Mean surface brightness μ₃₀₀ in the [0.5–2] keV band vs. M₅₀₀ for the XUCS sample. Clusters with a low surface brightness are spread over three out of the four mass quartiles. The large scatter between these two quantities strongly reduces the impact of the differences in mass on the brightness distribution. We plot the measurements of μ₃₀₀ from two different telescopes for three clusters.