As has been mentioned earlier, it is possible to make an exposure not in one go but in small steps to preserve sharpness. Doing it selectively for different parts of the picture would be much harder! Off the top of my head, one way would be to use (you might have to build) a traveling matte unit, which is a device generally used only with movie cameras to mask off parts of a scene which are exposed at different times (think of any movie where an actor appears on screen twice at the same time in different roles). This would work but would involve more effort than most folks would be willing to make! It wouldn't be too hard to make, though, taking a Cokin filter holder or similar as a basis.
Originally Posted by wager123