Context : Determination of the mass functions of open clusters of different ages allows us to infer the efficiency with which brown dwarfs are evaporated from clusters to populate the field .

Aims : In this paper we present the results of a photometric survey to identify low mass and brown dwarf members of the old open cluster Praesepe ( age 590 ^ { +150 } _ { -120 } Myr , distance 190 ^ { +6.0 } _ { -5.8 } pc ) from which we estimate its mass function and compare this with that of other clusters .

Methods : We performed an optical ( I _ { c } -band ) and near-infrared ( J and K _ { s } -band ) photometric survey of Praesepe covering 3.1 deg ^ { 2 } .
With 5 \sigma detection limits of I _ { c } = 23.4 and J = 20.0 , our survey is predicted to be sensitive to objects with masses from 0.6 to 0.05 M _ { \odot } .

Results : We photometrically identify 123 cluster member candidates based on dust-free atmospheric models and 27 candidates based on dusty atmospheric models .
The mass function rises from 0.6 M _ { \odot } down to 0.1 M _ { \odot } ( a power law fit of the mass function gives \alpha =1.8 \pm 0.1 ; \xi ( M ) \propto M ^ { - \alpha } ) , and then turns over at \sim 0.1 M _ { \odot } .
This rise agrees with the mass function inferred by previous studies , including a survey based on proper motion and photometry .
In contrast , the mass function differs significantly from that measured for the Hyades , an open cluster with a similar age ( \tau \sim 600 Myr ) .
Possible reasons are that the clusters did not have the same initial mass function , or that dynamical evolution ( e.g .
evaporation of low mass members ) has proceeded differently in the two clusters .
Although different binary fractions could cause the observed ( i.e .
system ) mass functions to differ , there is no evidence for differing binary fractions from measurements published in the literature .
Of our cluster candidates , six have masses predicted to be equal to or below the stellar/substellar boundary at 0.072 M _ { \odot } .

Conclusions :