Context :

Aims : We performed a deep wide field optical survey of the young ( \sim 100 - 150 Myr ) open cluster Blanco 1 to study its low mass population well down into the brown dwarf regime and estimate its mass function over the whole cluster mass range .

Methods : The survey covers 2.3 square degrees in the I and z -bands down to I \simeq z \simeq 24 with the CFH12K camera .
Considering two different cluster ages ( 100 and 150 Myr ) , we selected cluster member candidates on the basis of their location in the ( I,I - z ) CMD relative to the isochrones , and estimated the contamination by foreground late-type field dwarfs using statistical arguments , infrared photometry and low-resolution optical spectroscopy .

Results : We find that our survey should contain about 57 % of the cluster members in the 0.03 - 0.6 M _ { \odot } mass range , including 30-40 brown dwarfs .
The candidate ’ s radial distribution presents evidence that mass segregation has already occured in the cluster .
We took it into account to estimate the cluster mass function across the stellar/substellar boundary .
We find that , between 0.03 M _ { \odot } and 0.6 M _ { \odot } , the cluster mass distribution does not depend much on its exact age , and is well represented by a single power-law , with an index \alpha = 0.69 \pm 0.15 .
Over the whole mass domain , from 0.03 M _ { \odot } to 3 M _ { \odot } , the mass function is better fitted by a log-normal function with m _ { 0 } = 0.36 \pm 0.07 M _ { \odot } and \sigma = 0.58 \pm 0.06 .

Conclusions : Comparison between the Blanco 1 mass function , other young open clusters ’ MF , and the galactic disc MF suggests that the IMF , from the substellar domain to the higher mass part , does not depend much on initial conditions .
We discuss the implications of this result on theories developed to date to explain the origin of the mass distribution .