  A method to set constraints on the parameters of extended theories of
gravitation is presented. It is based on the comparison of two series
expansions of any observable that depends on H(z). The first expansion is of
the cosmographical type, while the second uses the dependence of H with z
furnished by a given type of extended theory. When applied to f(R) theories
together with the redshift drift, the method yields limits on the parameters of
two examples (the theory of Hu and Sawicki (2007), and the exponential gravity
introduced by Linder (2009)) that are compatible with or more stringent than
the existing ones, as well as a limit for a previously unconstrained parameter.
