Joint marginal error contours at 1σ and 2σ on the cosmological parameters for a flat f(R) model with |f_R0| = 5 × 10⁻⁶ in the optimistic setting. Purple is for GC_sp, blue for WL, orange for the combination GC_sp + WL, and yellow for all the photometric probes including their cross-correlation XC_ph, combined with GC_sp, namely GC_sp + WL + GC_ph + XC_ph. While the WL probe is unable to properly constrain the Hubble parameter h and the primordial slope n_s on its own, the orthogonality of the contours for GC_sp and WL in the subspaces involving h and n_s, helps lift degeneracies and further improves the fully marginalised constraints on log₁₀|f_R0|, when probe combinations are used.