  The surface density of populations of galaxies with steep/shallow source
counts is increased/decreased by gravitational lensing magnification. These
effects are usually called `magnification bias' and `depletion' respectively.
However, if sources are demagnified by lensing, then the situation is reversed,
and the detectable surface density of galaxies with a shallow source count, as
expected at the faintest flux densities, is increased. In general, demagnified
sources are difficult to detect and study: exquisite subarcsecond angular
resolution and surface brightness sensitivity are required, and emission from
the lensing object must not dominate the image. These unusual conditions are
expected to be satisfied for observations of the dense swarm of demagnified
images that could form very close to the line of sight through the centre of a
rich cluster of galaxies using the forthcoming submillimetre-wave Atacama Large
Millimeter Array (ALMA) interferometer. The demagnified images of most of the
background galaes lying within about 1 arcmin of a rich cluster of galaxies
could be detected in a single 18-arcsec-diameter ALMA field centred on the
cluster core, providing an effective increase in the ALMA field of view. This
technique could allow a representative sample of faint, 10-100 microJy
submillimetre galaxies to be detected several times more rapidly than in a
blank field.
