Studio-Like Modeling Light
Aside from their lower output, one of the main disadvantages of using portable flash instead of studio flash is the lack of a modeling light. A modeling light is a continuous light source that helps you visualize the shadows, highlights, and reflections the flash will produce before it fires. Nowadays, higher-end flash units by Canon and Nikon have built-in modeling flash, which fires a series of weak flashes to emulate a conventional modeling light (see workshop 35). Canon’s modeling flash is activated by pressing the depth-of-field preview button on the camera body.
This is fine if the flash is mounted ...