  Most of today's lattice simulations are performed in the isospin symmetric
limit of the light quark sector. Mass reweighting is a technique to include
effects of isospin breaking in the sea quarks at moderate numerical cost. We
will give a summary of our recent results on fine lattices with light quark
masses and will show how light quark masses can be extracted by introducing
suitable tuning conditions for the bare mass parameters.
  In general the reweighting factor introduces additional fluctuations and thus
increases the statistical uncertainties. In the case of isospin reweighting
this factor is a ratio of fermion determinants. The stochastic evaluation of
the determinants potentially leads to stochastic noise in observables. We show
the quark mass and the volume dependence of these fluctuations.
