  We present new JHKL'M' photometry on the MKO system for a large sample of L
and T dwarfs identified from SDSS and 2MASS and classified according to the
scheme of Geballe et al. (2002). We have compiled a sample of 105 L and T
dwarfs that are uniformly classified and measured on a single photometric
system. The scattered JHK spectral indices and colors of L dwarfs are likely
caused by variations in the altitudes, distributions, and thicknesses of
condensate clouds. Scatter in the H-K colors of late T dwarfs probably reflects
the sensitivity of the K-band flux to pressure induced H_2 opacity, which
itself is sensitive to surface gravity. The M' luminosities of late-T dwarfs
are 1.5-2.5 times fainter than predicted under conditions of chemical
equilibrium. We have computed L_bol and T_eff for 42 L and T dwarfs whose
trigonometric parallaxes have been measured. We find that T_eff ~ 1450 K for
types L7-T4, which supports recent models that attribute the changing JHK-band
luminosities and spectral features across the L-T transition to rapid changes
in the condensate clouds over a narrow range of T_eff. We compute T_eff =
600-750 K for 2MASS 0415-0935 (T9), which supplants Gl 570D as the coolest
known brown dwarf.
