TTL shooting modes (automatic exposure evaluations that are synced across the camera, lens and flash system) are great for capturing basic exposures of a scene, but these automatic camera evaluations restrict you from using light in more creative ways. Meter readings also can be very wrong, especially when working in tricky environments with mixed lighting situations or when using modification tools like bounce cards, reflectors, diffusion panels and other gear.
Learning to understand manual settings will give you extended control over flash output while also providing consistent results that you can modify as needed for the best image, even when using extra tools that will affect your total light output.
Even when using a TTL mode, you still have a lot of control over output. Most flashes allow you to adjust exposure value via a +/- button with a lightning bolt, usually by a 1?3- or 1?2-stop. Locking in flash exposure compensation also will let you change position without affecting the amount of light produced by the flash.
Shooting at full power will deplete batteries the fastest and requires the longest recharge times between bursts. You don’t always need full power, however, and choosing to shoot at lower output will increase battery life and recycle times dramatically.
Distance will affect the light throw, as well. Light falloff equals the square of the distance to the light source, which is known as the inverse-square rule. In other words, a subject that’s moved to twice the distance from a light source will require four times the amount of previous light for the same exposure. So moving your flash forward or backward is another great way to add or reduce light as needed, and light output is exponential as you get closer to the subject.