As a consequence of the electron continuity equation, ionospheric conductance derived from photometric measurements of the aurora represents a root-mean-square over the field of view and exposure time of the photometer, and not a true average. The magnitude of the discrepancy between these estimates depends on the statistical variance of the electron density within the sampling window and is therefore strongly dependent on both the resolution of the sensor and the activity of the aurora. We use high-resolution optical and incoherent scatter radar measurements to quantify the relationship among the instantaneous (true) conductance, the average conductance, and the photometrically derived conductance during an auroral substorm over Sondrestrom, Greenland. For a 36 s exposure time (typical for the ultraviolet imager (UVI) sensor on the Polar satellite), we show that the photometric estimate can be biased from the average value by 40% for Hall and 20% for Pedersen; the mean, in turn, can misrepresent the true conductance by more than 150% for Hall and 100% for Pedersen, owing to undersampling small-scale variability. We develop two schemes for correcting (in the former case) and understanding (in the latter case) these effects using conjunctive ground-based diagnostics.