  Holographic microscopy has developed into a powerful tool for 3D particle
tracking, yielding nanometer-scale precision at high frame rates. However,
current particle tracking algorithms ignore the effect of the microscope
objective on the formation of the recorded hologram. As a result, particle
tracking in holographic microscopy is currently limited to particles well above
the microscope focus. Here, we show that modeling the effect of an
aberration-free lens allows tracking of particles above, near, and below the
focal plane in holographic microscopy, doubling the depth of field. Finally, we
use our model to determine the conditions under which ignoring the effect of
the lens is justified, and in what conditions it leads to systematic errors.
