My original wording may have been a little misleading. Its not distance from the projector that needs to remain constant, it is the distance from the focal plane that needs to remain constant. THe focal plane is (ideally) perfectly flat. SO if you have a 1 foot difference, you can either have the whole screen a little out of focus except 2 vertical columns somewhere between the edge and the center where the focal plane and screen intersect, or you could have the center in focus and the edges progressively more out of focus. On my own projector, a 1 foot change throws the image unaccepatably out of focus for my eyes.