Through-the-lens, or TTL, flash fires a preflash, meters the reflected light, and calculates the main flash's power automatically. It is fast and convenient but not always perfectly consistent. Manual flash delivers a fixed output that is predictable and repeatable, ideal for studio work. Flash guide number, expressed as GN = distance × f-number at a given ISO, helps calculate exposure: at ISO 100 with a GN of 40, an aperture of f/4 reaches a subject 10 m away, and doubling ISO multiplies the effective guide number by roughly √2.
Flash sync speed is the fastest shutter speed at which the entire sensor is exposed at once—typically 1/200s or 1/250s on DSLRs, and up to 1/8000s on mirrorless cameras with electronic shutters. High-speed sync pulses the flash across the shutter slit so faster shutter speeds can be used, which is essential for fill flash in bright sun at wide apertures. Rear-curtain sync fires the flash at the end of a long exposure, leaving a trail of ambient motion behind a frozen subject—a creative technique for low-light shots. Flash exposure compensation adjusts the flash output in stops independently of the ambient exposure, brightening the subject without affecting the background. Fill flash uses a small burst—often minus one to minus two stops relative to the ambient exposure—to lighten shadows in backlit or harsh midday conditions.
Color can be controlled with gels, plastic sheets placed over the flash: CTB (color temperature blue) to match daylight, CTO (color temperature orange) to match tungsten, and creative colors for effect. The inverse square law means that doubling the flash-to-subject distance reduces illumination by two stops, so careful placement is essential when using off-camera flash. Mixed lighting—flash combined with ambient sources of different color temperatures—is often resolved by gelling the flash to match the ambient or by gelling the ambient to match the flash.