When much younger I was horrified when I had dust bunnies in the viewfinder. On the FM2 as the F4, I spent time blowing up the ground glass or trying to access the prism. Sometimes I succeed, sometimes not... A Nikon shop guy told me to use vacuum instead of blowing (an inverter you put on the usual canned air).
With age and zen training I learned to ignore those bunnies... And they really disappeared (getting somewhere else)
Recently I had one in my Df viewfinder, just before my holiday trip... I was annoyed but stayed calm (with the upper lip rigid
). Arriving in Malta, I found no trace of the culprit, haven't seen any since (there must be a huge stock of those hidden somewhere) !
The more we fret about those specks, the more they seem to cling and stay in our field of vision... If you ignore them, they'll go in hiding