  When an ultrashort laser pulse is tightly focused to a size approaching its
central wavelength, the properties of the focused spot diverge from the
diffraction limited case. Here we report on this change in behavior of a
tightly focused Petawatt class laser beam by an F/1 off-axis paraboloid (OAP).
Considering the effects of residual aberration, the spatial profile of the near
field and pointing error, we estimate the deviation in peak intensities of the
focused spot from the ideal case. We verify that the estimated peak intensity
values are within an acceptable error range of the measured values. With the
added uncertainties in target alignment, we extend the estimation to infer
on-target peak intensities of $\geq$ 10$^{22}$ W/cm$^{2}$ for a target at the
focal plane of this F/1 OAP.
