  A critical analysis and comparison of different methods for obtaining point
spread function (PSF) photometry are carried out. Deep ACS observations of
NGC3370 were reduced using four distinct approaches. These reductions explore a
number of methodological differences: software packages (DAOPHOT and DOLPHOT),
input images (individual and stacked frames), PSF models (synthetic and
empirical), and aperture correction methods (automatic and manual). A
comparison of the photometry leads to the following results: 1) Photometric
incompleteness between individual reductions shows only a minimal difference
(<10%). 2) Statistical errors are 20% to 30% smaller for DAOPHOT runs on
stacked frames than DOLPHOT runs on individual frames. 3) Statistical errors
assigned directly by the photometry codes are 25% to 50% smaller than the
errors measured from artificial star tests. 4) Systematic errors are magnitude
dependent and become larger at the faint end, at the level of $\sigma_s\sim0.1$
mag. 5) The automatic aperture correction routines in DOLPHOT result in a
significant systematic error ($\sigma_s \sim 0.05$ mag). 6) Individual
reductions agree well at the 0.02 mag level when the systematic errors are
properly corrected through artificial star tests. The reasonable agreement
between the reductions leads to important implications that i) the reduction
dependent errors can be reduced to a 1% level in the luminosity distance scale,
and ii) the stacked frame photometry can be a good means to study non-variable
stars in external galaxies.
