Automatically dimming rearview mirrors allow a vehicle operator to view a scene generally behind the operator at a reflectivity level automatically adjusted to ambient light and glare levels. An automatically dimming mirror includes a dimming element having a variably reflective surface, the degree of reflectivity based on a control signal. An ambient light sensor is positioned to receive light from a region generally in front of the vehicle. The ambient light sensor outputs a discrete ambient light signal based on the amount of light incident on the ambient light sensor over an integration period. A glare sensor is positioned to view the scene generally behind the vehicle operator. The glare sensor outputs a discrete glare signal based on the amount of light incident on the glare sensor over an integration period. A dimming logic determines an ambient light level based on the ambient light signal. The glare integration period is determined based on the ambient light level. The glare signal resulting from the glare integration period determines a mirror glare level used to set the dimming element control signal. The use of variable integration times increases the dynamic range of the light sensors, improving the mirror dimming response.