We present deep 450 and 850 mu m SCUBA images of the nearby spiral galaxy NGC 7331. Using the submillimeter emissivity inferred from COBE observations of Milky Way dust, we convert our SCUBA images into maps of optical depth. The opacity derived in this way is quite low at the visible limit of NGC 7331 (tau B<= 0.22 at the R25 radius for the disk seen face-on). In a similar fashion, we exploit SCUBA and ISOPHOT images of a further 10 galaxies and, collectively, these data indicate tau B=0.1-0.2 at the R25 radius. Our constraints on disk opacity are fed into a simulation of how light emanating from high redshifts is attenuated by foreground spirals. In making this calculation, we consider the possibility that galactic disks may have also contained different dust masses in the past. We estimate that less than 10% of the light emitted by Hubble Deep Field galaxies fails to reach the B-band observer due to intervening spirals.