  In this paper we study the Geodesic Deviation Equation (GDE) in metric f(R)
gravity. We start giving a brief introduction of the GDE in General Relativity
in the case of the standard cosmology. Next we generalize the GDE for metric
f(R) gravity using again the FLRW metric. A generalization of the Mattig
relation is also obtained. Finally we give and equivalent expression to the
Dyer-Roeder equation in General Relativity in the context of f(R) gravity.
