I'm not sure. The enlarger is really a macro camera with the subject (the film) much closer to the lens than the film (in this case, the paper!).

If it was focussing an image at infinity onto the paper then a change in the subject distance of a few hundred feet (think mountains in the distance) would be negligable.

In the case of a macro camera, there is a very narrow depth of field and this translates to the position of the film in an enlarger. Obviously this is ideal for an enlarger as the film is very narrow too as long as it is flat but it does have to be in the right place.

Doesn't really answer the question though!