The comparison between different estimations of the quantity FP = log R_e − 0.27 ⟨ SB_e ⟩  for all galaxies with HST photometry and a measured velocity dispersion. We plot ΔFP(a_eb_e)^1/2 = log ((a_eb_e)^1/2/R_e) − 0.27(⟨ SB_e ⟩ (a_eb_e)^1/2) −  ⟨ SB_e ⟩ ) (top), ΔFP = log (R_e(Ser)/R_e) − 0.27(⟨ SB_e ⟩ (Ser) −  ⟨ SB_e ⟩ ) (middle), and ΔFP = log (R_e(VLT)/R_e) − 0.27(⟨ SB_e ⟩ (VLT) −  ⟨ SB_e ⟩ ) (bottom) as a function of 1 − b_e/a_e. Symbols and color coding are as in Fig. 4.