  We use the timescale distribution of ~3000 microlensing events measured by
the OGLE-III survey, together with accurate new made-to-measure dynamical
models of the Galactic bulge/bar region, to measure the IMF in the inner Milky
Way. The timescale of each event depends on the mass of the lensing object,
together with the relative distances and velocities of the lens and source. The
dynamical model provides statistically these distances and velocities allowing
us to constrain the lens mass function, and from this to infer the IMF.
Parameterising the IMF as a broken power-law, we find slopes in the main
sequence $\alpha_{\rm ms}=1.31\pm0.10|_{\rm stat}\pm0.10|_{\rm sys}$ and brown
dwarf region $\alpha_{\rm bd}=-0.7\pm0.9|_{\rm stat}\pm0.8|_{\rm sys}$ where we
use a fiducial 50% binary fraction, and the systematic uncertainty covers the
range of binary fractions 0-100%. Similarly for a log-normal IMF we conclude
$M_c=(0.17\pm0.02|_{\rm stat}\pm0.01|_{\rm sys})M_\odot$ and
$\sigma_m=0.49\pm0.07|_{\rm stat}\pm0.06|_{\rm sys}$. These values are very
similar to a Kroupa or Chabrier IMF respectively, showing that the IMF in the
bulge is indistinguishable from that measured locally, despite the lenses lying
in the inner Milky Way where the stars are mostly ~10Gyr old and formed on a
fast $\alpha$-element enhanced timescale. This therefore constrains models of
IMF variation that depend on the properties of the collapsing gas cloud.
