Changes in ambient light intensity are detected by processing image data from an image capture device to determine, iteratively, a signal-to-noise ratio of the image data. The signal-to-noise ratio is a ratio of average to variance of pixel values for some of the pixels forming the image. A control output is generated, based on the signal-to-noise ratio, that is responsive to changes in ambient light intensity. The control output is used control a light source of a vehicle.