my test with 3 twenty-second exposures combined in camera with auto-gain turned off.
here's the deal...
last night i was playing around with two of the D200's cool features (multiple exposure and interval timing).
i got to thinking that joining the two would possibly be a good way to circumvent the need for a shutter release cord for long exposures up to five minutes (10 multiple exposures @ 30sec).
anyway, the interval shutter release only works if the total combined exposure time of all the multiple exposures is less than 30sec (it will rattle off 10 2sec exposures automatically). if you manually release the shutter you can do over 30 sec, all the way to 5 minutes equal to ten 30 second exposures.