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The first submm-wave surveys for distant galaxies exploited both the weak to moderate gravitational lensing magnification, by about a factor of 2-3, experienced throughout the inner few square arcminutes of rich foreground cluster of galaxies at a moderate redshift in the range z appeq 0.2-0.4 (Blain, 1997) and the greater magnification produced along critical lines for much smaller areas of the background sky. A 5-arcmin2 SCUBA field centered on a moderate redshift cluster includes both these regions, enhancing the flux density from all high-redshift background galaxies (Smail et al., 1997). More, and in some cases deeper, SCUBA images of clusters have been taken (Smail et al., 2002; Chapman et al., 2002a; Cowie et al., 2002), especially in Abell 2218, where a multiply-imaged source has been detected (van der Werf and Kraiberg Knudsen, 2001). Whether the magnification acts to increase the surface density of background galaxies on the sky depends on the form of their counts. Lensing by both galaxies and clusters could have significant applications in future submm surveys, especially those sampling the steep counts of bright submm galaxies in wide fields, including the all-sky survey from Planck Surveyor (Blain, 1998), and surveys using BLAST, Herschel-SPIRE and SCUBA-II (Tables 1 and 2) covering many tens of square degrees.