Of all the advanced photography techniques, I recommend learning the Inverse Square Law first. It takes a bit to wrap your head around it, but with just a bit of patience and some practice it will soon get in your reflexes. To help you get started, SLR Lounge does a great job at explaining the whole concept:

The inverse square law basically says that the further you are from your light source, the more of that light will be lost—and you’re losing even more light than you think.

Here’s the equation you need to remember regarding this effect:

LIGHT INTENSITY = 1 / DISTANCE²

What this means is that the light intensity decreases the further away you are from your source of light.

In practice, this means that if at 1 meter from the subject your light intensity is at 100 percent, at two meters it will be at 25 percent, and at 6 meters it will drop down all the way to 2.78 percent. Now, this is a huge drop from the first few meters onward, but this formula shows that the further away you go from the light source, the smaller the drop in intensity will be. If at 6 meters the intensity is at 2.78 percent, at 7 meters it will only drop down to 2.04 percent. Go a meter forward and the intensity will only drop to 1.56 percent.

The perfect example for this law in action is the sun; it’s so far away from all of us that it doesn’t matter if you’re on top of Mount Everest or if you’re at sea level—the sun will light you with pretty much the same intensity. If, however, you’re using a flashlight as your source and you move from right next to it to across the room, you’re going to see a significant drop in the light intensity.

What does this mean for us as photographers?

It means that when you shoot a photo of a group, make sure to have all of our subjects either at the same distance from the flash or light source you’re using.

For even better results, place the light source further away from the group, so that even though they might be at different distances from the source, they will still be evenly lit.