The Zeiss Ultracondenser 1.2-1.4 gives wonderful high contrast darkfield when oiled to the slide but over a limited field - it fills the field at 160X and above but not at 100x. It works ok at 40/0.75 but is becoming prone to fog (due to cover glass thickness?), and higher mag objectives more so, plus they need an iris to stop down to 0.8-0.9. So in use it is quite limited in mag range. This is a picture of the illuminated field viewed at 63x (I boosted the shadows so the edge of the field is just visible).
This means lower powers are of limited use for scanning the slide. The other problem I have with the condenser is that I can see the built-in illuminator in my peripheral vision below the eyepieces, which is both distracting and reduces contrast. Looking around on my desk for something simple to shade the illuminator, I picked up a short length of aluminium tube which worked perfectly. Unexpectedly it also markedly increased the size of the illuminated field! So the next day I had a dig around and made up a piece which fitted neatly around the illuminator and long enough to fill in the gap to the condenser while being easy to insert and remove. I polished the inside as best I could but might do more.
Here is the illuminated field now almost filling the view at 63x (again, with some contrast reduction). Note the central area is almost the same:
It isn't perfectly evenly illuminated but it does make scanning the slide at low mags entirely feasible. You might assume that the penalty would be reduced contrast especially at higher NAs, but I can see very little difference with the all the objectives I tried - 10/0.3, 16/0.4, 25/0.45 and 40/0.75. And in any case, it can easily be removed if causing a problem whereas changing condensers is not really feasible.
In effect it creates a much larger light source, which means a bigger illuminated field. But presumably the light path through the condenser mirrors still blocks light from reaching the objective directly. The tube does not need to be as long as mine for the effect to be created - worth a try!