Mixing ambient light with flash is a tough technique to get around. I can do it no problem, but when someone asks me to explain it I draw a blank. There’s a very good reason why it’s hard to understand: you’re smashing two distinct schools of lighting an image together.
A long time ago I was a young art student, being told about the “Rule Of Thirds.” I was told it’s one of the most important fundamentals of art and photography, as it helps you get the right composition in your images.
Flash is a complicated beast, let’s put it that way.
Say you’ve spent the last few years of your photography journey perfecting natural light, and you know the exposure triangle by heart. Flash comes in to the picture and ruins that perfect triangle, making it more of an exposure square.