What is TTL?
TTL stands for Through the Lens. At its most basic, TTL means that the light or flash for photography is automatically calculating and adjusting the output of the flash in real time. All modern auto flash systems use some version of the TTL concept to calculate the correct exposure. Whether they are battery-powered photography lighting equipment, like flashguns or those that need a power source like studio lamps.
The correct flash exposure for any scene depends on four variables: the shutterspeed, aperture, ISO (sensor sensitivity) and power (of the flash).
TTL is merely the Program mode for flash settings. In P mode, the camera calculates settings like ISO, aperture and shutterspeed by metering the scene, unlike Manual where the photographer needs to do it. Similarly, with TTL light system, the calculations are automated.
How TTL Works
A TTL-enabled light fires twice when you release the shutter. Let’s say you are at an outdoor wedding photography shoot. You have set up your studio lights for the portrait, the couple is posing, and you find the right moment. He laughs, she has a twinkle in her eye and you hit the shutter release. Here’s what happens next. First, a preflash (or a series of them) is fired, which helps the camera determine the ambient light and the light from the flash. The preflash travels to the subject and back to the sensor, providing it with the necessary information to calculate the correct exposure. Think of the preflash like a test shot for the camera.
Based on this, the flash power is adjusted, a correct value for the output is selected, and the flash is fired. Moreover, this time the camera records the image to your memory card.
For example, let’s say you are indulging in a portrait photography session. The sun is setting over the horizon in the background, so you decide to use a flash as a fill light to prevent a completely black silhouette. The flash would need to know how far the subject is from you and how much light is needed while not overpowering the ambient light.
Alternately, imagine a scenario where you need to bounce the flash off the ceiling, to avoid harsh reflections. The flash would need to determine the distance from the camera to ceiling, from the ceiling to the subject and all the way back to the camera. And then, calculate the output based on this more considerable distance. All of this gets calculated during the preflash exposure.
If you plan on keeping your eyes peeled for this preflash, you are going to be disappointed. While some preflashes may be white light, they can also be IR, and additionally, they occur microseconds before the actual flash. As a result, they are imperceptible to the human eye.
The use of the TTL preflash has allowed companies and photographers to experiment with more creative forms of flash photography. For example, multiple flashguns can be triggered by a single flashgun (designated as the “commander” unit) to create unique lighting effects for professional photography.
A Brief History of TTL
The use of the phrase Through the Lens in reference to flashguns and other light sources, such as pack-and-light systems, is a throwback to TTL metering, the form of metering cameras use.
Cameras with TTL metering were able to calculate the required exposure by measuring the intensity of light reflected into the lens, without needing to use a metering window or an external light meter. As a result, cameras could calculate the correct exposure settings on behalf of the photographer. TTL lights and flashguns work along the same analogy. The camera’s exposure meter figures out the intensity of the light and calculates the required settings accordingly.
Pros and Cons of TTL
TTL has the same advantages and disadvantages that Auto/Program mode offer for metering ambient light. Since the light is making the calculation decisions for you, as a photographer you can spend more time focusing on the scene in front of you rather than wasting time on settings. As a result, TTL can be quite useful for genres like wildlife, action, street and travel photography where the light changes almost as quickly as the action ahead of you. With Manual flash settings, you would need to calculate the output power of the flash for yourself, which can sometimes be quite a difficult game of trial and error.
However, the downside is that because metering can change with even a slight change in the frame, with TTL you may end up with a series of shots with different exposures for the light. Additionally, TTL does not allow for much control over the flash output, beyond flash exposure compensation.
A simple hack works well here. Many new TTL flashes and lights, like the Elinchrom ELB 500 TTL, can switch from TTL to Manual while remembering the TTL settings. You can further fine-tune the settings in Manual mode to capture the perfect light, giving you the best of both worlds.