z-band (AB) source counts from COSMOS and XXL-S for sources with S_1.8 GHz >  200 μJy and at z <  1.3 in both surveys. The y-axis is the number of sources per 0.5 mag per square degree and the x-axis is z-band apparent magnitude (AB) in bins of 0.5 mag. The error bars for each bin are calculated as deg⁻². The COSMOS source counts are within 3σ of the XXL-S source counts for each magnitude bin, but for 20 <  m_z <  22.5 the COSMOS counts are systematically higher the XXL-S counts, indicating that XXL-S potentially has less than ∼100% optical completeness for m_z >  20.