If I buy a new high quality APO enlarging lens from Rodenstock or Schneider, is there any focus shift at all when stopping down? I doubt it, but just want to get some comments on this. In the literature from either company, they point to the diaphragm preset feature as useful because you can focus wide open and then stop down to your working aperture. But I was always taught to try and focus at the working aperture if possible to avoid even the slightest focus shift. So what do you think? With these top-line lenses is it safe to focus wide open?