Flash Photography

TTL (Through The Lens) flash metering allows the camera to automatically calculate and set flash power for correct exposure. Modern TTL systems (Canon E-TTL II, Nikon i-TTL, Sony TTL) fire a pre-flash milliseconds before the shutter opens, measure the reflected light from the scene, and calculate the appropriate flash power for the main exposure — all invisibly and instantaneously. TTL is powerful for run-and-gun shooting (events, weddings, photojournalism) where scenes change constantly and manual flash adjustment would be too slow. TTL works in combination with the camera's ambient light exposure: in Aperture Priority with TTL flash, the camera exposes the background with the ambient settings and adds TTL flash to illuminate the subject. Flash Exposure Compensation (FEC) adjusts the TTL flash power independently of the ambient exposure. FEC +1 doubles flash output; FEC -1 halves it. Common use: -1 to -1.5 FEC for subtle fill flash that doesn't look obviously flashed. TTL limitations: it can be fooled by very reflective subjects (shiny surfaces, white backgrounds) or very dark subjects, similar to how ambient metering fails on extreme tones. Flash Guide Number (GN) describes a flash unit's power: GN = Aperture × Distance. A GN 40 (meters) flash at f/4 correctly exposes a subject 10 meters away (40 ÷ 4 = 10). Manual flash uses guide numbers to calculate exposure mathematically when TTL is unavailable or unreliable.