The formula of Gullstrand gives the Vergence complete system according to vergencies $V\_\; \{1\}$ and $V\_\; \{2\}$ of the two systems which compose it, of the index $n$ of the medium which separates them and the optical distance E: $e=\; \backslash \; overline\; \{H\_\; \{1\}\; \text{'}H\_\; \{2\}\}$ between their principal plans:

$V=V\_\; \{1\}\; +V\_\; \{2\}\; -\; \backslash \; frac\; \{E\}\; \{N\}\; V\_\; \{1\}\; *\; V\_\; \{2\}$

When a thin lens is considered, the principal plans object and image are confused, E are null and there is $V=V\_\; \{1\}\; +V\_\; \{2\}$.

It is used, for example, in the construction of objective photographic of short and long focal distances in order to respectively increase or decrease the obstruction by it by it. (Assemblies " Rétrofocus " and " Teleobjective ").