Upper panel: cumulative R-band magnitude distribution of optical counterparts of X-ray sources in XMM-COSMOS and CDF-N fields for the X-ray limiting fluxes of 10^-14, 10^-15, and 10^-16 erg s^-1/cm². Lower panel: cumulative number density of galaxies as a function of their R-band magnitude. The horizontal dashed lines show the number of “resolution elements” per deg² corresponding to angular error circle diameters of 1′′, 5′′, and 15′′. The approximate limiting magnitudes for various existing and future optical imaging surveys are also displayed.