An extensive PASP paper (Sirianni et al. 2005) that discusses the ACS Photometric Calibration and Photometric Transformations is now available. here
The ACS filters do not have an exact counterpart in any other 'standard' filter set and we strongly urge users to refer to ACS photometric results in a system based on the ACS filters themselves. Sensitivity curves can be used to translate physical quantities from theoretical models into the ACS observational system.
Nevertheless, the transformation of observed magnitudes to a standard system is sometimes the only way to properly compare data taken with different instruments. In the submitted paper we provide the coefficients to translate ACS photometry to other well known photometric systems. Coefficients are provided for transformations between WFC and HRC, providing internal consistency. We also provide conversion from ACS to WFPC2 and from ACS to the Landolt UBVRI system. In general, two sets of transformations are available: the first based on observation of two star clusters and the second based on synthetic photometry. We warn that all of these transformations should be used with extreme caution. The paper discusses the accuracy of these transformations and their sensitivity to details of the spectra being transformed, such as extreme color (intrinsic or due to reddening or redshift) or unusual SED.
The paper also contains a photometric calibration cookbook and detailed information about the aperture correction to be applied to point source photometry when using the acs zeropoints.