TTL stands for “Through The Lens,” a flash metering system where the camera measures the actual light passing through the lens to determine the correct flash exposure. It is the most common automatic flash mode in modern cameras and the foundation of convenient, accurate flash photography in changing conditions.
How TTL Flash Works
When you press the shutter button with TTL flash enabled, the camera fires a brief, low-power pre-flash an instant before the main exposure. This pre-flash bounces off the subject and returns through the lens to the camera’s metering sensor. The camera analyzes the reflected light, calculates the distance and reflectivity of the subject, and sets the flash power for the main burst accordingly. All of this happens in milliseconds, invisible to the subject in most cases.
TTL vs. Manual Flash
In manual flash mode, you set the flash power yourself (full, half, quarter, etc.) and it stays at that level for every shot. Manual flash gives you complete control and consistent results when your subject distance and lighting conditions do not change, making it ideal for studio work and situations where you can take test shots.
TTL flash adjusts automatically for each frame. If your subject moves closer or farther away, TTL recalculates and adapts. This makes TTL invaluable for events, weddings, and any fast-paced situation where you are moving and the scene is changing between shots. The trade-off is that TTL can sometimes be fooled by very bright or very dark backgrounds, producing inconsistent exposures in tricky lighting.
Flash Exposure Compensation
When TTL gets the exposure wrong, flash exposure compensation lets you override the system without switching to manual. Dial in positive compensation (+1, +2) to increase flash power, or negative compensation (-1, -2) to reduce it. This is especially useful when shooting subjects against very dark or very bright backgrounds, where the metering system tends to over- or underexpose.
Practical Applications
TTL works with both on-camera and off-camera flash setups, provided the flash and camera communicate wirelessly or through a dedicated cable. It is fully compatible with bounce flash techniques, where the longer light path is automatically compensated for by the metering system. For most photographers, TTL is the default mode for everyday flash photography, with manual flash reserved for controlled environments where consistency across frames is more important than automatic adaptation.