It says in the Gemorah in Chagigah Daf 16a
המסתכל בג' דברים עיניו כהות בקשת ובנשיא ובכהנים ...בנשיא דכתיב ונתת מהודך עליו
That the eyes of anyone who looks at the "Nasi" (a leader of the Jewish people) become "dim".
How does this Gemorah apply nowadays (if it does at all) in regards to looking at a Rav, Rebbe, Rosh Yeshiva, Teacher, Gadol HaDor, etc. Anyone that would assume a role of "leader" in Klal Yisrael (whether it be of a Kehillah, Chassidus, Yeshiva, etc.)