I believe trypan blue does work by exclusion but methylene blue and its kin by redox.
And I think we are talking of the same thing WRT the microscopes. Even though most oculars are going to be 10x (to go higher would run you into "empty resolution" unless you have objectives of high NA , i.e. oil immersion) you do have to know what the ocular power is in order to be able to specify the actual image magnification.
