Therefore, digital SLR cameras typically don't support any real-time TTL flash metering and must use preflash metering instead. The ambient and flash light metering is then carried out by a metering module located in the roof of the camera (see above).OverviewIn, through-the-lens metering (TTL metering) refers to a feature of whereby the intensity of light reflected from the scene is measured through the ; as opposed to using a separate m. Through-the-lens metering is most often associated with (SLR) cameras. In most film and digital SLRs, the light sensor(s) for exposure metering are incorporate.