TTL metering (Through-the-lens): The TTL flash metering system determines the flash exposure automatically. Pre-flash metering: A pre-flash fires before the main exposure. The preflash is metered (14 segment) and fed back to the flash exposure system to more accurately determine the flash exposure. ADI metering (Advanced Distance Integration): Flash metering is controlled by the guide number in addition to TTL metering. Flash output is less influenced by the background conditions or the subjects reflectance.