  The effects of seeing on Sersic r^{1/n} profile parameters are extensively
studied using a Moffat function. This analytical approximation to the point
spread function (PSF) is shown to provide the best fit to the PSF predicted
from atmospheric turbulence theory when beta=4.765. The Moffat PSF is
additionally shown to contain the Gaussian PSF as a limiting case
beta->infinity. The Moffat function is also shown to be numerically well
behaved when modelling narrow PSFs in HST images. Seeing effects are computed
for elliptically symmetric surface brightness distributions. The widely used
assumption of circular symmetry when studying the effects of seeing on
intrinsically elliptical sources is shown to produce significant discrepancies
with respect to the true effects of seeing on these sources. A prescription to
correct raw (observed) central intensities, effective radii, index n and mean
effective surface brightness is given.
