  We show that the optical force field in optical tweezers with elliptically
polarized beams has the opposite handedness for a wide range of particle sizes
and for the most common configurations. Our method is based on the direct
observation of the particle equilibrium position under the effect of a
transverse Stokes drag force, and its rotation around the optical axis by the
mechanical effect of the optical torque. We find overall agreement with theory,
with no fitting, provided that astigmatism, which is characterized separately,
is included in the theoretical description. Our work opens the way for
characterization of the trapping parameters, such as the microsphere complex
refractive index and the astigmatism of the optical system, from measurements
of the microsphere rotation angle.
