We present deep V and I photometry for the open cluster NGC 4815 and four surrounding Galactic fields down to a limiting magnitude V \sim 25 .
These data are used to study cluster spatial extension by means of star counts , and to derive the luminosity ( LF ) and mass function ( MF ) .
The radius turns out to be 3.6 \pm 0.3 arcmin at V=19.0 level , whereas the mass amounts at 880 \pm 230 m _ { \odot } down to V=20.8 . From the color-magnitude diagram , we obtain the LFs in the V and I bands , using both the standard histogram and an adaptive kernel .
After correction for incompleteness and field star contamination , the LFs have been transformed into the present day mass functions ( PDMF ) .
The PDMFs from the V and I photometry can be represented as a power-law with a slope \alpha = 3.1 \pm 0.3 and \alpha = 2.9 \pm 0.3 ( the [ ] MF in this notation has a slope \alpha = 2.35 ) respectively , in the mass range 2.5 \leq \frac { m } { m _ { \odot } } \leq 0.8 . Below this mass , the MF can not be considered as representative of the cluster IMF , as it is the result of the combined effect of strong irregularities in the stellar background , probable internal dynamical evolution of the cluster and/or interaction of the cluster with the dense Galactic field .
Unresolved binaries and mass segregation can only flatten the apparent derived IMF , so we expect that the real IMF must be steeper than the quoted slope by an unknown amount .