TTL FLASH AND DGITAL CAMERAS

In the days of lm photography, a separate TTL flash sensor in the camera continuously measured the amount of light that was reflected from the surface of the camera lm. Once the flash sensor had determined that the subject had been exposed correctly, the flash was extinguished.

The imaging sensors in digital cameras don’t reflect light in the same way that a frame of lm does, so TTL flash metering has had to evolve. Today’s advanced TTL flash systems, such as Nikon’s iTTL and Canon’s E-TTL II, use a low-power pre-flash to determine the optimum flash exposure before the main flash exposure begins. In some instances, you may notice this pre-flash — watch for people blinking in flash-lit portaits — but invariably the light from the pre-flash and the main flash are indistinguishable.

The light that’s reflected by the subject through the lens during this pre-flash hits the shutter curtains, and is measured by the camera’s normal metering system rather than by a dedicated flash meter.

01 QUICK FIRE

When you press the shutter release, the camera’s mirror flips up and a pre-flash is red. The subject and other parts of the scene that are illuminated reflect some of this light into the lens.

02 MEASURING UP

The llight strikes the shutter curtains that continue to remain closed in front of the camera’s sensor. The camera’s exposure meter then measures the light being reflected by the curtains.

03 CONTROLLING THE LIGHT

The camera then uses this information to determine the optimum power for the main flash. The shutter curtains now open to expose the sensor and the main flash is red.

04 FINE TUNING

Flash power falls away with distance, but a larger aperture or higher ISO will extend the flash’s reach. Try using flash exposure compensation for stronger or more subtle results, too.