  In a variety of astronomical situations, there is a relatively high
probability that a single isolated lens will produce a detectable event. The
high probability is caused by some combination of a large Einstein angle, fast
angular motion, and a dense background field. We refer to high-probability
lenses as mesolenses. The mesolensing regime is complementary to the regime in
which the optical depth is high, in that it applies to isolated lenses instead
of to dense lens fields. Mesolensing is complementary to microlensing, because
it is well suited to the study of different lens populations, and also because
different observing and analysis strategies are required to optimize the
discovery and study of mesolenses. Planetary and stellar masses located within
1-2 kpc are examples of mesolenses. We show that their presence can be detected
against a wide variety of background fields, using distinctive signatures of
both time variability and spatial effects. Time signatures can be identical to
those of microlensing, but can also include baseline jitter, extreme apparent
chromaticity, events well fit by lens models in which several independent
sources are simultaneously lensed, and sequences of events. Spatial signatures
include astrometric lensing of surface brightness fluctuations, as well as
patterns of time variability that sweep across the background field as the lens
moves. Wide-field monitoring programs, such as Pan-STARRS and LSST, are
well-suited to the study of mesolensing. In addition, high resolution
observations of the region behind a known mass traveling across a background
field can use lensing effects to measure the lens mass and study its
multiplicity.
