I've had my newer microscopes for nearly a year now without coming up with a proper dark-field image. It was always just a little off. First I tried half inch. Way too small but made it easy to center the condenser. Next I went to 5/8 inch. Better. Had dark-field at 10 and 20x but not at 4. Today I went up to 3/4inch. Wow! Dark-field possible from 4 to 40x. That's a really good result and I found the secret that works with my scope. That is carefully adjusting both the height of the diaphragm and the field condenser for each magnification.
Now I can finally take some good photos.
How frustrating this hobby can be yet how rewarding.
Greg