Camera Flash: An Introduction to TTL Flash

Cameras and flash units have gone together hand in hand forever. Since the beginning of photography cameras have been hungry for more light. When natural light couldn't do the job flash powders were employed to add an intense, bright light to fill the scene. Later, flash bulbs in shiny parabolic dishes took the place of the explosive flash powder. Photographers carried as many flash bulbs as they could cram into their pockets because once they ran out of bulbs their shooting day was done.

Then electronic flash was introduced, strobe lights powered by batteries or plugged into wall sockets. Soon the Xenon electronic flash became the standard means of adding light to a scene. From the 1950's through today electronic flash units have become smaller, more compact, and a lot more powerful. Along the way there have been a lot of refinements and improvements all of which have allowed flash photography to become more accurate, repeatable and automatic.

Don't worry; the purpose of this article isn't to delve deeply into the technical aspects of electronic flash. Rather we will take a very quick look at a term we hear so often and may not understand: TTL-Flash.

Today's camera flash is nothing less than a small computer with a flashtube attached. Working with the camera's computer the flash light is measured and metered out in just the right proportion to create a good exposure. In a very brief span of time the camera shutter opens, the flash fires and the shutter closes completing the exposure. The system that controls the amount of flash to use in order to create a proper exposure is generically referred to as TTL-Metering - Through the Lens Metering.

TTL-Metering gets its name because the flash exposure was measured by the camera's light meter through the camera lens. Flash units that support the system are called TTL-Flash. The name sticks with the system today even though the level of sophistication is much higher. In today's camera the metering system either points toward the imaging sensor reading the amount of flash light striking the sensor or the meter is the image sensor itself. Either way, technically the flash exposure metering system is still reading light that is coming through the lens.

While most camera manufacturers have their own brand name for the process the bare bones of the system are pretty much the same. However TTL-Flash has gone far beyond just measuring light entering the camera. Distance information from the camera's lens helps further adjust the amount of light released in the exposure burst. In fact many systems have a pre-flash burst and an exposure flash burst. The pre-flash burst allows the camera to measure the reflectivity of the subject, its color and the contrast to the surrounding components in the scene before opening the shutter to make an exposure. The pre-flash burst and the exposure burst happen so fast they may seem like one flash of light.

Some of the things that we take for granted about our cameras are possible only because of TTL-Flash. The "Green Zone" easy exposure mode is one example. The Green Zone is a fully automatic exposure mode that even turns flash on and off as it deems necessary. Because of the high degree of computerization in both the camera and the flash the photographer doesn't need to set anything in this exposure mode.

For all of the advantages of TTL-Flash there are a few snags too. First and foremost is that even though most TTL-Flash systems behave in the same way each camera manufacturer has a different means of deploying it in their cameras. The result is that a Canon TTL-Flash won't work on a Nikon camera. The second drawback is that the TTL-Flash must be connected to the camera in order for the TTL-Metering system to work. Thankfully today "connected" can be either a physical attachment or a wireless link, but either way there has to be a means for the flash and camera computers to talk to each other.

In conclusion: TTL-Flash has been a liberating factor for today's photographers. The camera's computer and the flash units computer work together to effortlessly create proper exposure shot after shot. Many TTL-Flash systems use a pre-flash burst to measure the subject and its environment for a more refined exposure computation. Even though TTL-Flash systems are all essentially the same each camera manufacturer has a proprietary means of achieving flash and camera communication.

Author: Stu Eddins

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