I recently replaced the head of my microscope from a monocular to a trinocular. I primarily wanted the new head so I could mount the camera and look through the eyepiece at the same time. However, I was getting an annoying "ring of light" in all my pictures from the camera port.
This was the place to look, a brief scan of the technical forums found me 2 workable answers. Putting a small ring of paper in the photo eyepiece tub got rid of it.
In a bit of serendipity, I saw jc maccagno's Closterium thread, and I found my own this evening, presented here without unwanted lighting artifacts:
