Exposure Lights: Photography Tips. Digital Photography Tips: Digital Photography School. Free Adobe Lightroom Tutorials. Strobist. Using External Flashes in the Manual (M) Mode. What is the Manual (M) Mode?
In the Automatic (A) mode, the sensor of an external flash takes over the control of flash exposure. In the Manual (M) mode, we return to the basics and do everything about flash exposure ourselves. More precisely, we choose an ISO speed and a lens aperture (resp., a flash-subject distance) and calculate the flash-subject distance (resp., lens aperture). The camera only triggers the flash, and when the flash fires, it always sends out its maximum power and won't cut off early.