With a single lens reflex camera, when is it recommended to use an assessory dioptric corrector lens?

answer/// priests singing psalms and the congregation singing responses;

i believe the answer is  the similarity is that both are sacrifices which have the same purpose and sacrifice - jesus christ, but the difference is in offering of that sacrifice.

the similarity is that both are sacrifices which have the same purpose (repenting our sins) and sacrifice - jesus christ.  the difference is in the way of the offering that sacrifice. the sacrifice of jesus on the calvary is sacrifice of his actual living body and blood, but the sacrifice of the mass is the sacrifice of the jesus through bread (body) and blood (wine). in the sacrifice on calvary jesus was offered for us; while at mass he offers himself through us.