Flash Fundamentals: Sync, Power, and Triggering
Off-camera flash opens a dimension of creative lighting control inaccessible when the flash is mounted on-camera. An on-camera flash illuminates from the same axis as the lens, producing flat, shadowless light with characteristic red-eye. Moving the flash off-camera — even a short distance — introduces directionality, shadow, and three-dimensionality that makes photographic subjects look solid and real.
Shutter speed controls the ambient light exposure in a flash photograph. Because the flash duration is extremely brief (typically 1/1000 second or faster), the flash exposure is controlled by aperture (how much of the flash's light is captured) and flash power (how much light the flash emits). The shutter speed, within the limits of the camera's sync speed, determines how much ambient light reaches the sensor. A fast shutter speed darkens the ambient exposure while the flash provides the primary illumination. A slow shutter speed allows ambient light to contribute equally or more than the flash.
Syncing the flash requires that the shutter curtain is fully open when the flash fires. At shutter speeds faster than the camera's X-sync speed (typically 1/200 or 1/250 second), the second shutter curtain begins closing before the first has fully opened, creating a dark band across the frame. High Speed Sync (HSS) solves this by firing the flash as a rapid series of pulses throughout the entire exposure, allowing sync at any shutter speed, but at a significant flash power cost.
TTL (Through-the-Lens) metering allows the flash to automatically calculate required power by firing a pre-flash and measuring reflected light. Manual flash mode requires the photographer to set power explicitly — typically expressed as fractions (1/1, 1/2, 1/4, 1/8, 1/16). Professional photographers often prefer manual flash for consistency: TTL can be fooled by highly reflective subjects, while manual produces identical results for identical setups.