Hello @Omnistic
The answer is simply that closing the diaphragm increases the effective focal depth of the condenser (i.e. the range, in depth, over which objects appear in focus). You can explain this in terms of increased spatial coherence or you can model it in terms of a narrower angular aperture - however you model it, the fact is that focal depth is increased with smaller aperture. So your condenser trick helped you to find the edge of the diaphragm by bringing the edge of the diaphragm (that was wildly out of focus - i.e. out of the range of the depth of focus - with a wide condenser aperture) into the (now greatly extended) focal field that you caused to be extended by closing the condenser aperture. You brought the field stop ‘into focus’ not by moving the field stop into the focal range of the condenser but by extending the focal range of the condenser till it reached the field stop.
This is also why you should always do your final focussing of the field stop with the condenser aperture opened as far as it can go (to match the objective NA) - because only then will you have the best resolution of focus (i.e. the focal depth will be the narrowest).
The ‘hotspots’ I mention in my video refer to localised areas of brightness and lower contrast seen in the image plane due to reflections of stray light off the insides of the objective and other microscope tubes. So, no, it is nothing to do with what is seen here.
Your last picture illustrates what I meant by various ‘shadow images’ in my first reply.
Note: You should always do your final field diaphragm focussing with a sample on the stage and in focus because the glass of the slide and mountant and coverslip will significantly alter the correct focal distance. The idea with focussing the field stop is that the field stop must be confocal with your specimen (as I explained in my first Köhler video).
All the best.
P