This problem just occurred to me while I was taking photos indoors: Light sources powered by mains fluctuate in intensity due to alternating current. In other words, a lightbulb will actually flicker on and off at 50Hz/60Hz (or, really, 100Hz/120Hz, since I guess it should be the absolute value) in Europe/US respectively. This is not noticeable to humans because it is faster than the refresh rate of our eyes, so we perceive a constant stream of light. So far, so good. But what if we take a camera and adjust its shutter speed to something significantly shorter, such as, say, 1/1000 seconds. We should be able to observe the fluctuation in that, in a succession of photos taken indoors, some will be brighter than others. I just tried this a number of times by taking photos of a lightbulb, and there is no visible difference (either in the photo or on the histogram) between any of the photos. Why is that?