Light bending in $Fleft[g(square)Rright]$ extended gravity theories

We show that in the weak field limit the light deflection alone cannot distinguish between different $R + F[g(square)R]$ models of gravity, where $F$ and $g$ are arbitrary functions. This does not imply, however, that in all these theories an observer will see the same deflection angle. Owed to the need to calibrate the Newton constant, the deflection angle may be model-dependent after all necessary types of measurements are taken into account.