Setting magnification with the ES-1 extension works because you are using a DX camera. The lens is never more than 1:1.5, therefore autofocus is feasible. At this point (e.g., the sample on the right), the frame would fill the image. If you were using an FX camera and wished maximize the magnification, auto-focus would not be an option since the ratio would be close to 1:1. AF might not be the best choice in your situation either if the slide were warped and the main subject off center. I use manual focus, even with an AF macro, and check it from time to time. It doesn't seem to change unless you accidentally disturb the setup In the example on the left (cropped), you are throwing away half of your pixels. That's not a good thing to do if you want the best resolution, comparable to a Nikon film scanner. I would expect the 40 mm lens to be diffraction limited above f/8. If you aren't seeing it, then diffraction is not the limiting factor in your setup. The root cause is probably film flatness (lack thereof). The film strip holder that comes with the ES-2 has cross bars between each frame. This should hold the film flat unless it is seriously curled or cupped. Storage in archival pages shortly after processing helps in that regard. It is gratifying to see that the ES-1 has enough extension to reduce the magnification as much as the image on the left. Hopefully the ES-2 will have similar capabilities.