Erratic light spot in optical projection I once had an old microscope, that included a projection screen that could be mounted instead the eyepiece. It showed a quite decent palm-sized image. 
Now I have a new microscope, and removed the eyepiece to mount a single lens reflex camera. However, the image projected on the ground glass, and also on the imaging sensor , shows a brightened fluff around the image center. By moving the microscope in respect to the camera, the spot can be moved a little, but would always stay inside the field of view. What is the reason of this effect ? 
 A: The type of microscope and illumination scheme are not described.  However, in general the microscope produces a real image down inside the tube just below the eyepiece.  For attaching a camera, a separate port is sometimes provided such that the camera image sensor can be placed at this real image plane, or at a relayed real image plane.  With an SLR type camera, however, it might be difficult to get the ground glass/ image plane down this low just by removing the eyepiece -- it depends on the microscope design.
By grossly adjusting the objective focus, it may be possible to move the nominal image plane up far enough beyond its usual position to reach the SLR image plane, but at that point both the microscope imaging and illumination optics may be operating very far from their design points, and this could possibly explain the observation of "fluff".
Absent an engineered camera adaptor, another approach which can produce usable results is to leave the eyepiece in place and place a camera (with its lens focussed at infinity) as close as possible to the eyepiece.  The camera objective aperture should be kept fairly wide open.
