You want to change the angle of the light to get better results.
yes you need to meter the light if you use flash
flash synchronization or flash sync is the synchronizing the firing of a photographic flash with the opening of the shutter admitting light to photographic film or electronic image sensor.
Evaluate through the lens
the second curtain starts to close before the first curtain has fully opened
softening the light
a flash unit set to slave mode which will fire when it detects the output of another flash unit
Narrow or open your aperture to allow less or more light to pass if your photos are coming out over or under exposed
