TTL Flash Metering
Understanding Through-the-Lens Flash Metering
Today’s DSLRs have light sensors that measure the ambient light, and the lens on the camera affects this. However, when you are using TTL flash metering to set your exposure, you need not worry. TTL flash metering doesn’t base the exposure reading on the ambient light, but rather on what the flash output light will be (based on a few factors). There are two basic ways to meter for flash. One way is to measure the flash light burst as it’s being projected. The second method uses a pre-flash as test light burst (of known brightness) and this is used to make the exposure calculations. In the three types of flash metering modes (TTL, automatic-TTL and evaluative-TTL), TTL and A-TTL use the first method, and E-TTL uses the second method. It should be pointed out that a flash unit capable of E-TTL, supports high flash sync. Now, let’s discuss the three modes in more depth.