you can use the MF setup (and some would argue that you'll get better quality doing so as it will only be using the centre of the lenses capability... debatable) but your enlargement size will be limited to smaller prints, and, I think, you may have longer exposure times (because the neg is further from the paper than if you had a 50mm lens). I've done this on occasion when I have wanted a quicky print from a 35mm neg but couldn't be bothered changing my enlarger to 35mm mode (different condensor, lens, neg carrier masks)