Estimated photo-zs vs. the spectroscopic redshifts for the source galaxies in our sample of real strongly lensed galaxy–galaxy systems. The error bars show the 1σ uncertainty on the estimated redshift, and the grey shaded region indicates where the estimated redshift is not an outlier. The outlier fraction for the 15 systems with Λ_i < 1 (blue points) is 20%, while the outlier fraction for the remaining 8 systems with Λ_i > 1 (yellow points) is 75%. The source photo-z estimation is much more accurate for the systems with Λ_i < 1, due to their significantly more accurate source colour measurements (compared to the systems with Λ_i > 1).