Additive light can be best defined as being in any situation where there is already light present and you make the decision for whatever reason, to add light. Certainly working with flash/strobe requires a great deal of understanding in order to be able to get exactly what you want each time, with predictable results. There are a lot of different reasons people will choose to use additive lighting but for me personally, it is best determined by whether or not I need to change the brightness of my background separately from my subject. I know that sounds strange but bear with me as I explain.
While the reciprocal, or reciprocity of photography is important to know and understand (i.e., 1/60 at f16 same exposure as 1/500 at f5.6) it only applies in an ambient or continuous light situation. When you introduce flash to the scenario, everything changes and a full range of understanding and control matters more than you might imagine.
In the first few years of becoming a photographer I stayed away from flash outdoors, not because I didn’t feel that I needed it, but because there was really not many talking about it and I had no one to learn from in my area. Then one day I was photographing a high school football players’ individual posed shot for a sports pack and had a “happy accident” where I accidentally increased my shutter speed on my old Hasselblad while using a Vivitar speedlight on top of the prism viewfinder, set to Blue/f8. The result was an almost magical deep rich and cloudy sky, dark sunset colors, and a perfectly exposed and lit player. I had no idea how it happened or how it worked. All I knew is that this was the single best football player picture I had ever taken and I needed to learn to do reproduce this.
Clearly, we all start out not knowing a lot of photographic technique and we tend to go through a lot of self-discovery. My education was very limited to the other photographers in my town and surrounding areas. Only one other photographer in my town was doing any flash work to speak of so I tried to learn what I could from him. What I learned by observation was that he was pretty good but couldn’t express very well what was happening nor did he know how to create repeatable and consistent results. Slowly, I learned that there is a very clear separation between the shutter speed and the aperture once flash was introduced.
The concept of how this works is simple and should not be confused within all of the noise in the photographic circles today that seem to have “foolproof” answers for everything. There are a lot of easy ways to achieve this type of work, but basically when using flash in an ambient light situation, the shutter speed controls the ambient light, and the aperture controls the brightness of the primary subject being lit by the flash. Once this single sentence soaks in and becomes part of your Photo DNA, you’ll have great success in then moving forward with the techniques and methods of making this work for you in your situation at any time of day or in any location.