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.

Through-the-Lens Metering Mode

TTL Flash Metering is the standard metering mode that your camera employs where you’re using the camera’s built-it or pop-up flash. You can also use this metering mode with some of the dedicated flash units available for your camera.

TTL flash metering measures the burst of flash output light reflecting back from the subject and takes its reading through the lens. It will take this reading from the section in your view where the active focusing point is set.

TTL manipulates the flash exposure with a dedicated sensor that measures the flash output reflected from the surface of the image sensor during active exposure. The one thing TTL does not employ is a pre-flash for the flash exposure calculation. Note: when you have your camera set in Program, TTL will choose an aperture setting based on the amount of available ambient light.

Advanced TTL Metering Mode

The automatic-TTL does all the same functions as the TTL metering mode in that it reads through the lens and concentrates its sensitivity on the section of your frame covered by the active focusing point. What A-TTL does that TTL doesn’t do is that A-TTL uses a pre-flash to calculate the appropriate f-stop based on the distance the flash output light must travel from the flash to subject.

This burst of light engages when the shutter release is halfway depressed, and the flash unit then sends the actual scene-illuminating flash output when the shutter is fully opened. In addition, when you have set the camera to Program mode and are using A-TTL, the camera quickly compares and contrasts the distance-based aperture information with ambient light-based aperture information (measured by the camera’s standard metering system) and chooses the larger f-stop (to ensure more accurate exposure, and increase sharpness and depth of field).

Evaluative TTL Metering Mode

Evaluative-TTL uses a different technology than TTL and A-TTL in that it uses the pre-flash to determine the optimal flash exposure (by measuring the scene’s reflective quality) to provide a mid-toned subject. The E-TTL pre-flash differs from A-TTL in that the E-TTL pre-flash happens immediately before the shutter opens (not when the shutter is halfway pressed with A-TTL).

So the exposure value is calculated a split second before the main flash fires, not during the metering of the ambient light. In addition, the pre-flash is analyzed by the camera’s main TTL sensor, not the external sensor on the flash unit. This allows the E-TTL to be more accurate. It’s remarkably difficult to notice the preflash when using E-TTL, because it fires so quickly… so even if you look for it, you might have trouble seeing it.

Conclusion

The TTL Metering System has been a great boon to photographers. If the image is overexposed (or underexposed), make the necessary adjustments and shoot again. TTL metering enables you to be as precise as possible with your photographs and have exacting control over exposure settings. Full understanding how the various TTL metering modes work will enable you to get more creative with your photography, because you’ll have better command of that all important element of photography… how you record the light.

Source: exposureguide.com