Photometric accuracy of the studied sample of star clusters from the Johnson et al. (2015) catalogue. The small grey dots mark the clusters rejected during the visual classification (see text) and not studied in the following analysis. Group 1 clusters (181 objects, large filled dots) have upper limits of the photometric errors indicated by long dashed lines in different filters. Group 2 clusters (1182 objects, large open dots) have photometric errors larger than these limits in at least one filter.