Left: Joint marginal error contours at 1σ and 2σ on the cosmological parameters for a flat f(R) model with |f_R0| = 5 × 10⁻⁶ (HS6) vs. |f_R0| = 5 × 10⁻⁷ (HS7) in the pessimistic setting. Lighter colours and solid contours correspond to HS6, while darker colours and dashed empty contours correspond to HS7. In purple the spectroscopic GC_sp, in red the combination of photometric probes (GC_sp + WL + GC_ph + XC_ph). These two HS models only differ by their value of σ₈ and log₁₀|f_R0|. Therefore the contours look very similar in the standard cosmological parameters. For log₁₀|f_R0| the ellipses become much more elongated in HS7. Right: One-dimensional fully marginalised posteriors on the |f_R0| parameter for HS7 (dashed), HS6 (solid), and HS5 (dotted) after applying the transformation in Eq. (37). We use the same colour convention as before. The shaded vertical areas correspond to the 1σ confidence intervals, while the Gaussian curves extend up to 3σ. Using the full combination of photometric probes, HS7 and HS6 can be distinguished at 2σ, and both of them can be distinguished from HS5 at 3σ or more.