Andrew Macpherson

if your subject is 1 m and and the light is1 m away you can get an exposure. If you double the distance from the light, not only will the shadows be harder, but you will need 2 stops more light Double the distance again to 4m and you will need another 2 stops or 4 stops morethan the original shot

This is the inverse square law in action. if the extra distance to the sitter's face is small the distance to where the light has dropped noticeably will also be small

Fill flash is intended to give a lift to darker areas in an image that is otherwise lit. As such it is usually at a lower intensity than that main illumination

The ambient light exposure will leave a blur where the subject has been, the the flash will freeze the subject at the front of the blur giving the "cartoon speed " blur

Another name for a GoBo is a Flag. Extremely useful in product photography. A DIY gobo will typically be made from corex and hoop&loop tape and employ an unused light stand. Commercial gobs are much more expensive

High Speed Sync is a proprietary method used within a camera system to fire a speedlight multiple times - a sort of continuous very high frequency strobe - as the shutter gap moves across the sensor to ensure the whole image is exposed on the sensor. There is a constraint that the power level has to be sufficiently low to allow all the multiple micro-discharges to take place from the charge in the capacitors of the speedlights. The speed light fires at about 40/50 thousand times per second for the duration of the gap between the curtains crossing the sensor. this means that even if the exposure is 1/8000 each strip of sensor will be illuminated 5 or 6 times

A white background is achieved by overexposing it so that it "blows out." If it is unevenly lit, the darkest part as to be 3 stops brighter than the subject, but it is easiest to strive for even lighting

th correct exposure is based on the incident light on the best lit parts of your subject

This answer may change if sensors get a greater dynamic range

To change the exposure the light coming into the Camera (or it's effect) has top change. As the light on the subject will remain constant all you can change is the Aperture, or the ISO, and usually you will be at your lowest ISO already.

Shutter curtains move across the film or sensor of your camera to let in light. For short exposures they are both moving and only a small part of the sensor is exposed at any instant. Longer exposures have a period when the whole sensor is exposed. Sync speed is the shortest exposure where the whole sensor will recieve light from a single flash