I have a question concerning the canonical topology of a topos.
By the definition I know, the canonical topology is given by universal effective epimorphisms as the covering sieves. Now I have read that for a topos, the canonical sieves are those that are jointly epimorphic. But I don't know how to prove that jointly epimorphic sieves are a covering in the canonical topology. Is the statement even true?
My problem is that for a morphism of topoi $f\colon T\to T'$ I want to show that $f^*$ is continuos with respect to the canonical topologies. If the above statement were true, it would be easy to prove.
Many thanks in advance