The origin and evolution of brown dwarfs remain a fundamental open question. For open clusters at different ages and environments the initial/present mass function is one of the main observational signatures which can be used to investigate this question. Here I will present my work on IC2391 and the Praesepe open clusters, including some ongoing projects. From our radial survey of the stellar and substellar population of IC2391 (age ~ 50 myr), we observe a radial variation in the mass function over the range 0.072-0.3Msun, but no significant variation in the mass function below the substellar boundary. This lack of radial variation for low masses is what we would expect with the ejection scenario for brown dwarf formation. In the Praesepe open cluster (age ~ 590 Myr), we observe that the mass function of Praesepe rises from 0.8Msol down to 0.1Msol and then turns-over at ~0.1Msol. The rise observed is in agreement with the mass function derived by previous studies, but we observe a significant difference with the mass function of the Hyades open cluster. This is surprising, since Praesepe and the Hyades share a comparable age, size, mass, metallicity and binary fraction.