In the 2-topos Cat, the pair of classes of morphisms (essentially surjective functors, fully faithful functors) form a factorization system in a 2-category. This factorization system can also be restricted to the (2,1)-topos Grpd.
In fact, an analogous factorization system exists in any regular 2-category and any (2,1)-exact (2,1)-category, including any Grothendieck 2-topos or (2,1)-topos; see here?.
The factorization of a functor in this factorization system is the construction of its full image.
(eso, fully faithful)-factorization system