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Recently, we have developed and calibrated the Synthetic Field Method (SFM)to derive the total extinction through disk galaxies. The method is based onthe number counts and colors of distant background field galaxies that can beseen through the foreground object, and has been successfully applied to NGC4536 and NGC 3664, two late-type galaxies located, respectively, at 16 and 11Mpc. Here, we study the applicability of the SFM to HST images of galaxies inthe Local Group, and show that background galaxies cannot be easily identifiedthrough these nearby objects, even with the best resolution available today. Inthe case of M 31, each pixel in the HST images contains 50 to 100 stars, andthe background galaxies cannot be seen because of the intrinsic granularity dueto strong surface brightness fluctuations. In the LMC, on the other hand, thereis only about one star every six linear pixels, and the lack of detectablebackground galaxies results from a ``secondary'' granularity, introduced bystructure in the wings of the point spread function. The success of the SFM inNGC 4536 and NGC 3664 is a natural consequence of the reduction of theintensity of surface brightness fluctuations with distance. When the dominantconfusion factor is structure in the PSF wings, as is the case of HST images ofthe LMC, and would happen in M 31 images obtained with a 10-m diffraction-limited optical telescope, it becomes in principle possible to improve thedetectability of background galaxies by subtracting the stars in the foregroundobject. However, a much better characterization of optical PSFs than iscurrently available would be required for an adequate subtraction of the wings.Given the importance of determining the dust content of Local Group galaxies,efforts should be made in that direction.Comment: 45 pages, 10 Postscript figure

The Opacity of Nearby Galaxies from Counts of Background Galaxies. II. Limits of the Synthetic Field Method