The camera as the manager of the flash unit
High-class analogue cameras are designed for TTL flash operation (through-the-lens) which is a special variant of the automatic flash mode. In the normal auto flash mode the amount of emitted light is controlled by a computer integrated in the flash unit. In the TTL mode, however, it is the camera that controls the light from the flash unit by way of a built-in sensor which measures the light that actually reaches the film.
Digital SLR cameras are now generally equipped with advanced TTL flash controls, such as Canon E-TTL or Nikon i-TTL. The necessary luminous energy for the shot is calculated by means of pre-flashes. Based on the pre-flash, the camera can measure the light reflected by the subject via metering through the lens (TTL = Through The Lens). This metering allows the camera to determine the actual exposure required and hence the luminous energy needed for the subsequent main flash.
The TTL flash control mode enables you to work with all of the available shutter setting on the camera. This makes it easier for you to solve problems with the depth of focus, for example.