  We present a statistical method to derive the mass functions of open clusters
using sky survey data such as the 2 Micron All Sky Survey and the Guide Star
Catalogue. We have used this method to derive the mass functions in the
stellar/substellar regime of three young, nearby open clusters, namely IC 348,
Sigma-Orionis and Pleiades. The mass function in the low mass range (M < 0.5
solar mass) is appreciably flatter than the stellar Salpeter function for all
three open clusters. The contribution of objects below 0.5 solar mass to the
total mass of the cluster is ~40% and the contribution of objects below 0.08
solar mass to the total is ~4%.
