-You would use external flash when you want to fill in shadows or create more light.

-If you use E-TTL mode you don’t have too.

-Flash synchronization is the synchronizing the firing of a photographic flash with the opening of the shutter admitting light to photographic film or electronic image sensor.

-ETTL means evaluate through the lens.

-What will happen is your picture will be brighter.

-They use it to get a soft light.

– Slave mode is mode that instructs a flash unit to monitor incoming light.

-If your picture is too dark or light you should add external flash to fill in dark spots and if it’s too bright use a diffuser for a softer light.