what is TTL functionality
We suggested that the external flash be set to the TTL mode. The TTL (i.e. , T hrough T he L ens) mode means that the camera will measure the output of external flash and stop the flash once illumination is sufficient.This is the easiest mode of using external flashes.However, do keep in mind that the Coolpix 990 is not true TTL cameras in terms of flash control. It measures flash illumination with a flash sensor next to the on-camera lens as shown below. As a result, what this sensor sees is usually not what the lens sees.his is the easiest mode of using external flashes. However, do keep in mind that the Coolpix 990 is not true TTL cameras in terms of flash control. It measures flash illumination with a flash sensor next to the on-camera lens as shown below.