## Contraction of curves on surfaces

Assume we have a surface $S$ (smooth if you want), and a map $f: S \to V$ that contracts a curve $C \subset S$. What condition would give a factoring of $f$ through a contraction $c : V \to V'$ contracting $X$ and is isomorphic otherwise?

What about the special case where $S$ is a flat family of curves and $X$ is the central fiber (all other fibers are assumed smooth). What criterion would give a factoring of $f$ through the contraction?

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