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We present the first results from our high-precision infrared (IR) astrometryprogram at the Canada-France-Hawaii Telescope. We measure parallaxes for 83ultracool dwarfs (spectral types M6--T9) in 49 systems, with a medianuncertainty of 1.1 mas (2.3%) and as good as 0.7 mas (0.8%). We provide thefirst parallaxes for 48 objects in 29 systems, and for another 27 objects in 17systems, we significantly improve upon published results, with a median (best)improvement of 1.7x (5x). Three systems show astrometric perturbationsindicative of orbital motion; two are known binaries (2MASSJ0518-2828AB and2MASSJ1404-3159AB) and one is spectrally peculiar (SDSSJ0805+4812). Inaddition, we present here a large set of Keck adaptive optics imaging that morethan triples the number of binaries with L6--T5 components that have bothmulti-band photometry and distances. Our data enable an unprecedented look atthe photometric properties of brown dwarfs as they cool through the L/Ttransition. Going from \approxL8 to \approxT4.5, flux in the Y and J bandsincreases by \approx0.7 mag and \approx0.5 mag, respectively (the Y- and J-band"bumps"), while flux in the H, K, and L' bands declines monotonically. Thiswavelength dependence is consistent with cloud clearing over a narrow range oftemperature, since condensate opacity is expected to dominate at 1.0--1.3micron. Interestingly, despite more than doubling the near-IR census of L/Ttransition objects, we find a conspicuous paucity of objects on thecolor--magnitude diagram just blueward of the late-L/early-T sequence. This"L/T gap" occurs at MKO(J-H) = 0.1--0.3 mag, MKO(J-K) = 0.0--0.4 mag, andimplies that the last phases of cloud evolution occur rapidly. Finally, weprovide a comprehensive update to the absolute magnitudes of ultracool dwarfsas a function of spectral type using a combined sample of 314 objects.

The Hawaii Infrared Parallax Program. I. Ultracool Binaries and the L/T Transition