Comparison of I-band magnitudes computed with the differential photometry (I_diff) method and the “all-sky” method (used in the current work). The comparison is based on two Coma fields (C5 and D5). The difference between I-magnitudes calculated with these methods increases monotonically with growing R − I and reaches  ~ 0.25 at R − I = 2.5. This difference could be due to different passbands of the TLS and Cousins/Bessel photometric systems and partly because the colours of the Landolt standards are not red enough for BD/VLM objects.