You would use external flash when there isn’t enough natural light to fit what you want, or you don’t want to natural light, and want the sharper flash
no cause the meter works for only natural light
flash sync is synchronizing the firing of a photo flash, when the shutter open giving light to the photo
it’s a mode where the camera uses information from the lens to calculate how much light needs to be used for good brightness
half the picture will be black the other will be exposed
to get a sharper image since they don’t need to adjust the ISO to the natural light
a slave is a feature used to instruct a flash unit of incoming light, and fire when it senses light produced by another flash unit
change the ISO either up or down to capture more, or less light