ 3 min + 5 exp @ 5 min etc). Otherwise, if all of your work is at 30 sec exposures or less, you can do with the existing USB interface.
Anthony.
PS. (Sorry Dave for jumping in ... I just happened to be online).
  