We present the analysis of the faint galaxy population in the Advanced Camera for Surveys ( ACS ) Early Release Observation fields VV 29 ( UGC 10214 ) and NGC 4676 .
These observations cover a total area of 26.3 arcmin ^ { 2 } , and have depths close to that of the Hubble Deep Fields in the deepest part of the VV 29 image , with 10 \sigma 
detection limits for point sources of 27.8 , 27.6 and 27.2 
AB magnitudes in the g _ { F 475 W } , V _ { F 606 W } and I _ { F 814 W } 
bands respectively .

Measuring the faint galaxy number count distribution is a difficult task , with different groups arriving at widely varying results even on the same dataset .
Here we attempt to thoroughly consider all aspects relevant for faint galaxy counting and photometry , developing methods which are based on public software and that are easily reproducible by other astronomers .
Using simulations we determine the best SExtractor parameters for the detection of faint galaxies in deep HST observations , paying special attention to the issue of deblending , which significantly affects the normalization and shape of the number count distribution .
We confirm , as claimed by Bernstein , Freedman and Madore ( 2002a , b ; hereafter BFM ) , that Kron-like magnitudes , such as the ones generated by SExtractor , can miss more than half of the light of faint galaxies , what dramatically affects the slope of the number counts .
We show how to correct for this effect , which depends sensitively not only on the characteristics of the observations , but also on the choice of SExtractor parameters .

We present catalogs for the VV 29 and NGC 4676 fields with photometry in the F 475 W,F 606 W and F 814 bands .
We also show that combining the Bayesian software BPZ with superb ACS data and new spectral templates enables us to estimate reliable photometric redshifts for a significant fraction of galaxies with as few as three filters .

After correcting for selection effects , we measure slopes of 0.32 \pm 0.01 for 22 < g _ { F 475 W } < 28 , 0.34 \pm 0.01 for 22 < V _ { F 606 W } < 27.5 and 0.33 \pm 0.01 for 22 < m _ { F 814 W } < 27 .
The counts do not flatten ( except perhaps in the F 475 W filter ) , up to the depth of our observations .
Our results agree well with those of BFM , who used different datasets and techniques , and show that it is possible to perform consistent measurements of galaxy number counts if the selection effects are properly considered .
We find that the faint counts m _ { AB } > 25.5 can be well approximated in all our filters by a passive luminosity evolution model based on the COMBO-17 luminosity function ( \alpha = -1.5 ) , with a strong merging rate following the prescription of Glazebrook et al .
( 1994 ) , \phi ^ { * } \propto ( 1 + Qz ) , with Q = 4 .