  The ``Synthetic Field Method'' (SFM) was introduced by Gonzalez et al. (1998)
to calibrate numbers of distant galaxies as a probe of extinction in a
foreground spiral disk. Gonzalez et al. (2003) studied the effect of the
foreground disk on these numbers using simulations of current and future
instruments for fields in the LMC, M31 and NGC 4536, a galaxy in Virgo. They
concluded that: (1) the brighter centers of disks were unsuitable, (2) the
granularity of the disk at a fixed surface brightness is the limiting factor in
the detection of distant galaxies, and (3) the optimum distance for
measurements would be that of the Virgo cluster for the current instruments on
board HST. At this distance the foreground disk is smoothed with distance,
improving detection of distant background galaxies. Holwerda et al. (2005a)
automated the SFM and Holwerda et al. (2005b) applied it to a large set of
WFPC2 fields. In this paper, the quality of the extinction measurement in these
fields is compared to their distance, granularity, surface brightness and
structure. The average surface brightness of the of a field is shown to
directly influence the accuracy of the SFM. This restricts meaningful
measurements to the disks of spiral galaxies. Large structures such as spiral
arms have a similar effect. The granularity or small scale structure in a field
influences the detection of distant galaxies, limiting the SFM measurements in
nearby disks. From the trends in the accuracy and maximum practical
field-of-view considerations, the minimum and maximum distance for SFM
application, approximately 5 and 35 Mpc respectively. Using the same instrument
and detection method, the relations with SFM parameters and field
characteristics can be used to forgo the synthetic fields altogether.
