> which i assume for most devices varies a lot Most will flicker at a multiply of the power line frequency (60 or 50 Hz depending where you live). It's so com

> which i assume for most devices varies a lot Most will flicker at a multiply of the power line frequency (60 or 50 Hz depending where you live). It's so common, that most digital cameras have a setting to set the power line frequency, so they can reduce the flickering in the footage. Incidentally - many alternating current engines are also working with multiples of the powerline frequency, because it's easier that way.