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A total of four bright microlensing events (<26.5 mag AB) of individual luminous stars at redshift z=1-2 have recently been discovered in Hubble Space Telescope observations of galaxy-cluster gravitational lenses. The frequency of these events appears to be greater than expected comma and a sufficiently elevated rate would directly imply the existence of compact objects consisting of even 1-2% of dark matter. Galaxy-cluster fields also highly magnify the galaxies responsible for the reionization of the universe comma yet lens models require improvement to predict the magnifications of high-redshift galaxies accurately. Finally comma we lack constraints on the stellar luminosity function at z=1-2. We propose two epochs of unfiltered ACS WFC and WFC3 UVIS imaging of each of the six Hubble Frontier Field (HFF) galaxy clusters fields with a single-visit five-sigma limiting magnitude of 31 AB. The observations will detect of order 100 microlensing events at three-sigma and two dozen at five-sigma significance comma as well as an equal number of always-visible pairs of stellar images. If only 2% of dark matter consists of primordial black holes comma then we will detect five times as many microlensing events. The pairs of stellar images will stringently constrain lens models comma probe the abundance of low-mass dark-matter halos comma and evaluate the possibility that dark matter consists of ultra-light bosons. The relative rates of microlensing events across lensed galaxies will measure variations in their stellar luminosity functions. The proposed imaging program will comma finally comma identify low-redshift contaminants in James Webb Space Telescope Lyman-break galaxy samples comma and detect ultra-faint galaxies.