Magnitudes within the SDSS are expressed as inverse
hyperbolic sine (or "asinh") magnitudes, sometimes referred to informally as luptitudes .
The transformation from linear flux measurements to asinh magnitudes
is designed to be virtually identical to the standard astronomical
magnitude at high signal-to-noise ratio, but to switch over to linear behavior at low S/N and even at negative values of flux,
where the logarithm in the Pogson magnitude fails. Details can be found in the Lupton et al. 1999 AJ paper.