In this informative video from Jay P. Morgan of The Slanted Lens, Jay goes back to the basics to show off the principles of light, and how they affect a sphere. Whenever a light is added to a subject, five things happen, and this video explores what is created, and how to control it, which ultimately will help you to craft your final image in a photo or video.
A few weeks ago I shared a video called "The Light in Your Eye" which went through, in great detail, the nuances of how to create a catch light and the kind of feelings they can impart. Studying light and seeing creative examples of how it can be controlled never seems to get old.
Back to Jay P.'s video above, which is like a lesson pulled from an intro to studio lighting class, he touches on adding another light to the sphere, which creates another five effects (incident, highlight, core, shadow, cast shadow) on the sphere. While he does note that the newly created core might not be desirable (and he moves to using a reflector), for me that brings up an important point. Every time you add a light, you add more shadows and highlights, and when they mix with the shadows and highlights of your other lights, it can sometimes create more problems than it solves.
In this video example, open-face lights are used (sometimes with bounce cards or reflectors) which is good for showing how they affect the sphere. In practice though, I think really manipulating that light comes from the skilled use of modifiers. Controlling spill, placement, shape, and spread is really where you begin to craft the look of light in a scene and on a person.
Be on the lookout for more videos from the "Laws of Light" series from the Slanted Lens.