Formal uncertainties of the optical positions of ICRF sources versus magnitude and number of observations per source. σ_{pos, max} refers to the semi-major axis of the error ellipse in position. Left: position uncertainty versus the G magnitude for 2152 sources with calibrated photometry. Right: position uncertainty of the 2191 sources versus the number of detections (field-of-view transits) used in the astrometric solution. Symbols show defining sources as blue squares, non-VCS sources as red crosses, and VCS-only sources as pale grey plus signs.