Top: measured magnitude differences (m_meas − m_true) versus “true” input magnitudes m_true, for two simulated images populated with extended sources (HRI has FWHM = 0.2′′ and HST H-band-like fluxes, LRI has FWHM = 1.66′′ and IRACch1-like fluxes), using three different methods: SExtractor dual-mode aperture, SExtractor dual-mode “best”, and t-phot. Vertical lines show the 5σ (dashed) and 1σ (dotted) limits of the simulated LRI. Bottom: magnification of the top panel, showing only t-phot results, colour-coded as a function of the covariance index. See text for more details.