  We explore the limits of photometric reductions of crowded stellar fields
observed with the Wide Field and Planetary Camera 2 on board the Hubble Space
Telescope. Two photometric procedures, based on the DoPHOT and DAOPHOT/ALLFRAME
programs are tested, and the effects of crowding, complex sky background and
cosmic-ray contamination are discussed using an extensive set of artificial
star simulations. As a specific application of the results presented in this
paper, we assess the magnitude of photometric biases on programs aimed at
finding Cepheids and determining distances. We find that while the photometry
in individual images can be biased too bright by up to 0.2 mag in the most
crowded fields due to confusion noise, the effects on distance measurements
based on Cepheid variables are insignificant, less than 0.02 mag (1% in
distance) even in the most problematic cases. This result, which is at odds
with claims recently surfaced in the literature, is due to the strict criteria
applied in the selection of the variable stars, and the photometric cross
checks made possible by the availability of multiple exposures in different
filters which characterizes Cepheid observations.

Astrophysics