In this paper we present a method to automatically control the exposure of multispectral cameras. To this end, we use an input image taken using camera pre-sets. This pre-set exposure time and the input image are then used to recover an updated exposure time making use of the spectral power. This yields a setting where the spectral power is computed using a photopic function. Moreover, the exposure time can then be recovered using a bounded least squares optimisation effected upon the histogram equalised spectral power. We illustrate the utility of the method for exposure time recovery on multispectral images using a calibration chart, where we compare the timings yielded by our method against those recovered through manual photometric calibration. We also show results on real-world imagery under several lighting conditions.