A
ayesane
Guest
this is what i was thinking but thought there must be an obvious reason not for doing it so kept schtum.
The logic behind multiple long exposures: The longer you have the shutter open the more light you will get from your foreground (what ever you put in your composition) so taking lots of long exposures and merging them will give you a fixed exposure for as long as you like. As opposed to a huge exposure with potentially well over exposed detail.
I think anyways..